Despite being surrounded by a multitude of people, urban birds may be picky about who can approach them, new research suggests.

After surveying over 37 city bird species in five European countries, experts found that the avians fled sooner when approached by women than by men. The findings, published in December 2025 in the journal People and Nature, suggest that the birds can differentiate between the sex of the person approaching them.

The researchers used each bird's distance from a person before flying away as a metric of fearfulness. But why the birds seem to be more fearful of women remains a mystery.

"As a woman in the field, I was surprised that birds reacted to us differently," study co-author Yanina Benedetti, an ecologist at the Czech University of Life Sciences Prague, said in a statement. "This study highlights how animals in cities 'see' humans, which has implications for urban ecology and equality in science. Many behavioural studies assume that a human observer is neutral, but this wasn't the case for urban birds in our study."

Outside experts agree that these findings are puzzling, but also preliminary.

"Until we have a good reason to hypothesize such differences, I remain a bit skeptical," John Marzluff, a professor emeritus in ecology at the University of Washington, told Live Science in an email. "But I am not at all skeptical that birds pay a lot of attention to us and respond to humans in ways that are important. We just need more research here to better understand why this effect was so consistent."

Baffling bird behavior

To understand whether this puzzling response was a bigger pattern rather than a few skittish encounters, the researchers used a standard measure of urban wildlife wariness: flight initiation distance, or how close a person can get before an animal flees.

The team studied the flight initiation distance in 37 species — including the common wood pigeon (Columba palumbus), carrion crows (Corvus corone), house sparrows (Passer domesticus), hooded crows (Corvus cornix), and blackbirds (Turdus merula) — across cities in the Czech Republic, France, Germany, Poland and Spain.

Some types of birds, like pigeons, seem more comfortable with human behavior and fled later or had a shorter flight initiation distance. Others, like magpies, fled early and had a longer flight initiation distance.

During the study, a man and a woman who were of comparable height and were wearing similar clothing tried to approach a bird in a city's green space by walking in a straight line and keeping their eyes on the avian. The researchers measured the distance after the bird fled. Four men and four women, all expert ornithologists, participated in the research, so the birds interacted with different pairs of people.

From 2,701 observations collected between April and July of 2023, the team found that men could get around 3 feet (1 meter) closer to the birds than women could, on average.

"I fully believe our results, that urban birds react differently based on the sex of the person approaching them, but I can't explain them right now," study co-author Daniel Blumstein, a professor in the Department of Ecology and Evolutionary Biology at UCLA, said in the statement.

Possible theories

The researchers proposed a few ideas for why the city birds flee faster from women. For example, they suggested that pheromones, body shape or gait could be factors.

"If I had to guess, I would agree with the supposition that gait is am important cue the birds are using," Marzluff said. "What I find puzzling is that one would expect such effects to be learned by a bird’s experience with various humans in their environment. If so, then there would be no reason for birds to have only experienced more threatening women. Some surely would have experienced threatening men and therefore the overall response should be no difference in response to sex."

However, the team noted in their paper that they did not have any female participants collect data while they were menstruating — when specific scent compounds in a woman's body odor intensify — an aspect that could be investigated further.

The researchers cautioned that this study is preliminary and that more data is needed to confirm this behavior isn't just a fluke.

"Follow up studies could focus on individual factors such as movement patterns, scent cues, or physical traits, testing them separately rather than grouping them under observer sex," Benedetti said.

"But how do we test this? Perhaps a study resembling Monty Python's Ministry of Silly Walks," Blumstein joked.

Are you a bird nerd? Find out with our bird quiz!

Changing just one "letter" in the DNA of female mouse embryos triggers the development of male genitalia and testes, scientists have found.

"This is a remarkable finding because such a tiny change — just one DNA letter out of ~2.8 billion — was enough to produce a dramatic developmental outcome," senior study author Nitzan Gonen, a senior investigator at Bar-Ilan University in Israel who studies how sex is determined during embryonic development, said in a statement.

In early development, the emergence of male sexual characteristics relies on two key genes: SRY and SOX9. The first gene carries instructions for a protein that then interacts with and switches on the second. This activation of SOX9 triggers a chain reaction that causes testes and sperm-making cells to develop.

Back in 2018, Gonen and colleagues pinpointed another stretch of DNA that is important for this chain reaction. This snippet of DNA, called enhancer 13 (Enh13), doesn't carry instructions for any proteins. Instead, it acts as an "on-off" switch for SOX9. The SRY protein latches onto this switch, flipping it and sending SOX9 into overdrive.

In previous work, the researchers also found that, by eliminating this on-off switch, they could reverse the sex of male mouse embryos. Even though they carried XY sex chromosomes, the mice without Enh13 developed as females. Enh13's absence causes SOX9 activity to fall by about 80%, which prevents the development of testes and instead ushers the development of ovaries, the researchers reported. Tweaking only select bits of Enh13 has the same effect, the team found in a later study.

The scientists suspect that mutations in this on-off switch may contribute to certain differences of sexual development (DSD) in humans, which can cause a person's sex chromosomes and sexual characteristics to be mismatched. In particular, it was clear Enh13 might play a role in cases where XY individuals develop female characteristics, as they'd explored in mouse studies.

But some studies also hinted at a potential role in conditions that cause XX individuals to develop male features. And the new research, published Thursday (April 9) in the journal Nature Communications, backs up this latter idea.

The researchers tweaked Enh13 in female mouse embryos by either deleting three letters or inserting one letter into the portion of the on-off switch that SRY latches onto. Both of these modifications caused small testes and male external genitalia to develop in the female mice, although they also grew some ovarian tissue.

For male sex organs to develop, the mutation had to affect both copies of Enh13; cells carry two copies of chromosome 17, which Enh13 is found on. If only one copy was mutated, the XX mice developed normal ovaries and no male organs.

Typically, the SOX9 gene must be switched off for ovaries to develop properly, and XX embryos use various mechanisms to achieve that. The new study suggests that mutations in Enh13 can lift the brakes on SOX9, allowing the gene to activate to a small degree even without any SRY protein present.

Once activated, SOX9 can sustain and amplify its own activity, so "this minimal activation would be enough to trigger the self-amplification loop," the study authors wrote.

In the long run, these results could help scientists better understand how DSDs emerge in humans, the researchers say. For now, though, the work raises a number of hypotheses about Enh13's role in sex development in both males and females, and more research is needed to fully unpack its effects.

"Our findings show that it is not enough to look only at genes," Elisheva Abberbock, a doctoral student at Bar-Ilan University who led the research, said in the statement. "Important disease-causing mutations may also lie in the non-coding genome, in DNA regions that control gene activity rather than encode proteins."

This article is for informational purposes only and is not meant to offer medical advice.

On average, human males tend to be about 5 inches (13 centimeters) taller than females. But why?

In short, we're not sure. Even though this sex-linked height difference exists across the world, we still lack a full scientific understanding of why. But we do have a few clues from sex-linked genes and hormones that are helping to unravel this mystery.

Height is a complex trait influenced by many factors. One of these is genetics: Genes on both sex-linked and regular chromosomes help determine height. Every baby receives two sets of chromosomes: 23 from their mother and 23 from their father. Two of these chromosomes are sex chromosomes: Females tend to be XX and males are usually XY. Parents often contribute genes consistent with their own features, so usually taller parents will have taller kids than shorter parents do.

Studies of identical twins have found that height is about 80% heritable, meaning about 80% of the differences in height can be attributed to genetics.

But disentangling which genes contribute to height is complicated. "Height is a famously polygenic trait," Alexander Berry, a bioinformatics scientist at Geisinger College of Health Sciences in Pennsylvania, told Live Science. This means that many genes influence it. Height can vary wildly based on the thousands of genes affecting it throughout the genome. "Genome-wide association studies that look at height find hits all over the genome," Berry said.

One 2022 paper published in the journal Nature found 12,111 places in the genome where a single nucleotide — the G, A, T, or C in a DNA sequence — varies and is significantly associated with height.

Related: What determines a person's height?

One of the best-studied genes connected with height is called SHOX, which is on both the X and Y chromosomes. SHOX is estimated to account for about 25% of the 5-inch-average disparity between males and females, Berry said. He added that gene variants in which SHOX loses its function on the X or the Y chromosome can lead to significantly reduced height. For example, the genetic disorder Léri-Weill dyschondrosteosis, which is caused by a mutated SHOX gene, affects bone growth and results in short stature.

Another place to investigate height disparities is in people with sex chromosome aneuploidy, or an atypical number of chromosomes, such as XXX or XXY. In a paper published in May 2025 in the journal PNAS, Berry and his team studied 928,605 people using data from three biobanks. Of this group, 1,225 people had sex chromosome aneuploidy. In these people, the researchers studied how the dosage — or number — of genes affects these sex chromosomes. In other words, how did more or fewer specific genes on these extra or missing Xs and Ys affect height?

The findings shed some light on how SHOX affects height in the average male and female. The team found that more Y chromosomes impacted height more than more X chromosomes did. In general, those with extra Ys tended to be taller than those with extra Xs. This is in part because those with XX chromosomes have one inactive X, meaning it doesn't transcribe all the same genes, including SHOX. But someone with XY chromosomes has two sets of transcribing SHOX genes. For the average male and female, this finding supports that SHOX expression occurs at a higher rate in those with XY chromosomes than in those with XX.

But Berry cautioned that this finding doesn't mean that the Y chromosome itself is responsible for this disparity.

"Our data doesn't show that at all," he said. SHOX occurs on both the X and Y chromosomes, so its impact on height isn't unique to males. However, Berry said there could be Y chromosome-specific codings that impact hormones, like testosterone, which, in turn, can influence vertical growth. Testosterone is a hormone that both male and female sex organs produce, though males naturally make more of it. An androgen, testosterone is responsible for development of typically male characteristics like a deeper voice and more body hair.

"It's likely that other genes on the Y chromosome influence hormone production, directly or indirectly," he added. However, "the relationship between hormones and height is complicated."

Related: Why do women tend to outlive men?

Other hormones contribute to growth, as well. Human growth hormone comes from the pituitary gland, promoting growth in children. Insulin-like growth factor 1 (IGF-1) mediates the effects of human growth hormone, with both peaking in puberty.

But in the story of hormones and height, "estrogen is a star," Holly Dunsworth, an anthropology professor at the University of Rhode Island, told Live Science. Produced in both male and females during puberty, estrogen "is a major driver of long bone growth," she said.

When it comes to bone growth, estrogen works in two phases. The first phase involves driving height. During the second phase, eventually there's so much estrogen that the bones ossify, or fuse, Dunsworth said. Once estrogen production peaks in adolescents, their bones ossify. Estrogen production peaks sooner in females compared to males, who continue growing.

Dunsworth authored a 2020 paper published in the journal Evolutionary Anthropology exploring estrogen's role in the height difference between males and females. Crucially, this hypothesis asserts that estrogen may partially explain the gulf in growth, as opposed to evolutionary speculation that males, as the supposedly more aggressive sex, simply evolved to be taller.

That said, a person's sex isn't the only factor that influences height; of course, some females are taller than males.

Beyond genetics and hormones, environment plays a significant role, too. Since height is 80% heritable, 20% is attributed to environmental factors, such as nutrition and climate. This confluence of drivers has made it difficult for researchers to definitively answer why men tend to be taller than women. But Berry thinks that another factor — gene expression — could be the next key to this mystery.

"I think we might find, with some larger gene expression studies, that height variation even among people with the same chromosomes varies because of gene expression."

Iron deficiency during pregnancy can cause a male mouse embryo to develop female features, a new study reveals.

The low iron disrupts the activation of a key gene that spurs the development of male sex organs. This causes embryos with XY chromosomes — the most common combination seen in males — to develop female sex organs instead.

"This is a completely new and totally unexpected finding," study co-author Peter Koopman, a professor emeritus of developmental biology at the University of Queensland in Australia, told Live Science. "It's never been shown before that iron can flip such an important developmental switch."

Earlier research established that the SRY gene on the Y chromosome is the "master switch" for turning on the development of male organs in mammals. An enzyme called JMJD1A plays an important role in flipping this master switch, and it requires iron to function properly. However, the connection between iron levels and sex determination was not fully understood.

Now, in a study published June 4 in the journal Nature, researchers report that iron is essential for the development of testes in XY mice. The results show that maternal iron deficiency disrupts the activity of JMJD1A, which lowers SRY expression and drives the development of ovaries in XY mouse embryos.

However, it's too early to say whether this finding in mice might translate to human pregnancy and sex development, Tony Gamble, an associate professor of biological sciences at Marquette University in Milwaukee who wasn't involved in the study, told Live Science.

Related: Is there really a difference between male and female brains? Emerging science is revealing the answer.

In the study, the researchers used pharmaceutical treatments and low-iron diets to manipulate the iron levels in pregnant mice. When the pregnant mice experienced iron deficiency, this caused six out of 39 total XY embryos to develop ovaries instead of testes. Investigating further, they found that genetics appear to be a factor in which embryos are sensitive to this effect.

To confirm this mechanism, the team also grew embryonic gonads — structures that develop into testes or ovaries in the womb — in lab dishes so they could directly observe the impact of iron depletion. These lab analyses showed that reducing the iron in cells to 40% of normal levels led to a large increase in histones on the SRY gene. Histones are proteins that bind DNA and help control which genes are switched on, and this effect almost completely blocked the SRY gene's expression.

Normally, the JMJD1A enzyme rids the SRY gene of histones, allowing it to turn on. The researchers hypothesize that when iron levels drop, the enzyme's activity is compromised, so suppressive histones build up on the SRY gene.

These results suggest that "some important developmental traits that were previously thought to be purely genetically controlled can also be seriously impacted by nutrition and metabolic factors," Koopman said. And "if iron can have such an impact on sex development, then maybe other organ systems may also critically depend on iron or other dietary factors in a similar way," he added.

Because the research was conducted solely in mice, the question of whether iron may have similar effects in humans is still open. Although sex determination follows a broadly similar blueprint across mammals, there are some important differences between mice and humans, Gamble said.

For example, while both species rely on the same genes to drive the development of testes, the consequences of mutations in these genes differ between the two species. Their similarities to humans make mice important models for studying development and disease, Gamble said, "but the differences urge caution in simply assuming processes are acting identically across both species."

Testing the new finding in humans won't be easy, since many of the experiments possible in mice can't ethically be done in humans, Koopman said. "So, the way forward will have to involve doing biochemical, cell culture and gene expression experiments to build a body of indirect evidence that what holds true in mice is also the case in humans," he said.

Small pieces of tooth enamel from deep in a South African cave have begun to reveal secrets held for 2 million years by a distant human relative, a new study finds.

Archaeologists recovered teeth from four members of the species Paranthropus robustus, a two-legged human relative who lived between 1.8 million and 1.2 million years ago, from Swartkrans, a fossil-bearing cave in Africa's Cradle of Humankind World Heritage site. Using cutting-edge techniques that can analyze fossils’ amino acid sequences, the researchers were able to determine the sex of the individuals and discovered surprising genetic variation that could point to the existence of a previously unknown species.

These techniques are part of the field of proteomics, or studying sets of preserved proteins — a relatively new area of science that is shedding much-needed light on the evolution of early hominins, a group that includes humans and our closest relatives.

"Figuring out the human family tree using proteins is the goal," Claire Koenig, a postdoctoral researcher at the University of Copenhagen and co-author of a study published Thursday (May 29) in the journal Science, told Live Science in an email co-written with lead author Palesa Madupe and co-author Ioannis Patramanis. But currently "our ability to distinguish between different species is limited by the small number of different proteins present in enamel."

Although DNA has been recovered from ancient skeletons in Africa, so far that technique has only successfully worked on hominin material dating to no more than 20,000 years ago — well within the lifetime of our own species, Homo sapiens — because DNA degrades quickly in that environment. To get at the roughly 6 million-year history of hominin evolution, analysis of the harder and more stable tissue of dental enamel is needed.

In the new study, an international team of researchers led by Madupe employed paleoproteomic analysis to move beyond the limits of ancient DNA and understand the genes of four hominins who lived around 2 million years ago.

"Proteomics is inherently a destructive technique, but we take great care to minimize impact, especially when working with rare or precious specimens," Koenig said.

Related: In a 1st, ancient proteins reveal sex of human relative from 3.5 million years ago

The researchers focused their proteomic analysis on four P. robustus individuals who likely all died around the same time. They were able to identify AMELY-specific peptides, which are found in the tooth enamel of males, in two individuals. The other two individuals had a high AMELX intensity, meaning they were likely female.

Correctly determining the sex of a fossil is important in paleoanthropology because most hominins are sexually dimorphic, with males being, on average, larger than females. Experts therefore expect that any species will have some larger and some smaller individuals.

But Madupe and colleagues discovered a surprising result: one P. robustus individual who was thought to be female, based on tooth size and shape, was actually male, based on proteomic data. "Our results thus indicate that measurements of dental size are not necessarily accurate for correct sex estimation," the researchers wrote in the study.

Diversity in our ancient relatives

Since sex alone could not explain the differences in the appearance of P. robustus, the team investigated whether the diversity they were seeing could be the result of different groups or species they didn't know about, or the result of interbreeding, as P. robustus overlapped in time with australopithecines and early members of the Homo genus.

The researchers found a couple amino acid sequence positions that varied among the P. robustus specimens they examined, and that were different from the amino acid sequences seen in present-day humans, Neanderthals and Denisovans. This analysis revealed that one of the individuals — SK-835, whose molecular sex and morphological sex did not match up — was more distantly related to the other three individuals than they were to each other.

"It would be premature to classify SK-835 as a member of the newly proposed Paranthropus [capensis] taxa," Koenig said, but it remains a possibility that the amino acid difference reflects its position in a different species than the rest.

It could also be explained, however, by microevolution at different sites, study co-author Rebecca Ackermann, a biological anthropologist at the University of Cape Town, told Live Science in an email. "We need to analyse more Paranthropus material from different sites to get a better handle on the variation within southern African Paranthropus," she said.

Because the enamel proteome is so much smaller — and provides less information — than a full genome, reconstructions of fossil human relatives need to be cautiously interpreted, Ackermann said.

Koenig expects that further methodological developments will be beneficial, including less invasive methods such as acid etching to remove an extremely thin layer of dental enamel, and the development of faster and more sensitive protein-sequencing instruments.

"It remains to be seen, for example, whether or not we can molecularly tell apart a Paranthropus robustus from an Australopithecus africanus," Koenig said, "because these species are closely related and therefore their proteins are going to look very similar."

Evolution quiz: Can you naturally select the correct answers?

Sperm cells have a monumental task, but their days are numbered. From the moment they're made in the testes to the time they may spend winding their way up the labyrinthine of the female reproductive tract, their internal clocks are ticking.

So exactly how long do human sperm survive, in order to complete this journey?

The answer depends on where the sperm is and conditions such as temperature, moisture and energy levels. For instance, from the testes, they migrate up a narrow, coiled channel called the epididymis for up to 10 days, until they reach a reservoir at the end, where they await ejaculation.

In this protected bubble, "The sperm can be stored there in a viable but functionally suppressed state for anywhere up to around two weeks," Brett Nixon, a researcher in the School of Environmental and Life Sciences at the University of Newcastle, Australia, told Live Science in an email.

Outside the cozy confines of the testes, sperm face a more unpredictable world. While it's often said that sperm die when they come into contact with oxygen, that's not true — but moisture levels are critical to their survival.

If left on a dry surface outside the body without a liquid medium to swim in, sperm will wither and die within a few minutes to an hour. In ideal laboratory conditions where sperm may be incubated in a controlled environment — for instance, during assisted reproduction processes like in vitro fertilization — these cells can survive for up to 72 hours. Their lifespan increases significantly with the help of other technological advances, however. "If frozen appropriately, sperm can survive for decades," Nixon said.

But sperm can live naturally inside the body, after they leave the testes, for an impressive amount of time.

"There are one or two reports of sperm surviving in the female tract for the next menstrual cycle, so up to 28 days, but they're few and far between," Christopher Barratt, a professor of reproductive biology at the University of Dundee in the U.K., told Live Science. "So most people say [sperm survive] up to seven days. We know this because a person still has a roughly 5% chance of getting pregnant even if they have sex as long as seven days before ovulation, he explained. "So the presumption is that the sperm cells can survive for that period of time," Barratt said.

Related: Is the Y chromosome dying out?

How do sperm last this long? Their longevity comes down to several factors, many of which researchers are still trying to understand. The seminal plasma that carries sperm into the body could play a role. This fluid contains ingredients that support and nourish sperm, including protein molecules and nutrients like zinc. And although sperm cells burn a lot of energy, they can switch to a more efficient method of using and generating energy when supplies are low, which might help them survive longer, Barratt said.

The female reproductive tract also plays a critical role in sperm's survival by supplying them with energy in the form of glucose as they travel toward the egg, Barratt noted. "[Sperm] do have their own energy resources, but they won't last five days or seven days," Barratt said. "They do need to get it from the female tract."

What's more, the cells of the tract appear to provide crucial rest stops for sperm along the way — for example, in the lower fallopian tube, or oviduct. Rather than racing to the egg, there’s evidence that sperm slow down and dock at these reservoirs, Barratt said. Here, they attach to surface cells and seem to take refuge, until they wriggle free for a final dash to the egg.

In fact, when researchers investigated this phenomenon in pigs, they found that sperm cells bind to special sugars, called glycans, within oviduct cells and that the sperm that did so lived longer than those that did not. Research on other mammal species also shows that sperm that bind to oviduct cells move, fertilize and live longer than those that don't bind to these cells.

It's possible that the oviduct cells deliver ingredients and trigger cellular changes that help to maintain the sperm and increase their staying power in the reproductive tract. "There's a lot of data on different molecules that might pass between the sperm and oviduct, and in fact, that you have the sperm signaling to the oviduct, themselves," Barratt noted. However, studies on this process are difficult to do in humans, and are therefore limited. "I don't think the mechanism is yet clear," Barratt said.

What is clear is that longer-living sperm provide a reproductive advantage in humans. Sperm that linger for several days in the body have a greater chance of overlapping with ovulation, the brief window of time when an ovary releases an egg to be fertilized. The sperm's ability to wait it out in the oviductal reservoirs essentially ensures that hundreds of sperm are prepped and ready to go when an egg arrives.

"There's more chance of getting pregnant closer to ovulation, and the day of ovulation," Barratt said. "But if the sperm cells can be in there, in waiting, then that's an advantage. Surviving for a longer period of time gives that flexibility."

This benefit isn't exclusive to humans. In fact, sperm show remarkable survival skills in other species, where they can last many times longer than human sperm. "We know that the sperm of some species of bats are able to survive for up to six months in the female reproductive tract," Nixon said. "There are also some extraordinary examples of sperm survival of several years in the female reproductive tract of reptiles."

How those particular sperm clung onto life is another fascinating mystery to unpick. "Scientists have still yet to work out how this extended survival is accomplished," Nixon said.

When archaeologists find ancient human remains, they often try to determine if the person was male or female based on their bones.

So how do archaeologists figure out the sex of the individual from their skeleton, and how accurate are their techniques?

"Overall, we're looking at shape and size differences between the sexes," Sean Tallman, a biological anthropologist at Boston University, told Live Science, but "no one method is 100% accurate."

Archaeologists often take measurements of long, slender bones, like the femur and tibia (which make up the leg), and then use statistical methods to predict the person's sex.

"On average, males are about 15% larger than females," Kaleigh Best, a biological anthropologist at Western Carolina University, told Live Science. But many variables — such as diet, genetics, disease and environment — go into body size, so there can be wide variation even among people of the same sex.

Related: What is the maximum number of biological parents an organism can have?

Most measurement-based techniques assume that males are larger and taller than females, and sex predictions from measurements are 80% to 90% accurate. But if the skeleton's pelvis is well preserved, simply looking at certain features of it is generally a more accurate method than relying on measurements of leg bones.

The main method of estimating the individual's sex from the pelvis is called the Phenice method, named after the anthropologist who proposed it in the 1960s. Differences in the shape of the pubic bone at the front of the pelvis correlate with a person's sex — a taller pubic bone, for example, is more likely to be from a male individual, while a wider one is more likely to be from a female. A well-trained archaeologist can predict the sex of a skeleton with about 95% accuracy with this method.

Ancient DNA analysis is also an accurate chromosomal sex estimation method, in which scientists identify the sex-linked variant of a gene related to tooth enamel production. This technique now reaches about 99% accuracy, even in archaeological skeletons. However, since DNA degrades over time, not every archaeological skeleton can be analyzed in this way.

In spite of this high accuracy rate, many archaeologists say that estimating whether a past person was male or female based on their bones alone may miss other aspects of biological sex, which is a result of the interplay between chromosomes, hormones, gonads and gametes. (Gender, in contrast, is a cultural construct that reflects self-identity, societal roles and pressures.)

"Sex is not binary, but it may be bimodal," Donovan Adams, a biological anthropologist at the University of Central Florida, told Live Science. Bimodal in this context means that if you were to plot sex on a graph, there would be two "humps" for male and female on each end of the graph. But the overlap between the two groups in the middle would represent people who are described as intersex.

"About 1.7% of the population is some form of intersex," Virginia Estabrook, a biological anthropologist at the University of Maryland, Baltimore County, told Live Science, which is "slightly less than 1 in 50 people."

Some examples of intersex conditions include congenital adrenal hyperplasia (CAH), an over-production of male hormones that can make female genitalia look ambiguous at birth; Klinefelter syndrome, or XXY sex chromosomes, resulting in small testes and enlarged breasts in people born male; androgen insensitivity syndrome, in which a person may be born with female-type external genitalia but no internal reproductive organs; and 5α-Reductase 2 deficiency; in which an infant that appears female at birth later develops a penis and testes. And people may have other forms of sex chromosome mosaicism, with XX chromosomes in some cells and XY in others.

For example, Estabrook studied the skeleton of Revolutionary War hero Casimir Pulaski, who died in battle in 1779. His skeleton showed several bony traits that are more typical in female-patterned growth and development, Estabrook said, but historical records clearly show he lived his life as a man. One possible explanation for this discrepancy may be CAH, in which chromosomally female infants have genitals that look more like male genitals. People with CAH produce increased androgens and can grow facial hair.

The case of the intersex general is relatively unique, Estabrook said, "because ordinarily when we encounter skeletons in archaeology, we don't know who these people were."

Understanding who an ancient person was can be stymied not just by the limitations of osteological sex estimation but also by the variable of gender.

Most aspects of a person's identity — from the sports team they support to the gender they adopt — are not something they are born with. "You have to perform identity all throughout your life," Adams said. Those life experiences, including behaviors like wielding a bow-and-arrow or kneeling to grind grain that are often gendered, may leave marks on an ancient skeleton that can muddy the waters — especially because we only imperfectly understand past cultures.

The complexity of both sex and gender means that sometimes archaeologists' interpretations are wrong.

At Pompeii, for example, DNA analysis revealed that a set of skeletons assumed to be a mother and her biological child were actually a man and unrelated child, and in 2019, a Viking burial replete with weapons was found to be chromosomally female rather than male.

Though DNA analysis can dramatically increase the accuracy of chromosomal sex assignment, that doesn't necessarily mean archaeologists have solved the problems of estimating sex from ancient human remains.

"It's really hard to separate ourselves from that binary system," Tallman said, "but there's a ton of overlap between females and males."

Estabrook agreed. "Every way that we try to put a strict, solid line of demarcation on biological sex, there are people who fall outside of those lines," she said.

Another issue is that archaeologists still lack information on intersex conditions because there hasn't been much research into the potentially 1-in-50 people who have one.

"Future work will be greatly affected by the availability of federal funding to do this type of research," Tallman said, "and that could limit these nuanced perspectives that we need to interpret behavior and biology from skeletal remains and from archaeological sites."

Scientific advances have made it easier to determine limited aspects of sex from ancient skeletons, Best said, but figuring out a person's identity from their skeleton "is actually a lot more complicated than we once thought it was."

General anesthesia is given during surgery to make people unconscious, eliminate pain during the procedure and prevent reflex movements. Many factors are known to impact how anesthesia affects a given person, from their weight and age to whether they have kidney or liver diseases that alter how drugs are metabolized.

But does anesthesia affect males and females differently? And if so, should that change the doses each sex receives?

Current research does hint at some sex-based differences in how general anaesthesia affects patients. However, sex "doesn't drive dosage decisions," Dr. Elisha Peterson, a pediatric pain medicine physician and an associate professor at George Washington University, told Live Science.

She noted that, historically, most research on anesthesia has been done with white, male subjects, and researchers do not yet have sufficient data from female participants of different backgrounds to provide tailored health care based on these factors. As such, a female patient who previously experienced some level of consciousness during surgery would be treated in the same way as a male patient with a similar history: Doctors would monitor the patient's brain waves to monitor how well the anesthetic was working and adjust the dose accordingly throughout the procedure.

Related: How does anesthesia work?

Current guidelines on the use of general anesthesia do not differ by sex, in part because existing research on sex-based differences in anesthesia sensitivity shows mixed results.

For example, data collected after the approval of propofol — one of the most widely used general anesthetics — suggested that men woke up more slowly from the drug than women do. Further research over the years backed up this idea that women emerge faster from general anesthesia than do men, regardless of the drug used.

In addition, a 2023 review of 33 studies suggested that women under general anesthesia had, on average, 38% higher incidences of "anesthesia awareness" than men did. That means they experienced some moments of consciousness, ranging from a dreamlike state to full awareness, despite being under general anesthesia. Although patients recall having some level of consciousness, they do not typically feel pain during anesthesia awareness, according to the American Society of Anesthesiologists (ASA).

Women also woke up from surgery and were able to respond to questions sooner than men did, the 2023 review suggested.

However, some studies, such as this 2002 paper from Thailand, do not report the same pattern. Out of about 800 patients in the study, most of them female, five experienced "intraoperative awareness" — three females and two males. Ultimately, the study authors concluded that inadequate anesthesia was the most likely reason for these five cases.

Peterson noted that differences in male and female physiology, hormone levels and metabolic rates may contribute to differences in the way each sex processes general-anesthetic drugs. Early data from both animal and human research buttress that idea.

In 2024, for example, researchers showed that female mice and adult women took more time to fall unconscious under anesthesia than their male counterparts, and they also woke up faster. Additionally, male mice that were castrated were more resistant to the effects of anesthesia, but upon receiving testosterone injections, became more sensitive to it.

Growing evidence shows that anesthetics modulate the activity of parts of the brain that control the sleep-wake cycle, which are primarily located in the hypothalamus. There's evidence that sex hormones, such as estrogen and progesterone, can affect the sleep cycle, potentially via the hypothalamus. These hormones might also make a difference to how well general anaesthesia works, according to a 2011 study in the British Journal of Anesthesia.

There's also research in mice showing sex differences in the dopaminergic system — major pathways in the brain that rely on the chemical messenger dopamine. This system helps regulate arousal, or wakefulness and attention, and thus may influence how quickly someone wakes up from anesthesia, said Dr. Raveesh Sunkara, a consultant neurosurgeon at Arete Hospitals who was not involved in that past research. However, it's not yet known for sure how those brain differences might actually impact sleep and anesthesia sensitivity.

Again, even though some research hints at sex-based differences, doctors' application of anesthesia does not vary by sex, Peterson emphasized. The overall rate of anesthesia awareness is estimated to be about 1 to 2 per 1,000 medical procedures involving general anesthesia, according to the ASA.

Moreover, there are potential risks associated with increasing anesthetic dosages; for instance, they could lead to dangerous cases of severely low blood pressure, said Dr. Kai Rabenstein, an anesthesia adviser at SEDSConnective, a charity supporting individuals with hypermobility, neurodivergence and associated conditions.

Until further research sheds light on concrete and actionable sex-based differences, some measures could be taken to reduce incidents of anesthesia awareness in women, Peterson suggested. For example, providers should use brain-monitoring tools more frequently for procedures with a high risk of anaesthesia awareness, such as emergency Caesarean sections.

In addition to analyzing the patient's history with anesthesia, it can be worth assessing their alcohol consumption patterns. "Alcohol sensitivity can be taken as a guide," Rabenstein said, as alcohol is metabolized similarly to propofol in the body. That means people with a high alcohol tolerance might also be tolerant to the anesthetic's effects. The ASA also advises patients to inform their doctors of past drug and alcohol use, as it can "increase the risk of anesthesia awareness."

Editor's note: This story was updated on April 29, 2025, to clarify a quote from Dr. Kai Rabenstein.

This article is for informational purposes only and is not meant to offer medical advice.

Be it a salmon swimming upstream or a turtle mounting a shoe, creatures obey the impulse that commands them to reproduce. But do they, at the end of the great energy expenditure, feel anything akin to what humans call an orgasm?

There's no clear list of animals that can orgasm. For one, research on physiological and neurological activity in animals during sexual behavior looks mostly at primates and rodents, David Puts, a professor of anthropology and psychology at Penn State, told Live Science in an email. And because animals can't describe their own experiences, we can't know for sure if they feel what humans feel during climax.

We can, however, compare observed physical and neurological behaviors. "Many female primates exhibit behaviors that are similar to those exhibited during orgasm in women," Puts said. These behaviors include a clutching reaction; changes in bodily tension and facial expressions; changes in respiratory patterns; vocalizations; and contractions of the vagina, anus, pelvic muscles and uterus, he said.

"We can also look at behavior and brain activation patterns, which appear similar across male mammals," Puts said.

The definition of an orgasm has evolved over time to reflect both the physiological process as well as the characteristic experience of intense pleasure. A 2016 paper published in the journal Socioaffective Neuroscience & Psychology defines the physiological process as "a spinal reflex that results in rhythmic muscle contractions of the pelvic floor and anus." In males, this response also typically entails a urethrogenital reflex that coincides with semen emission and ejaculation, while females have contractions of the uterus and cervix.

Related: What were the first animals to have sex?

But even with these observations, it's difficult to conclude with certainty whether these animals are having what humans describe as an orgasm. "We can never know if they feel or interpret it the exact same way," James Pfaus, a professor of neuroscience at Charles University in Prague and director of research at the Czech National Institute of Mental Health's Center for Sexual Health and Intervention, told Live Science in an email. Pfaus is also the first author of the 2016 paper.

However, the term "orgasm-like response" cuts through the uncertainty of how other animals experience what appears to be a sexual climax, Pfaus said. Ultimately, even if we can't assert that other animals have orgasms, they have something that closely resembles an orgasm.

"My feeling is that all mammals probably have [orgasm-like responses]," he said. Our understanding of anatomy across species informs his belief. While clitorises and penises vary in structure across mammals, the distribution of nerves throughout those anatomical features "is virtually the same" in all mammals and across sex. As mammals have evolved, the hormonal and neurochemical mechanisms involved in sex have been largely conserved, Pfaus explained.

That's not to say nonmammals don't feel pleasure or have an orgasm-like response during sex. Rather, because research on orgasms has favored mammals, it leaves the opportunity for other scientists to investigate physiological and neurological responses during sex in reptiles, amphibians, birds and other nonmammals.

Why do orgasms exist in the first place? "The 'why' would take us many days to discuss," Pfaus said. Put simply, however, "Orgasms serve two masters: reproduction and reward," he said.

Thankfully, one doesn't need to understand the purpose of an orgasm to enjoy one.

Egg fertilization is often described as an epic swim meet: Millions of sperm swim as fast as they can toward the egg until one — the fastest, strongest, healthiest sperm of them all — wins the race and wriggles into the egg, with the prize of passing its genes to future offspring.

But is this really how it happens? Do sperm really race to the egg?

Yes and no, David J. Miller, a professor in the animal sciences department at the University of Illinois Urbana-Champaign, told Live Science. "The important player in all of this is actually the female reproductive tract."

Sperm do swim during this process, but "the major movement is actually provided by contractions of the female tract," Miller explained. "There are contractions of the uterus, for example, that are much like contractions of the GI tract that can move fluid through the uterus."

A 1996 study illustrated just how efficient these contractions are, Miller noted. Scientists deposited sperm-size beads into the uteruses of 64 women, and some of the beads traveled all the way to the fallopian tubes — where fertilization usually takes place — within minutes.

Related: Are you genetically more similar to your mom or your dad?

It makes sense that sperm would need some extra help, because while the sperm swim in one direction, the egg needs to travel in the opposite direction to meet them, Sabine Koelle, a full professor of anatomy and developmental biology at the University College Dublin School of Medicine and Medical Sciences, told Live Science. The egg can't swim, so instead tiny hairs called cilia help it along .

"Cilia beat to transport the oocyte," or egg, Koelle said. "Because the sperm are coming from the opposite direction, they have to struggle" against the current created by the cilia.

In fact, an individual sperm's movement is less of an effort to move forward and more of an effort to move inward, toward the middle of the tract, Koelle said. If the sperm get too close to the sides, they stick and lose their forward momentum.

However, just because a sperm is the first to arrive doesn't mean it gets to do the fertilizing. "Sperm require some final maturation that occurs in the female tract, and it's time dependent," Miller said. "So the sperm that 'win the race,' so to speak, need more time before they can actually fertilize the egg."

"They may not be there when they've completed that maturation," Miller added. "They might be replaced by some of the other more slowly transported sperm that have had time to complete that maturation."

But even those less-mature sperm are more successful than the vast majority of sperm that get deposited. As the female reproductive tract pushes the sperm along, it also cuts unlucky individuals from the swim team.

"Less than 1% — maybe up to 2 or 3% of the sperm that are actually deposited — make it all the way to where the egg is," Miller said. "A lot of them are flushed back out from the tract. Some are eaten up by immune cells in the uterus, because sperm are foreign."

Up to 70% of sperm don't even make it past the cervix, Koelle noted. "The sperm are stuck there and can't free themselves," she said.

For those few sperm that make it into the fallopian tube, the goal is to get as far as possible and then stick to the wall as they wait for the egg to arrive. This is another place where the female reproductive organs are choosing winners: Scientists have noticed that normal-looking sperm are more likely to bind to the wall, Miller said, and binding to the wall provides some metabolic benefits that increase their lifespan.

Then, once the egg arrives, the fallopian tube — also known as the oviduct — allows only healthy-looking sperm to unstick from the wall. "As soon as a sperm is not OK, the oviduct doesn't release it," Koelle said. "It's the main selector of good sperm."

This isn't a perfect system, of course. " Obviously, we have genetic diseases that come through sperm. So it's not always true that the fittest are the best ones genetically," Miller said.

Every step of the way, the female reproductive tract is doing its best to weed out the less-fit sperm so that only healthy sperm reach the egg. In that way, fertilization is less like a race and more like a job interview.

"There's certain qualifications that you need to be able to apply for the job," Miller said. "But also, the sperm that have those qualifications would have to have them at the time the job is open — the time when the egg is ovulated." But in the end, it's the female reproductive tract that chooses the best candidate.

You're holding two wrinkly human brains, each dripping in formaldehyde. Look at one and then the other. Can you tell which brain is female and which is male?

You can't.

Humanity has been hunting for sex-based differences in the brain since at least the time of the ancient Greeks, and it has largely been an exercise in futility. That's partly because human brains do not come in two distinct forms, said Dr. Armin Raznahan, chief of the National Institute of Mental Health's Section on Developmental Neurogenomics.

"I'm not aware of any measure you can make of the human brain where the male and female distributions don't overlap," Raznahan told Live Science.

But the question of how male and female brains differ may still matter, because brain diseases and psychiatric disorders manifest differently between the sexes. Disentangling how much of that difference is rooted in biology versus the environment could lead to better treatments, experts argue.

There are many different disorders of the brain — psychiatric and neurologic diseases — that occur with different prevalence and are expressed in different ways between sexes, said Dr. Yvonne Lui, a clinician-scientist and vice chair of research in NYU Langone's Department of Radiology. "Trying to understand baseline differences can help us better understand how diseases manifest."

Now, thanks in part to artificial intelligence (AI), scientists are starting to reliably distinguish male and female brains using subtle differences in their cellular structures and in neural circuits that play a role in a wide range of cognitive tasks, from visual perception to movement to emotional regulation. Other studies point to sex-based differences in human brain structure that may be present from birth, and still other, lab-based research in animals points to sex-based differences in how brain cells fire at a molecular level.

What's still completely unclear is to what extent these differences matter. Do they change how people's brains function or how susceptible they are to disease? Should they dictate which treatments doctors offer to each patient? Even as scientists pinpoint subtle brain differences between females and males, their research inevitably runs up against tricky questions of how sex, gender and culture interplay to sculpt human cognition.

Right now, it's impossible to answer these big questions. But ongoing and future research — focused on lab animals, human chromosomes and brain development, and subjects followed from youth through adulthood — could start to reveal how these sex-based differences concretely affect cognition, and ultimately, the development of diseases of the brain.

Why study sex-based brain differences?

Historically, scientists used purported brain differences to make sweeping statements about how men and women think and behave and to justify sexist beliefs that women were innately less intelligent and less capable than men.

While that early research has been discredited, modern studies still find cognitive differences between men and women — at least on average. For example, men reportedly perform better on tests of spatial ability, while women are better at interpreting the facial expressions of others. But men and women are raised and treated very differently in society, so what's at the root of these differences? Is it nature or nurture, or both?

"It's actually incredibly difficult in humans to … causally distinguish how much of a sex difference is societally or environmentally driven," Raznahan said. "We have all of these assumptions and biases that sort of slip into our heads through the back door without us realizing."

Given the dubious history of studying sex differences in the brain, and the logistical difficulty of doing it the right way, one might wonder why scientists bother. For many, it's because neurological diseases and psychiatric conditions seem to play out differently in males and females, and both biological and environmental factors could explain why that is.

Data suggest women experience higher rates of depression and migraine than men do, while men have higher rates of schizophrenia and autism. About twice the number of men develop Parkinson's disease than women do, but women with the condition tend to have faster-progressing disease. All these data come from studies that don't necessarily distinguish sex from gender — "sex" describes biology, while "gender" reflects self-identity, as well as societal roles and pressures. Lumping the two concepts together muddies our understanding of why a given difference exists.

For instance, pubescent girls are more likely to experience depression than boys are, which may be related to how their maturing brains handle stress or the possibility that they encounter more stressful events than boys do at that age. Conversely, do boys' brains make them resilient against depression, or are they actually going underdiagnosed due to social stigma? The answers to these questions point to different solutions.

Large-scale structures, negligible differences

Thanks to brain-scanning techniques like MRI, scientists have found subtle sex differences in the size, shape and thickness of various brain structures, as well as differences in networks that link different parts of the brain.

But these differences are small to negligible when you account for the average size difference between males and females, argues Lise Eliot, a professor of neuroscience at the Rosalind Franklin University of Medicine and Science and author of "Pink Brain, Blue Brain" (Houghton Mifflin Harcourt, 2009).

Eliot and colleagues recently looked at about 30 years of studies, finding that, on average, male brains are 6% larger than female brains at birth and grow to be 11% larger by adulthood. This makes sense because average brain size scales along with average body size, and male bodies tend to be larger. But when you take this overall size difference into account, subtler structural differences between male and female brains shrink to the point of negligibility, the researchers concluded.

"There are maybe species-wide sex differences in the brain, but so far, they haven't been proven," Eliot told Live Science. "And so if they exist, they must be pretty small."

Nonetheless, some scientists have reported differences that they say don't scale with body size. Some examples came from a research group who'd crunched MRI data from over 40,000 adult brains scanned for the UK Biobank, a repository of medical data from 500,000 adults in the United Kingdom.

In that study, males had a larger thalamus, a relay station for sensory information. They also had a larger putamen, which helps control movement and forms part of a feedback loop that tells you whether a movement was well executed. Females, on average, had a larger left-side nucleus accumbens, part of the brain's reward center, and a bigger hippocampus, the storage site for short-term memories of facts and events that also helps transfer the information to long-term memory.

But neither this nor other studies have revealed a specific feature that reliably distinguishes a given male brain from a female brain, since the size ranges seen in each sex largely overlap, Raznahan and colleagues noted in a letter responding to that study.

For the few size differences that do exist, it's currently impossible to say whether they explain any differences in cognition linked to sex, or alternatively, whether they actually make males' and females' cognition more similar, the letter authors noted. Perhaps male and female brains operate slightly differently to reach the same output — to "counterbalance" differences in hormones or genetics that may affect brain function, they wrote.

"When we're just talking about describing a difference in a measurement, that's not saying anything about whether it's got any functional relevance at all," Raznahan emphasized.

AI finds subtle differences

While large-scale structural features might not distinguish male and female brains, AI is helping to uncover other, subtler features that may differentiate the two. Some of these differences appear on the level of the brain's microstructure, meaning its individual cells and components of those cells.

For instance, a study published in May 2024 used different AI models to analyze brain scans from 1,030 young adults ages 22 to 37 years old. The research primarily focused on white matter, the bundles of insulated wiring that run between neurons. "I believe ours is the first study to detect brain microstructural differences between sexes," said Lui, who co-authored the study.

The AI models analyzed differences in both local landmarks in the brain — such as the corpus callosum, which connects the brain's two halves — and the highways that connect distant cells. It also looked at differences in how the white matter was bundled together, as well as in how dense and well insulated those bundles were.

The algorithms accurately predicted the sex of the subject tied to a given scan 92% to 98% of the time. That remaining gap in accuracy likely comes down to the "huge amount of variance in humans," Lui said.

No single part of the brain could be used to make predictions; one model relied on 15 distinct regions of white matter. All models showed some consistencies, though, with the largest white matter structure that crosses the midline, the corpus callosum, standing out as key.

From birth

Lui and colleagues' study was not designed to address how an individual's upbringing or environment shapes the brain. Nor did it aim to disentangle biological differences in the brain from those rooted in gender.

Sex describes biological differences in anatomy, physiology, hormones and chromosomes. Sex traits are categorized as male or female, although some people's traits don't fit neatly in either category. Gender, on the other hand, is cultural. It encompasses how people identify and express themselves, as well as how they are treated and expected to behave by others. Genders include man and woman, as well as others, including those that fall under the umbrella term nonbinary or are unique to specific cultures, like the māhū of Hawai'i.

Historically, studies have conflated sex and gender. To tease these factors apart and see how each manifests in the brain, it would be helpful to follow people over time as their brains are developing — and new research is beginning to do just that.

For example, a 2024 study looked at average brain volume in over 500 newborns: Males' brains were 6% larger overall, even after accounting for differences in birth weight, and females had larger gray-to-white matter ratios. (Gray matter, the cell bodies of neurons, is primarily found in the outer layer of the brain, called the cortex.) That average difference in gray matter is also seen in adults, which makes sense given that larger brains need more white matter to relay signals between far-apart cells.

Statistically, these big-picture brain differences were more significant than differences seen in smaller structures. Females had larger corpus callosa, as well as more gray matter around the hippocampus and in a key emotion-processing hub called the left anterior cingulate gyrus (ACG). Males had more gray matter in parts of the temporal lobe involved in sensory processing, as well as in the subthalamic nucleus, key for movement control. But sex could only explain a fraction of the variance seen in these structures.

Some of these brain differences are "present from the earliest stage of postnatal life" and persist into adulthood, the authors noted. This applies mostly to the global differences, but also potentially to some of the smaller ones. For example, some studies — but not all — show that the left ACG is also larger in adult females, not only in babies.

Durable differences present from birth are likely sex-based. But differences that emerge or disappear in later life, like those in the hippocampus, may be influenced by the environment, or else reflect sex differences in development, including hormonal shifts in puberty.

Gender and sex

Studies like this can help tease apart the influence of sex and gender on the brain. At present, there's a "massive gap" in our understanding of how these factors shape the brain independently and in tandem, said Elvisha Dhamala, an assistant professor of psychiatry at the Feinstein Institutes for Medical Research in New York.

Dhamala and colleagues recently aimed to fill in that gap using data from the Adolescent Brain and Cognitive Development (ABCD) study, an enormous U.S.-based study of brain development and child health. They incorporated functional MRI (fMRI) scans from nearly 4,800 children; fMRI tracks blood flow in the brain to give an indirect measure of brain activity. Each child joined the study at age 9 or 10 and will be followed for 10 years, which will enable follow-up studies.

The fMRI scans highlighted linked brain areas, or networks that lit up as the children did different tasks, including memory tests that required them to recall several images. The children and their parents also answered questions about the kids' feelings about their genders and how they typically play and express themselves. "It's not anything clinical," Dhamala noted. "It's just an aspect of behavior that represents your gender."

These answers were used to generate "scores" for each child that the AI algorithm could use as data points.

The algorithm ultimately revealed two largely distinct brain networks tied to sex and gender. The brain differences most strongly tied to sex were found in networks responsible for processing visual stimuli and physical sensations, controlling movement, making decisions and regulating emotions. Differences tied to gender were more widely dispersed, involving connections within and between many areas in the cortex.

After pinpointing these networks, the researchers trained their AI algorithms to "predict" a child's sex or gender based on brain activity. They accurately determined most children's sexes, similar to the results of Lui's study. Gender proved trickier: With the children's questionnaire answers, the AI couldn't predict where they landed on a continuum of gender, whereas with the parents' answers, its predictive power exceeded chance but was still "much lower" than the predictions for sex, Dhamala said.

Nonetheless, the study highlighted an understudied idea: that gender sculpts the brain in ways that are distinct from sex, she said.

Interestingly, some tentative lines can be drawn between Lui's and Dhamala's AI-powered studies. They can't be directly compared, as the two studies used different types of analyses and focused on different features of the brain. But many of the physical white matter tracts flagged in the former study correspond with functional networks highlighted in the latter, Dhamala told Live Science.

As an example, the cingulum — a white-matter tract that encircles the corpus callosum — seemed key for making predictions in Lui's study. It also links together various networks flagged in Dhamala's study, including circuits involved in emotional processing. That hints that sex differences exist in both the physical anatomy of these networks and in their activation patterns, Dhamala said.

The future of the sex-difference field

Scientists have made some progress at teasing out sex differences in the brain, but to truly understand these distinctions, researchers will need to do more animal studies to allow for more experimental control, according to a 2020 paper co-authored by Raznahan.

Various studies in lab rats have already revealed differences in how males and females form connections between neurons, and how each sex processes fearful memories, for example.

Related: 'Let's just study males and keep it simple': How excluding female animals from research held neuroscience back, and could do so again

In humans, scientists can collect more brain data right at the time of birth, to pinpoint baseline differences that might exist before a child encounters any cultural influences, and then track the child over time, Raznahan and colleagues added.

Another option is to study human genes that are unique to either the X or Y chromosome. By looking at people with extra or missing sex chromosomes, for example, scientists have started to unravel how these genes either inflate or shrink brain structures, contributing to sex differences in size. Chromosomes may also raise or lower the risk of disorders — for instance, carrying an extra Y raises the likelihood that a person has autism, whereas an extra X does not. That may help to explain why males, who usually carry one X and one Y, have higher autism rates than females, who typically have two Xs.

Right now, the fate of such research is uncertain in the U.S.

Prompted by executive orders from the new presidential administration, the National Science Foundation has been combing through active research projects to see if they include words that might violate said orders, such as "woman," "female" and "gender," and the National Institutes of Health appeared to archive a long-standing policy requiring both male and female lab animals in studies.

"There's just a lot of uncertainty," Dhamala told Live Science. If the worst case scenario comes to pass, "removing that gender component, or making it harder to study sex differences, is going to push us backward rather than forward."

But if the field survives, future work could incorporate gender the way the ABCD study did, using questionnaires to generate composite scores, Dhamala said. As a start, scientists could at least ask study participants what gender they identify as, she added. Other experts agree.

By adopting these strategies, scientists could dramatically advance this research field that dates back to Aristotle. Their efforts could lend new talking points to the endless debate of nature versus nature. They could uncover meaningful sex differences that pave the way to better treatments for depression, Alzheimer's and more. Or they could highlight the ways members of the "opposite sex" are actually more alike than they are different.

Actions taken by the Trump administration have spurred changes at America's major science agencies — and if worst comes to worst, these shifts may jeopardize our understanding of how the brain works and how to treat neurological conditions, experts told Live Science.

For years, the National Institutes of Health (NIH) has upheld a policy that requires scientists who work with vertebrate animals to consider "sex as a biological variable." To earn grants, researchers must explain how their study designs account for sex, such that any differences between the sexes will be made apparent. Researchers must provide strong scientific justification to include only one sex in a study.

But recently, The Transmitter reported that the NIH seems to have archived this policy. The NIH has yet to issue an official statement on the matter, but the move followed executive orders issued by the Trump administration that called for overturning "gender ideology" and "radical" diversity, equity and inclusion programs.

In apparently archiving its "sex as a biological variable" policy, the NIH could be signaling a shift away from requiring both males and females in research. And such a shift could be particularly dire for basic neuroscience research.

Related: Is there really a difference between male and female brains? Emerging science is revealing the answer.

Emerging animal research is revealing fundamental differences between male and female brains. These differences manifest in how sex hormones influence brains at the basic level of memory formation and neuronal firing, for example, and evidence suggests these differences may be relevant not just to lab animals, but also to humans. Failing to include both sexes in lab research could lead us to miss fundamental forces that shape the human brain and how drugs affect it.

"If the fundamental mechanisms by which molecules sculpt neurophysiology differ between the sexes, then we need to know that as early as possible in the process," said Catherine Woolley, a professor of neurobiology at the Northwestern University Weinberg College of Arts and Sciences.

A long legacy of bias

Are male and female brains wired differently? The question has beguiled humanity for centuries, but scientists have only recently begun to find answers.

That's partly because, to uncover differences between males and females, you must study both sexes, including in research with lab animals. But scientists haven't done that until relatively recently.

Animal studies enable experiments that would be impossible with humans. Researchers can't slice living people's heads open, pluck out their neurons or implant electrodes with wild abandon.

Human subjects also come with baggage — namely, men and women are raised differently and have different life experiences. These cultural factors shape the brain alongside biological factors, like hormones and chromosomes, making it difficult to separate nature from nurture.

Historically, however, scientists simply ignored the variable of sex by excluding females altogether. This bias was particularly pronounced in neuroscience — one 2009 review found that studies with exclusively male lab animals outnumbered those with females 5.5-to-1.

There was a concern that the reproductive cycle of female lab animals would "mess up the data, make it too variable," said Lise Eliot, a professor of neuroscience at the Rosalind Franklin University of Medicine and Science in Chicago. That idea has been thoroughly debunked, but for a time, the prevailing viewpoint was, "let's just study males and keep it simple," Eliot told Live Science.

Only in recent years has that attitude begun to shift.

Inclusive research breeds discovery

Including female lab animals has led to findings that unseated long-held assumptions about how the brain works.

For instance, Woolley uses lab rats to study how the sex hormone estrogen drives synaptic plasticity, the brain's ability to dynamically strengthen or weaken connections between neurons over time. This process enables the ability to learn and form memories, and it also underlies the brain changes behind psychiatric conditions like addiction.

Woolley has shown that plasticity works differently in males and females.

Connections between two neurons grow stronger via a process called long-term potentiation, which unfolds in two phases: an early phase, which lasts a few hours, and a late phase, which lasts longer. A key enzyme — protein kinase A (PKA) — was thought to participate in only the later, long-lasting phase.

"We've shown that that's true only in males," Woolley said. For females, PKA is crucial for the early phase of long-term potentiation, she and her colleagues revealed.

In another study, the team uncovered a sex-specific difference in the hippocampus, a brain region key for learning and memory. They showed that, exclusively in female rats, a specific type of estrogen makes neurons more likely to trigger a signal, thanks to a "previously unknown" mechanism. This particular estrogen is made in the brains of both males and females, and it doesn't follow the same cycle as estrogen made by the female reproductive system.

In that research, the team was "doing an experiment that has been done before … and our results were different," Woolley told Live Science. "It dawned on me that perhaps the results that we were getting differed from the published literature because we were using female animals." Previous work had included only males.

Many drugs act at the connection points between neurons, so it's important to understand how those connections are formed and maintained, Woolley said. Therefore, findings from these animal studies could help inform future treatments and diagnostics for people, she added.

There are some key differences between rodents and people, including that the human menstrual cycle is about seven times longer than the comparable cycle in rodents. But many studies have probed the similarities and differences in how estrogens act on the rodent brain and the nonhuman primate brain.

Based on these data, Woolley suspects many observations in the rat brain will translate to humans, even if they're not exactly the same.

Related: Faster brain aging tied to X chromosome inherited from Mom

Differences in fear processing

Beyond Woolley's lab, scientists have also uncovered nuances in how male and female mice store fearful memories.

When the researchers blocked signals from part of the amygdala — a key emotion-processing center involved in fear conditioning — it stopped fear memories from being "saved" in male mice, but not in females.

Then, they studied people with a genetic mutation in that same signaling pathway. Males with the mutation had trouble remembering that a cue on a screen would come with a mild electric shock, but the same mutation had no effect on females' memories.

That hints that males and females store fearful memories differently, and that could have implications for post-traumatic stress disorder (PTSD), the authors speculated. For instance, using a drug to block those amygdala signals after a traumatic event could help prevent PTSD in males but likely not in females.

Another study revealed cell-level differences in the nucleus accumbens, the core of the brain's reward system. More excitatory, or "activating," signaling occurs in the nucleus accumbens of female mice and rats, compared with males.

And several methods of damping that excitement in males do not work in females, the team found. That finding could be relevant for better understanding depression and addiction, since signaling in the reward system underlies both conditions, the study authors suggested.

The road ahead

A better understanding of these sex differences could lead to improved psychiatric and neurological treatments tailored to each sex, scientists argue. But the future of this research is uncertain given the changes that seem to be unfolding at America's premier science institutions.

The fate of the NIH's "sex as a biological variable" policy is currently unknown. But if including both sexes in research is no longer prioritized, it could set back the whole field of biomedicine, Woolley emphasized.

"It is very important to understand that the Sex as a Biological Variable policy is not about sex differences research," Woolley told Live Science. "This policy is about all biomedical research and ensuring that the results of taxpayer-funded biomedical research are relevant to everyone: both men and women, both boys and girls."

Male and female sexual partners leave traces of their "genital microbiomes" during intercourse, even when they use a condom, a new study suggests.

In the study, researchers had 12 monogamous heterosexual couples collect swab samples after a period of abstinence and then shortly after having sex. The subsequent analyses led the scientists to confirm that, at baseline, each of the male and female participants had a unique collection of microbes in their nether regions.

After penetrative sex, though, these distinct microbial communities, or microbiomes, were transferred to their partners in a reciprocal way. These microbiome shifts occurred even in couples that used condoms, and they could be detected using simple lab tests.

The researchers who conducted the study, which was published Wednesday (Feb. 12) in the journal iScience, have dubbed this sexual microbiome the "sexome."

Related: Scientists are building an ultimate atlas of the vagina. Here's why.

With further research, the scientists hope that sexome analysis could somehow be used to help pinpoint perpetrators of sexual assault. Particularly, such analyses could be applied to assaults against women, who are significantly more likely to be victims of such attacks than men. In these instances, investigators would typically analyze samples of DNA from sperm found in a woman's genital area. But sometimes no sperm is detected in these swab tests, which can affect the results of the exams.

In their study, Chapman and colleagues asked a dozen heterosexual couples to abstain from having sex for at least two to four days. After this period of abstinence, they had the participants take swabs of their genital regions to be sent to the lab for analysis.

Overall, the female participants had a greater volume of bacteria in their genital microbiomes than the male participants did — equating to an average of 8,038 bacterial genetic sequences in females, compared with 6,661 in males. However, males showed a greater diversity of bacterial species, with approximately twice the number of species represented compared with females.

In the second round of the study, the couples were asked to wait between two and 14 days before having sexual intercourse. Then, within three to 12 hours of doing so, the participants were asked to take a second series of swabs for analysis. This subsequently revealed that a participant's unique genital microbiome could be identified in their partner's swab.

"When we compared the before and after samples we could see bacterial DNA signatures from the female on the male and the male on the female," said study co-author Brendan Chapman, a forensic scientist at Murdoch University in Perth, Australia.

"In forensic science this is what we call a 'trace' or 'transfer' and that's the sort of thing that we ultimately use to show that there was contact," he told Live Science in an email.

What's more, three of the 12 couples reported using a condom during intercourse, which impacted how many, and which, microbes were transferred between partners, but did not completely prevent the formation of the sexome.

Other things that didn't seem to affect microbial transfer included whether males were circumcised or if either partner had pubic hair. However, the researchers did note that the composition of the genital microbiome in females changed at different points in their menstrual cycle, which could affect the results of future swab tests.

More experiments are now needed to replicate these findings in larger groups, given that this initial study included only 24 people, the researchers said. Future studies could also aim to answer questions such as how long the sexome lingers after intercourse.

"We’ve only scratched the surface in demonstrating this as a technique for use in real cases," Chapman said. "We still need more participants to make sure that we can reliably develop a test that's suitable for the robust validation that forensic science requires."

The patient: A 20-year-old woman in the United Kingdom

The symptoms: The patient had unprotected vaginal sex with her habitual male partner, and shortly afterward, her vulva and vagina began itching and swelling. She developed angioedema — swelling of the tissues below the surface of the skin — and hives appeared across her body. She also felt faint and short of breath.

What happened next: The woman visited a hospital and was given 10 milligrams of cetirizine, an oral antihistamine that's typically used to treat hives and allergies. She was already aware that she was severely allergic to Brazil nuts, a type of tree nut, but it was unclear what triggered her allergic reaction in this instance.

She noted that her partner had eaten Brazil nuts about two to three hours before they had sex, but he had taken a bath and thoroughly cleaned his teeth and fingernails prior to intercourse.

The diagnosis: Due to the patient's medical history, doctors at the hospital suspected that vaginal sex introduced allergy-triggering proteins from Brazil nuts into the woman's body by way of her partner's semen.

They conducted skin prick tests on the patient using different semen samples from her partner; this involved exposing small regions of skin to each sample. For one test, the doctors used a sample taken when the man had not recently eaten nuts. The other test used a sample collected about 2.5 hours after he ate Brazil nuts. In the latter test, a 0.28-inch-long (7 millimeters) welt appeared on the woman's skin. That suggested that Brazil nut allergens were behind her severe postcoital allergic reaction.

The treatment: Within 45 minutes after she took the cetirizine, the woman's symptoms began to improve. She felt tired the next day but did not have any hives or feel dizzy, nor did she have trouble breathing. The doctors instructed her to keep antihistamines and an adrenaline pen (otherwise known as an EpiPen) close by and suggested that, in the future, she should abstain from sexual intimacy with her partner if he had recently eaten Brazil nuts.

What makes the case unique: There are other documented cases of individuals with severe allergies developing local allergic reactions after intimate contact. However, the culprit is usually the direct transfer of allergens through touch or kissing. In other words, the allergen is present on a person's hands or mouth and then gets passed over.

"To our knowledge this is the first case of a severe food allergic reaction transferred by normal vaginal intercourse," doctors at the hospital wrote in a report.

This article is for informational purposes only and is not meant to offer medical advice.

Our planet is filled with a dazzling variety of creatures that bump uglies to reproduce. Cats do it. Dogs do it. The birds and the bees definitely do it. But what were the first animals to have sex? 

Animals have been sexually reproducing since they evolved, so the first animals to have sex were the first animals to exist. Researchers are still searching for direct evidence of the first animals, but they likely emerged within the last 800 million years, lived in the ocean and looked like sponges. 

The sponges in our oceans today reproduce sexually by ejecting sperm and egg cells into the water, which combine to form new sponge larvae, according to the Exploring Our Fluid Earth website hosted by the University of Hawaii.

But while ancient sponges may have been among the first animals to reproduce sexually, the act itself long predates them. That's because life forms were having sex before animals came onto the scene. 

"The first animals to have sex were already having sex before they were animals," John Logsdon, an associate professor of biology at the University of Iowa, told Live Science. 

Related: Do animals have friends?  

Logsdon traces sexual reproduction by looking for the presence of meiosis, a form of cell division that creates reproductive cells in eukaryotes — organisms with a nucleus in their cells, such as animals, plants and fungi. 

"It's pretty clear that all eukaryotes either had the ability to do meiosis or have the ability to do meiosis," Logsdon said. "The logical inference there is that a common ancestor of all of us did."

So when did the first eukaryotes evolve? According to Logsdon, the answer is around 2 billion years ago, when simple bacteria would have participated in some kind of genetic exchange. 

But sex among sea sponges and bacteria is quite different from the sexual intercourse, or copulation, that humans and many other animals engage in, which relies on a more intimate internal fertilization. For the first evidence of that, scientists look to ancient fish fossils. 

"The earliest evidence of intimate sexual reproduction using copulation is from placoderm fishes of the Devonian period [419.2 million to 358.9 million years ago], like Microbrachius dicki," John Long, a paleontology professor at Flinders University in Australia and author of "The Dawn of the Deed: The Prehistoric Origins of Sex" (The University of Chicago Press, 2012), told Live Science in an email.  

Fossils reveal that M. dicki males had paired claspers to inseminate females internally, while the females had reciprocal genital plates. Long and his team found that the male and female fish would have hovered side by side during copulation with their arm-like limbs linked, so the first sex act would have looked like square dancing. 

"We have placoderms to thank for both the joy of sex and the labors of childbirth," Long wrote in his book, "The Secret History of Sharks" (Ballantine Books, 2024). 

Sexual reproduction has many benefits. For one thing, offspring get genes from both parents, unlike in asexual reproduction, in which offspring get only the genes of one parent. This mix of genes enables animals to better adapt to changes in their environment.

"Sexual reproduction means that the genetic make up of offspring are more diverse than asexual creatures that just clone themselves (like jellyfish), so it's far less likely that the entire population of the species is susceptible to being wiped out by diseases," Long said. "This greater variability in the gene pool enhances survival not only [against] pathogens, but also for environmental changes, e.g. climate changes, or even better tolerance of chemical toxicity if say volcanic eruptions alter sea water chemistry."

Gender reveal parties are best known as celebrations involving pink and blue, cake and confetti, and the occasional wildfire. Along with being social media hits, gender reveals are a testament to how society is squeezing children into one of two predetermined gender boxes before they are even born.

These parties are often based on the 18- to 20-week ultrasound, otherwise known as the anatomy scan. This is the point during fetal development when the genitals are typically observed and the word "boy" or "girl" can be secretly written on a piece of paper and placed into an envelope for the planned reveal.

Now there is a new player in the gender reveal game: genetic screening.

Advancements in genetic research have led to the development of a simple blood test called cell-free DNA prenatal screening that screens for whether a baby has extra or missing pieces of genetic information — chromosomes — as early as 10 weeks into pregnancy. Included in this test are the sex chromosomes, otherwise known as X and Y, that play a role in the development and function of the body.

This blood test is more informally called noninvasive prenatal testing, or NIPT. Many people refer to it as "the gender test." But this blood test cannot determine gender.

As genetic counselors and clinical researchers working to improve genetic services for gender-diverse and intersex people, we emphasize the significance of using precise and accurate language when discussing genetic testing. This is critical for providing affirming counseling to any patient seeking pregnancy-related genetic testing and resisting the erasure of transgender and intersex people in health care.

Distinguishing sex and sex chromosomes

Sex and gender are often used interchangeably, but they represent entirely different concepts.

Typically when people think of sex, they think of the categories female or male. Most commonly, sex is assigned by health care providers at birth based on the genitals they observe on the newborn. Sex may also be assigned based on the X and Y chromosomes found on a genetic test. Commonly, people with XX chromosomes are assigned female at birth, and people with XY chromosomes are assigned male. Since cell-free DNA, or cfDNA, prenatal screening can report on sex chromosomes months before birth, babies are receiving sex assignments much sooner than previously possible.

While cfDNA prenatal screening can offer insights into what sex chromosomes an infant may have, sex determination is much more complicated than just X’s and Y’s.

For one, sex chromosomes don’t exactly determine someone’s sex. Other chromosomes, hormone receptors, neural pathways, reproductive organs and environmental factors contribute to sex determination as well, not unlike an orchestra with its ensemble of instruments. Each cello, flute, tympani and violin plays a crucial role in the performance of the final musical score. There is no single instrument that defines the entirety of the symphony.

RELATED: Fetal testosterone may program boys' behavior

Intersex people, or those with variations in sex characteristics that deviate from societal norms of binary sex, exemplify the complexities of sex. These variations can manifest in various ways beyond X and Y chromosomes, such as differences in hormone levels, genitalia or secondary sexual characteristics.

The oversimplification of sex based on societal norms has led many to believe that there are only two discrete sexes. The binary framework of sex excludes intersex people and perpetuates their erasure and mistreatment within both health care and society at large.

For instance, many intersex individuals face unnecessary surgeries, such as nonconsensual genital procedures, to conform to binary norms, violating their bodily autonomy.

Where gender comes in

While sex typically describes someone’s anatomical characteristics, gender is an umbrella term that encompasses the way someone views and presents themselves to the world. Countless aspects influence how someone defines their own gender and how the world views their gender, including clothing, haircuts and voice tone. Similar to how Western cultures have historically confined sex to two buckets, it has also created two gender categories: man and woman.

Gender is not dependent on anatomical parts or chromosomes. People are not math equations, and having certain combinations of biological parts does not equal someone’s gender. For example, some people may be transgender, meaning their assigned sex is not congruent with their socially or self-defined gender. Nonbinary people do not identify exclusively with either of the two genders in the binary, regardless of their assigned sex.

Just like sex diversity, gender diversity is not rare. A 2022 Pew Research Center analysis found that approximately 5% of adults in the U.S. under the age of 30 are transgender or nonbinary.

These estimates will likely increase as societal awareness and acceptance of gender-diverse individuals increases. Anti-transgender legislation often oversimplifies gender as strictly binary, conflating it solely with sex assigned at birth.

Intersex and gender-diverse people show that sex and gender are both multidimensional. Gender is not solely determined by biology, and it is erroneous to define someone’s gender by their sex, much less by their sex chromosomes.

Challenging sex and gender norms

The idea that biology plays the largest role in determining who an individual is, or bioessentialism, has governed misconceptions about sex and gender for many years. This concept is used to confine people to buckets and limit their self-determination.

For instance, societal norms dictate that women should be nurturing and gentle, while men are expected to be protective and assertive. Such rigid gender roles, often enforced through the lens of biology, serve to uphold notions of evolutionary destiny and a purported natural order.

Marketing strategies for children’s toys often adhere strictly to gender roles, steering girls toward dolls and domestic play sets while steering boys toward action figures and construction sets.

Educational systems often reinforce gender norms by directing girls toward subjects such as literature and arts while steering boys toward science and mathematics. This perpetuates the notion that certain traits and interests are inherently linked to one’s sex and gender, thereby reinforcing societal norms and sustaining inequality.

Upholding binary constructs of sex and gender does not allow for individuality and gender fluidity. Categorizing people from the time their chromosomes are analyzed or the moment their genitals are observed at birth restricts their autonomy and authenticity. These simple assumptions set expectations that can be harmful.

Letting children define themselves

If you’re a parent offered cfDNA prenatal screening during pregnancy, remember that it is commenting only on one instrument in the orchestra of sex. It cannot examine all of the other factors that determine sex as a whole. And it most certainly cannot determine gender, which is an entirely different concert.

In recent years, Jenna Karvunidis, the mother considered the inventor of gender reveal parties, shared her regrets for starting the trend and noted that her views on sex and gender have shifted. In a 2019 Facebook post, Karvunidis wrote, "PLOT TWIST. The world’s first gender reveal party baby is a girl who wears suits!" She had also gone on to say, "Celebrate the baby … Let’s just have a cake."

When the envelope is opened, the balloons are popped and the crafty cake is cut, consider how these practices perpetuate social confinements and a gendered destiny for your little bundle of joy. Perhaps opt simply for a celebration that leaves space for your child to one day define who they are.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Mammals' chromosomes have a huge influence over whether an individual develops as male or female — but a new study reveals that the pull of these sex chromosomes can be overridden by tiny molecules called microRNAs. 

The study, published May 7 in the journal Nature Communications, showed that deleting the genes behind specific microRNAs could transform male mice into females in the womb, sparking a complete sex reversal.

"We did not expect that the results would be as spectacular as they are," study co-author Rafael Jiménez, a professor of genetics at the University of Granada, told Live Science. 

Sex determination in mammals relies on a fine balance between "opposite" sets of genes — one that drives the development of female characteristics, such as ovaries, and another that produces male characteristics, such as testes. Early in an animal's development, the scales tip one way or another, leading to an irreversible cascade of steps that ends in the development of either set of sex organs.

"In a very early stage of our development, all the mammals have the capacity of being male or female, potentially," said study co-author Francisco Barrionuevo, a professor of genetics at the University of Granada.

Related: The human Y chromosome has finally been fully sequenced, 20 years after the 1st draft

A gene called SRY, which is only found on the Y chromosome, triggers the series of events that forms testes. The gene's absence in individuals with only X chromosomes results in the formation of ovaries. Scientists know a great deal about the genes involved in making the proteins needed for these processes. But a huge portion of mammals' DNA — including about 98% of the human genome — doesn't code for any proteins, so scientists were unsure what role these other genes play in sex determination, if any.

Long considered "junk DNA," these stretches of genetic material are converted into molecules called non-coding RNA, rather than proteins. The RNA can affect many biological processes. About one-quarter of these molecules are microRNAs, which can attach to numerous genes and regulate their activity levels.

Out of thousands of known microRNAs, the team focused on a group of six known to interact with genes involved in sex determination. They deleted these molecules from growing mice fetuses that had either XY or XX chromosomes. The XX mice developed ovaries, as expected, but the XY mice showed early signs of developing uteruses and had ovaries indistinguishable from those in XX mice.

"We saw the gonad [under the microscope] and it was full of the signal for this female marker," Alicia Hurtado, first author of the study and a postdoctoral researcher at the Andalusian Center for Development Biology in Seville, told Live Science. To confirm the results, they repeated the experiments multiple times, using different strategies to delete the microRNAs.

For testes to develop properly in XY animals, the protein made by the SRY gene must be made in the appropriate amounts and at the right times. The absence of the six microRNAs in XY mice caused this protein to be made about 12 hours later than normal, the researchers found. This, in turn, impacted the production of a different protein that's essential for the growth of male sex organs. Ultimately, this chain of events led to the mice's sex reversal.

"[These findings] fit very well with what we know, except it brings another layer of complexity," said Serge Nef, a professor of genetic medicine and development at the University of Geneva who was not involved in the study. "It is one additional brick in our understanding of the whole [sex determination] process in mammals," he told Live Science.

While the study has only been done in mice, the six key microRNAs are found in all vertebrates and date back to the first vertebrates, about 500 million years ago. Therefore, it is very likely that this cluster of microRNAs works similarly in other mammals, as well — including humans. 

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

In an unusual case, a man's premature, painful ejaculations turned out to have a rare cause: a cyst interfering with nerves near the base of his spinal cord.

According to a description of the case, published in February in the journal Archives of Sexual Behavior, the 32-year-old man went to the doctor after roughly two years of experiencing premature ejaculation (PE), broadly defined as a condition in which a person orgasms and releases semen sooner than desired — generally, in less than one minute after starting intercourse. Estimates for the condition's prevalence vary, but research suggests that between 30% and 40% of men experience PE at some point in their lives, according to the Cleveland Clinic.

Causes of PE vary widely and may be physical — for example, they may be tied to levels of certain hormones involved in sexual function. Or, they may be psychological, related to stress or performance anxiety, for example. But in this patient's case, a body scan revealed that a cyst near his spinal cord was the likely culprit.

Related: Our 10 oddest medical case stories from 2023

In his time experiencing PE, the man's condition had slowly worsened and was eventually accompanied by prolonged erections in the absence of stimulation — a phenomenon medically known as priapism. He also started experiencing pain in his pelvis, as well as pain during ejaculation.

Doctors scanned the man's lower back with magnetic resonance imaging (MRI) and revealed "one oval lesion" around the bottom of his lumbar spine and the start of his sacrum, the fused bones at the base of the spine.

The lesion was a so-called Tarlov cyst, a type of fluid-filled sac that sometimes forms around the sacrum. These can emerge for various reasons, such as inflammation or physical trauma in the spine, although the case report didn't note the exact cause of this patient's cyst. Typically, the sacs appear among the roots of the nerves that grow out of the spinal cord.

Often, these cysts cause no symptoms. However, in this case, the man's oval-shaped cyst was compressing the root of his sacral nerves, which, among other functions, relay movement instructions to and receive sensory information from various structures in the pelvis. Such compression can lead to sexual dysfunction.

The doctors assessed the severity of the man's PE with questionnaires about his symptoms, as well as his pelvic pain. Based on these assessments, "the patient had obvious PE and severe pain around the pelvis," his doctors wrote in the report. Therefore, they determined that "prompt surgical treatment" was necessary.

After the cyst was completely removed via surgery, "PE and pain around the pelvis were significantly relieved," the doctors reported. The man was assessed six months post-surgery and reported significant improvements.

In general, when a patient sees a doctor because of PE, "nerve symptoms and effective image-assisted examination are often ignored during diagnosis and treatment," the report authors added. "In our opinion," examinations of the spine and surrounding structures should be considered after doctors rule out other potential causes in otherwise healthy males who have recently developed PE, they wrote.

If a PE case is tied to a Tarlov cyst, surgical removal of the lump may help, they concluded.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

For the first time ever, researchers have documented penetrative sex between two humpback whales. What’s more, both whales were male.

A private vessel spotted the adult whales off the Hawaiian island of Maui in January 2022. Citizen scientists on board the boat noticed one of the animals was an unusual, brownish color and deployed cameras underwater to record the encounter. What they captured is the first known case of a male humpback whale (Megaptera novaeangliae) inserting its penis into another male's genital slit.

"This is the first report of penetration by a humpback whale, and the first report of sexual activity between two male humpback whales," researchers wrote in a study published Tuesday (Feb. 27) in the journal Marine Mammal Science. 

The whales swam in circles around the boat for 30 minutes, with one of the humpbacks (Whale B) seemingly pursuing the other (Whale A), according to the study. Whale A looked emaciated, had a significant jaw injury and was covered in whale lice (Cyamus boopis) — parasites that indicate poor health and are the probable cause of the skin discoloration noted by citizen scientists at the start of the encounter. Whale B, on the other hand, appeared healthy and strong.

Related: Kelping is a 'global phenomenon' sweeping the world of humpback whales, scientists say

Whale B also had its penis extruded from its usual hiding place inside the genital slit during the entire encounter. "Whale B repeatedly approached from the rear of Whale A and penetrated the second whale, appearing to hold Whale A in place with its pectoral fins," the researchers wrote.

In response to Whale B's advances, Whale A contorted its body into an "S" shape — a posture which previous research suggests humpbacks in Hawaii adopt when they feel threatened or stressed.

"It is possible that the use of this posture by Whale A meant that this behavior was unwelcome, but that Whale A lacked the energy to engage in avoidance strategies," the researchers wrote in the study. "It was slowly attempting to swim away from Whale B but was not making any sudden or powerful movements and did not dive out of sight at any point."

After penetrating Whale A several times, Whale B dove and did not reappear, according to the study. Whale A remained at the surface for a few minutes before also plunging out of sight. It is unclear what caused Whale A's jaw injury, but the researchers suggested it may be the result of a collision with a ship.

Male-male sex in cetaceans — a group of marine mammals that includes whales, dolphins and porpoises — is well documented for common bottlenose dolphins (Tursiops truncatus) and several other species. Research suggests cetaceans engage in such homosexual activity to establish or reinforce dominance among males, to learn or practice reproductive behaviors, to form social alliances and even to reduce social tensions. 

Related: Orcas are learning terrifying new behaviors. Are they getting smarter?

Whale B may have mistaken Whale A for a female, the researchers suggested, or it could have been trying to reinforce their social relationship. But the sexual behavior may also have been "an expression of dominance over a weak and injured competitor," they wrote in the study.

While this is the first documented case of homosexual penetration in humpback whales, a similar incident reported in 1998 described a male humpback whale extruding its penis as it swam near the corpse of another male that had died shortly before.

"It is striking that the only two observations of such behavior in the scientific literature involve ailing or deceased whales," the researchers wrote in the new study. "Whether such behavior would occur between two healthy males is unknown."

Drugs taken to treat erectile dysfunction have once again been tied to a reduced risk of Alzheimer's disease. 

A new study of almost 670,000 men in the U.K., published Feb. 7 in the journal Neurology, revealed that those who take so-called phosphodiesterase 5 inhibitors (PDE5I) — which include Viagra (generic name sildenafil) — may have a lower risk of developing this common form of dementia than men of the same age who don't take the drugs. Another study, published March 19 in the Journal of Alzheimer's Disease, found that people who took sildenafil were 30% to 54% less likely to be diagnosed with Alzheimer's disease compared to those who didn't, adding to this idea. 

But this isn't the first time that drugs like the "little blue pill" have been linked to this reduced dementia risk — since the late 1990s, rodent studies have suggested that PDE5I drugs could have beneficial effects on cognition, and in 2021, a study published in the journal Nature Aging suggested that taking Viagra was associated with a 69% reduced risk of Alzheimer's. That 2021 study included more than 7 million people in the U.S. who on average were around 71 years old, 116,000 of whom took Viagra. 

PDE5I drugs like Viagra work by increasing blood flow to the erectile tissues of the penis. They do this by preventing the breakdown of a signaling molecule called cyclic guanosine monophosphate (cGMP), which keeps smooth muscles within the penis relaxed. This relaxation allows blood to flow into the erectile tissue — the corpus cavernosum — as its blood vessels dilate upon sexual arousal. When cGMP gets broken down too quickly, the erection can't be maintained. 

Related: Viagra alternatives? Study of mouse erections hints at new ways to treat erectile dysfunction

While Viagra's effects on the penis are well understood, it's unknown how this and similar drugs may influence brain disorders such as Alzheimer's. However, several potential mechanisms have been proposed. 

Other than influencing erections in the penis, PDE5I drugs dilate blood vessels throughout the body, including in the brain, Dr. Sevil Yasar, an associate professor of medicine at Johns Hopkins University, told Live Science. Indeed, Viagra was originally developed to treat high blood pressure and chest pain. It's therefore possible that increased blood flow in the brain may somehow reduce the burden of Alzheimer's, she said. 

In the 2021 Nature Aging study, Feixiong Cheng, a principal investigator at the Cleveland Clinic, and colleagues found that Viagra boosted the growth of brain cells that had been grown from the stem cells of patients with Alzheimer's. The drug also turned down the production of proteins associated with the disease — namely, phosphorylated tau, which accumulates in the brains of people with Alzheimer's. 

Increased blood flow to the brain that results from these drugs might therefore help clear these proteins in some way, Cheng told Live Science. 

Another explanation could be that PDE5I drugs strengthen the connections, or synapses, between neurons in the brain, as this process partially relies on cGMP, Atticus Hainsworth, a reader in cerebrovascular disease at St George's, University of London, told Live Science. The brain stores memories by strengthening synapses, helping linked neurons "talk" to one another. This could explain the link between PDE5I drugs and a reduced risk of Alzheimer's, which causes memory loss.

In addition, people with high blood pressure or type 2 diabetes have a higher risk of experiencing erectile dysfunction and of developing Alzheimer's, compared with people without either condition. So it could be that erectile-dysfunction drugs are helping to manage these other conditions, rather than directly targeting the causes of Alzheimer's, Yasar said. 

For now, though, none of these theories has been definitively proved — in fact, it's still unclear whether PDE5I drugs have any effect on Alzheimer's risk. 

In 2022, for example, a study in people with pulmonary arterial hypertension — a form of high blood pressure in the lungs that can be treated with PDE5I drugs — found that the medications were not associated with a reduced risk of Alzheimer's. 

A big issue is that the largest studies to date have been observational, meaning they retroactively compared the rates of disease between different people without accounting for other factors that could affect people's Alzheimer's risk. Such studies can't definitely prove that erectile-dysfunction drugs influence the risk. For example, the findings of the 2024 Journal of Alzheimer's Disease study may have been skewed by genetic or socioeconomic factors which weren't considered by the authors in their analyses, and by the doses of drug they used in lab-dish experiments. 

Related: Erectile dysfunction risk may rise on lengthy space missions, rat study reveals

To do so, scientists would need to conduct a gold-standard clinical trial with comparison groups who don't take the drug but are matched to those who do in other ways, such as age and sex. This would help ensure that other lifestyle factors or medical conditions don't skew the results. 

For example, "it could be that the people who are still cognitively well enough to have sex and want the drugs are biasing the [study] population," Hainsworth said. 

A few clinical trials have assessed the potential cognitive effects of PDE5I drugs in humans. However, these have been limited in size; one trial included only 10 people, for instance. Some have tested only the short-term impacts of the treatment — for example, with the cognitive effects of a single dose measured over the course of a day.  

To get closer to proving that erectile-dysfunction drugs can help prevent Alzheimer's, future trials would need to run for potentially three to five years and include people with clinically confirmed Alzheimer's diagnoses, Yasar said. This would go beyond relying on, say, insurance claims data, as previous observational studies have done, she said. 

It would also be important to look for telltale markers of Alzheimer's disease during the trials, using brain imaging to measure changes in a person's blood flow after they took PDE5I drugs, she added. 

Long-term trials would allow scientists to look out for any long-term side effects of taking erectile-dysfunction drugs. Such side effects could stem from taking a drug that reduces the pressure of blood flowing through your arteries, Francesco Tamagnini, a lecturer in pharmacology at the University of Reading in the U.K., told Live Science. 

On a mechanistic level, the drugs may need to be modified slightly so they are more likely to accumulate in the brain, to achieve the most robust effects, Tamagnini said. 

Future trials could also include women, as well as men, to see if similar cognitive effects are seen in them, Hainsworth said. Currently, Viagra is only approved by the U.S. Food and Drug Administration (FDA) to treat erectile dysfunction in men. Several early studies have investigated whether it could be used to treat sexual dysfunction in women, but with conflicting conclusions about effectiveness. 

If these drugs eventually turn out to be an effective guard against Alzheimer's, they could fuel the fire of research striving to repurpose approved drugs for other uses. This is happening with the diabetes drug metformin, for example, which doctors are trying to repurpose for cancer and heart disease, Tamagnini said. As these drugs are already licensed for use in humans, this could speed up the drug-development process, he said. 

For the little blue pill, though, there's a long way to go before it would be prescribed as dementia prevention.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

Editor's note: This story was last updated on March 27, 2024, to include information on the Journal of Alzheimer's Disease study. The article was first published on Feb. 16, 2024.

By studying male mice's erections, scientists discovered that two types of cells in the penis are essential for triggering and maintaining a boner. 

Although the cells are also abundant in the human penis, their crucial role in erections was previously unknown, according to a new study published Thursday (Feb. 8) in the journal Science. 

"Because the mechanisms of penile erection are similar in mice and humans, these findings may be relevant to erectile dysfunction in aged men," Ji-Kan Ryu of the Inha University School of Medicine and Gou Young Koh of the Institute for Basic Science in South Korea, wrote in a commentary of the new study.

The findings may point to new ways of treating erectile dysfunction in humans, the study authors wrote in their paper. Standard drugs like Viagra don't work for up to 30% of patients and they can't be used by those who take certain medications or have specific medical conditions.

Related: Move over, Viagra — this spider's boner-inducing venom could treat people let down by the blue pill 

Viagra and similar erectile-dysfunction drugs work by blocking an enzyme and making tissues of the penis more sensitive to nitric oxide. The release of nitric oxide is a crucial, early step for triggering an erection. The signaling molecule kicks off a chemical chain reaction that causes smooth muscles in the penis to relax. This then allows a sponge-like tissue — the corpora cavernosa — to fill with blood. 

Viagra helps those smooth muscles stay relaxed, keeping the erection aloft.

While nitric oxide is a key player in this process, it's not the only one. In their new study, the researchers were interested in studying fibroblasts, cells that make connective tissue and that are the most abundant type of cell in the human corpora cavernosa. The team wanted to understand whether these cells help control blood flow to the penis.

In mice, the team identified two large groups of fibroblasts adjacent to blood vessels in the corpora cavernosa. Through experiments with genetically modified mice, the researchers messed with the activity of these fibroblasts, to see how blood flow to the penis changed. These tests revealed that the cells help trigger erections by sucking up a hormone that keeps the penis flaccid: norepinephrine. By stashing the norepinephrine away, the cells help the relevant blood vessels dilate.

The researchers found that the cells' ability to perform this function varied depending on how concentrated they were in the penile tissue. The more fibroblasts present, the more robust the results. And notably, compared with young mice, old mice had fewer fibroblasts — this hints that aging may contribute to erectile dysfunction by reducing these cells' numbers over time, the authors theorized. 

The team also figured out that there's a molecular switch within the fibroblasts that can boost their numbers, if switched off, or decrease their numbers, if switched on. This switch is known as the "Notch signaling pathway." By completely switching off the pathway and dramatically increasing the number of fibroblasts, the researchers found that they could trigger "atypical long-lasting erections" in the mice, characteristic of a painful condition called ischemic priapism. 

Taken together, the findings suggest that flipping these switches in fibroblasts "might open new therapeutic alternatives for treatment of erectile dysfunction," the study authors wrote.

Besides altering Notch signaling, another approach could be to boost the amount of norepinephrine that the fibroblasts absorb, the commentary authors added. That said, any of these new approaches would need to be thoroughly tested to make sure they didn't have unwanted effects in nearby tissues, they added.

"These new approaches will require rigorous preclinical and clinical testing to translate observations made in transgenic mice into therapies that are safe and effective in men," Ryu and Koh concluded.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

Male antechinuses live through just a single, extremely intense mating season. So to maximize their chances at sex, these little carnivorous marsupials waste as little time on sleep as possible, new research has found.

During a three-week breeding period, males belonging to two species — dusky (Antechinus swainsonii) and agile (A. agilis) antechinus — lost an average of three hours of sleep every night, or around 20% of their total sleep. "This level of sleep loss would make humans perform very poorly," lead author Erika Zaid, a graduate researcher in sleep eco-physiology at La Trobe University in Australia, told Live Science in an email. "Yet, antechinus endured doing so for three weeks."

Males sacrificed sleep to mate with as many females as possible and father a maximum number of offspring before dying, Zaid said. 

In the wild, male antechinuses drop dead in a single "programmed" event following the mating period. Males often survive for longer in captivity, Zaid said, but they become sterile. In the new study, published Thursday (Jan. 25) in the journal Current Biology, 2 captive males out of 10 died at the end of the breeding season. "They died in the same night, only [a] few hours apart," Zaid said.

Recent research suggested male marsupials of the dasyurid family — which live and reproduce for only one year and include antechinus — died due to extreme sleep deprivation, based on the observation that severely sleep-deprived laboratory rats eventually die of exhaustion.

However, Zaid thinks this may not be the case with antechinuses. "After the breeding season, antechinus males do develop skin lesions and fur loss that resemble those observed in sleep deprived rats," Zaid said. "However, we don't think sleep loss is the cause of the male die-off," because the two males that died weren't the most sleep-deprived.

Related: 'They mated like mad': Low-flying helicopter sparks massive crocodile orgy in Australia

Another theory is that male antechinuses die from a surge in testosterone and corticosteroid levels during the mating period, which can induce organ failure and leave males more susceptible to infections and parasite infestation, according to the study. But no such surge has been detected in males northern quolls (Dasyurus hallucatus) — another dasyurid species — prompting the researchers to think another, unknown trigger causes male antechinuses to die off. 

Unlike the males, female antechinuses survive for up to two years and can give birth to two litters. They might even benefit from the males dying en masse, as the corpses may provide a rich and abundant source of food to fuel their pregnant or lactating bodies, according to a study published Jan. 18 in the journal Australian Mammalogy.

The study described a sighting of a mainland dusky antechinus (A. mimetes) eating a dead member of its species in New England National Park, New South Wales, suggesting these critters indulge in cannibalism.

While cannibalism has been documented in some dasyurids, it is very rarely seen in the wild, Andrew Baker, an author of the Australian Mammalogy paper and an associate professor specializing in Australian mammals at the Queensland University of Technology in Australia, said in a statement. 

Zaid and her colleagues housed the antechinuses used for their study in individual enclosures and therefore didn't observe any cannibalism. "I guess cannibalism is a good strategy they might adopt to have easily accessible energy," Zaid said. But Baker and colleagues could not identify the sex of the cannibalistic antechinus, meaning it could have been a male eating another male, she said.

Much remains to be learned about antechinuses — including the details of their cannibalistic tendencies and how males cope with extreme lack of sleep during the mating season.

It may be that antechinuses feel the negative effects but simply get on with it, or that these critters are resilient to sleep deprivation, the researchers said in a statement shared with Live Science.

Roughly 2,500-year-old DNA has revealed the first ancient person on record with Turner syndrome, a genetic condition in which a person has just a single X chromosome rather than two, a new study finds.

The individual, who died when they were 18 to 22 years old, likely hadn't gone through puberty, an analysis of the bones revealed. A further investigation of the remains revealed that the individual had mosaic Turner syndrome, as some cells had just one X chromosome while others had two.

The research looked at historical DNA gathered in the Thousand Ancient British Genomes project — a database of DNA being collected from skeletons in the U.K. The team identified a total of six people with sex chromosomal conditions, according to a study published Jan. 11 in the journal Communications Biology.

The researchers made the discoveries after developing a computational method to find atypical numbers of chromosomes in DNA from skeletons.

Related: Europeans' ancient ancestors passed down genes tied to multiple sclerosis, Alzheimer's risk

The individual with Turner syndrome, who died in the early Iron Age (750 to 400 B.C.) likely had a partially missing second X chromosome. This condition can often lead to symptoms and characteristics such as shorter-than-average height, cardiac defects and small or absent ovaries, leading to fertility issues.

Of the other five people with aneuploidies, or genetic disorders in which a person doesn't have 46 chromosomes, three individuals showed signs of Klinefelter syndrome — a genetic condition in which a person has an XXY set of sex chromosomes. Of these three individuals, one died in the Iron Age (circa 750 B.C. to A.D. 43), one in the high Middle Ages (around A.D. 1050 to 1290) and one in the early 19th century. Klinefelter syndrome often stunts the growth of a person's testicles, leading to lower testosterone levels, lower muscle mass, less body hair and larger breast tissue than typical XY individuals.

The three skeletons identified with this condition were buried in ways typical for their times, according to the study, showing that "their burials did not reveal any differences in how they were perceived by their contemporaries," the researchers wrote.

Another male individual from the early medieval period (eighth century) had an extra Y sex chromosome, known as XYY syndrome. Most people who have XYY chromosomes have no physical features that are different than people with XY chromosomes, other than often being taller than average.

The researchers also identified a male infant from Iron Age Britain who had Down syndrome. This condition can result in neurodevelopmental problems, and identifying skeletons with the syndrome "can provide insights into care within ancient societies, as well as how people with these conditions, which have characteristic physical manifestations, were perceived by their peers," the researchers wrote in the study.

Although the number of people with chromosomal differences revealed in this study is small, the researchers' new method gives them the opportunity to observe genetic diversity to "provide another layer of information that can contribute to a more detailed reconstruction of the human past," they wrote in the study.

In particular, making it easier to study variations in sex chromosomes in ancient DNA can help move the field of skeletal analysis beyond binary sex estimations and into a more complex understanding of social gender.

"It is difficult to know an ancient individual's conception of their own gender identity, and gender norms in the past may not align with those of the present day," the researchers wrote in their study. "It is possible that an elevated proportion would have been seen to transgress gender boundaries."

Researchers have discovered an extraordinary, vibrantly colored tropical bird with "female" feathers down one side of its body and "male" feathers down the other.

Female green honeycreepers (Chlorophanes spiza) typically have grassy green feathers, while the males are a brilliant blue. However, this individual was green on the left and blue on the right. Amateur birder and lead study author John Murillo first spotted the two-toned honeycreeper at his farm near Villamaría in Colombia in 2021; the bird was last spotted in June 2023 and its current status is unknown.

The bird was a bilateral gynandromorph, meaning it had features characteristic to both sexes and could theoretically reproduce as either. This is only the second record of bilateral gynandromorphy in green honeycreepers and the first in more than 100 years, according to a new study about the bird published in the December issue of the Journal of Field Ornithology.

Study co-author Hamish Spencer, a professor of zoology at the University of Otago in New Zealand, saw the same bird in January 2023 when he visited Murillo's farm during a birdwatching tour.

"I couldn't really believe it," Spencer told Live Science. "Here was this bird with two halves: one male half and one female half."

Related: Rare bee has a body that's half-male, half-female, and split exactly down the middle

Green honeycreepers' habitat range extends through Central and South America, from southern Mexico to Brazil. Murillo feeds honeycreepers and many other birds at his property, which doubles as an ecotourism business and fish farm called Reserva Natural Demostrativa Don Miguel. He documented the mixed-sex bird by photographing and filming it at a feeding station.

"He had these incredible photos," Spencer said. "They're the best photos I've ever seen of a [wild] gynandromorphic bird."

The honeycreeper hung around Murillo's feeding station for weeks at a time between 2021 and 2023, according to the study. The authors noted that the bird didn't exhibit any abnormal behavior at the feeding station and wasn't harassed by other honeycreepers for its unusual appearance. However, the bird typically waited for other honeycreepers to leave before using the feeders and would sometimes block other birds from using them when it did.

The honeycreeper's tendency to avoid other members of its species suggested it would have difficulty finding a mate, although it's unknown whether it did or could reproduce. Researchers have previously dissected gynandromorphic birds of other species and found their sexual organs can match their plumage — an ovary on one side and a testis on the other, according to the study.

The females of most avian species only have one functional ovary on their left side, anyway. So if a gynandromorphic bird is female on the left, like this honeycreeper, then it can potentially use the left side to reproduce as a female or the right side to reproduce as a male. Most birds don't have external genitalia and mate by rubbing an opening called the cloaca, which is present in both males and females, against another bird's cloaca.

Other gynandromorphic birds have been observed mating in the past, the study authors noted. For example, a behaviorally male gynandromorphic zebra finch (Taeniopygia) courted and mated with a female zebra finch, while two behaviorally female gynandromorphic Gouldian finches (Chloebia gouldiae) mated with males, producing both infertile and fertile eggs.

Gynandromorphy can also occur in other animals, including insects, crustaceans and lizards, according to a statement released by the University of Otago. In birds, researchers suspect it's caused by an error that occurs during the division of cells in a female's unfertilized egg that enables one egg cell to develop two nuclei, which are then fertilized by two separate sperm.

Female birds carry both avian sex chromosomes (ZW) while males carry two copies of the same chromosome (ZZ) — the opposite pattern is seen in mammals, with females carrying XX and males carrying XY. In a gynandromorphic egg cell with two nuclei, one nucleus has a Z chromosome and the other has a W chromosome. Double fertilization then occurs when sperm arrives carrying male Z chromosomes, with the Z nucleus taking a Z chromosome to become ZZ (male) and the W nucleus taking a different Z chromosome to become ZW (female).

The male and female nuclei go on to form the two separate halves of the same bird. In this scenario, the bird can be either sex on either side, depending on how the female's chromosomes are split between the two nuclei.

"The reason our observation is important is because the previous known example in the green honeycreeper was the other way around; it was male on the left and female on the right," Spencer said. "What that tells us is that both outcomes are possible, and that fits with the double fertilization scenario."

Deep space exploration may increase the risk of astronauts developing erectile dysfunction, new research on rats suggests. 

In experiments in the lab, male rats' exposure to simulated galactic cosmic radiation (GCR) and weightlessness impaired the function of the erectile tissue in the penis, according to a new study published Wednesday (Nov. 22) in The FASEB Journal.  

These effects were observed even after a year-long recovery period, suggesting that deep space exploration could have long-lasting health impacts for astronauts. 

With crewed missions to outer space planned for the coming years, "this work indicates that sexual health should be closely monitored in astronauts upon their return to Earth," co-senior study author Justin La Favor, an assistant professor who studies neurovascular dysfunction at Florida State University, told Live Science in an email. 

Related: Move over, Viagra — this spider's boner-inducing venom could treat people let down by the blue pill

As the space industry prepares to send astronauts around the moon as early as 2024 and to Mars by 2040, more attention is being paid to the potential long-term impacts of deep space exploration on the human body. During such missions, astronauts would be exposed to weightlessness due to the low levels of gravity in space, as well as high levels of GCR. 

On Earth, the planet's thick atmosphere helps to deflect GCR — energetic subatomic particles — back into space, protecting life below. But in space, there is less protection, so astronauts are more exposed to its effects. For example, a six-month stay on the International Space Station will give astronauts a dose of radiation from cosmic rays equivalent to about 25 lifetimes on Earth's surface, Live Science previously reported. High exposure to GCR could lead to certain cancers, neurological damage and cardiovascular disease.

Both GCR and weightlessness "are associated with adverse health outcomes, although the effects on erectile function had not previously been investigated," La Favor said. 

On Earth, erectile dysfunction — the inability to establish and maintain an erection — affects more than half of men between 40 and 70 years old. The effects of space exploration could therefore be an important consideration for male astronauts, especially if the condition persists in the long term. 

Over four weeks, the authors of the new study exposed 86 adult male rats to either hindlimb unloading, an experimental way of simulating weightlessness by lifting them up by their tails so that they are suspended with their head down at a 30 degree angle, or a control where they were able to touch the floor. Hindlimb unloading mimics two of the major physiological effects of low gravity, namely that the animals don't bear weight so there is no resistance on their muscles and bones, and it mimics shifts in body fluid and pressure that happen in an absence of gravity, La Favor said. 

At the same time, using the ground-based GCR simulator at the NASA Space Radiation Laboratory in New York, the rats were exposed to one dose of either low or high amounts of cosmic radiation. 

Around 12 to 13 months later, the authors euthanized the mice and extracted samples from the corpus cavernosum, the erectile tissue of the penis and the internal pudendal artery, the major artery that regulates the flow of blood into the penis during an erection. They discovered that GCR, and to a lesser extent weightlessness, impaired the function of both types of tissue mainly by increasing oxidative stress — when there is a build up of free radicals in the body which can damage cells. 

Follow-up experiments, however, showed that it was possible to counteract some of the effects caused by GCR using certain types of antioxidants. 

The study only considered the health effects of GCR and weightlessness following a long recovery period, so it is possible that the short-term effects could be more severe, the authors wrote in the paper. 

Going forward, the authors would like to look further into the exact causes of the observed effects and investigate ways to prevent them. Female astronauts will also be on board upcoming missions, so it will also be important to investigate the potential impacts of deep space exploration on their sexual function too, they wrote. 

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

Bats with "disproportionately large" penises that don't fit inside the females' vaginas are the only mammals known to have sex without penetration, scientists have discovered.

Mammals usually mate via penetrative sex, but serotine bats (Eptesicus serotinus) have evolved a different strategy. Instead of inserting their penis into the female's vagina, males use their oversized genitalia to push aside their partner's tail sheath, before pressing up against her vulva for up to 12 hours, according to a study published Monday (Nov. 20) in the journal Current Biology.

"We had observed that these bats have disproportionately long penises, and we were always wondering 'how does that work?'," lead author Nicolas Fasel, a postdoctoral researcher and evolutionary biologist at the University of Lausanne in Switzerland, said in a statement shared with Live Science.

It turns out, it doesn't work — at least not like in other mammals. Their penises are seven times longer and wider than the females' vaginal openings when erect, making post-erection penetrative sex impossible.

"We thought maybe it's like in the dog where the penis engorges after penetration so that they are locked together," Fasel said. "Or alternatively maybe they just couldn't put it inside, but that type of copulation hasn't been reported in mammals until now."

Related: 10 phallic flora and fauna that look just like penises

Fasel and his colleagues analyzed footage from cameras they had placed on a grid that the bats could climb, and where the animals' genitalia were visible during copulation. They recorded a total of 97 mating events in the attic of a Dutch church and in a bat rehabilitation center in Ukraine. None of the sex sessions appeared to involve penetration.

Instead, the footage showed males grasping females by the nape and moving their pelvis until their erect penis was pushed firmly against their partner's vulva. The males then remained completely still in this embrace for an average of 53 minutes, with the longest interaction lasting almost 13 hours.

The researchers noted the penises were enlarged before coming into contact with the vulva, excluding the possibility of even pre-erection penetration, which is known to occur in dogs. 

The team also measured and described the erect penises of live serotine bats. These animals develop erections under anesthesia, according to the statement, enabling researchers to document a "heart-shaped" swelling at the tip of the penis.

"The terminal swelling was adorned with a few short hairs and composed of two large erectile tissues," the researchers wrote in the study. "We postulate that the hair present on the terminal swelling serves as a sensor to assist in finding the vulva." 

Serotine bats may have evolved an oversized penis so they could push aside a membrane that covers the females' genitals. "Bats use their tail membranes for flying and to capture the insects [they eat], and female bats also use them to cover their lower parts and protect themselves from males," Fasel said. "But the males can use these big penises to overcome the tail membrane and reach the vulva."

The fur on female bats' abdomens appeared wet after mating, indicating the presence of semen, but samples are needed to confirm that males ejaculate during the long embrace, according to the study.

This is the first documented evidence of non-penetrative sex, also known as "contact mating," in mammals. This behavior has been described in birds, however, and involves males and females pressing together openings called cloacae to perform "cloacal kisses," resulting in a transfer of sperm.

To better describe the mating behavior of serotine bats, Fasel and his colleagues "are trying to develop a bat porn box, which will be like an aquarium with cameras everywhere," he said.

Name: Mangrove leaf slug (Elysia bangtawaensis)

Where it lives: Shallow pools of water in the mangrove forests and swamps of southeast Asia and Australia

What it eats: The content of tiny algal cells

Why it's awesome: In addition to looking like a starry night sky, this fabulous slug belongs to the sacoglossans, also known as the "solar-powered sea slugs" — the only known group of animals that can photosynthesize by retaining chloroplasts from algae.

Mangrove leaf slugs have seven to nine sharp teeth, which they use to puncture algal cells to then suck out the good stuff inside. That includes chloroplasts, the cellular structures plants use to transform sunlight into sugars during photosynthesis. Mangrove leaf slugs, which grow up to 2 inches (5 centimeters) long, use the chloroplasts they ingest to power themselves, enabling them to survive without food for two to three months.

Related: This sea slug can chop off its head and grow an entire new body, twice

As well as producing energy, chloroplasts tint these slugs a dark, leafy-green color. This helps mangrove leaf slugs blend into their surroundings and go unnoticed by potential predators, such as crustaceans and birds, as they lounge in shaded pools amid the mangrove trees.  

After a few months, however, the chloroplasts start to shrink and the slugs turn a yellowish color, at which point they have to replenish their photosynthetic machinery with another meal of algae. Once they have fed, the famished slugs regain their leafy-green color.

And if their photosynthetic abilities weren't weird enough, mangrove leaf slugs are simultaneous hermaphrodites, meaning each slug has the sexual organs of both sexes. To reproduce, two slugs hug tight and insert their penises into each other's vaginas at the same time. After sex, the slugs sometimes form a ball as they withdraw their penises. They then lay egg strings up to 28 inches (71 cm) long and coil them around solid features on the swampy substrate. The eggs hatch within a week, releasing a fresh batch of tiny mangrove leaf slug larvae.

 Image courtesy of Nick Volpe Wildlife Photography. 

A roadside burial discovered in Israel may hold the cremated remains of an ancient Greek courtesan who accompanied Alexander the Great's armies on their campaigns.

The woman, who was buried with an ornate bronze mirror, was laid to rest 2,300 years ago on the road to Jerusalem and far from any settlement, suggesting she may have been a professional escort, or "hetaira," traveling with military men — the first discovery of its kind, archaeologists said in a statement shared with Live Science.

"It is most likely that this is the tomb of a woman of Greek origin who accompanied a senior member of the Hellenistic army or government," the researchers said in the statement. Her client may have fought in one of Alexander the Great's campaigns, they added, or in a series of conflicts called the Wars of the Diadochi, which saw Alexander's generals battle to succeed him after he died in 323 B.C.

The woman was 20 to 30 years old when she died, researchers told the Israeli newspaper Haaretz, and her remains indicate she was cremated before being buried alongside a "very precious" bronze mirror and four iron nails.

Related: Did Alexander the Great have any children? 

The mirror is enclosed in a folding box of a type previously found in Greco-Hellenistic burials, hinting at the woman's Greek origin. While these accessories often feature engravings or reliefs of idealized female and goddess figures, the newly discovered object is decorated on the outside with a simpler pattern of concentric circles.

"This is only the second mirror of this type that has been discovered to date in Israel," Liat Oz, an archaeologist who led the recent excavation in Jerusalem's Talpiot neighborhood on behalf of the Israel Antiquities Authority, said in the statement. The hetaira may have received it as a gift from her powerful client, the researchers said.

Women also acquired bronze mirrors as part of their dowry — but married women at the time seldom left their homes in Greece, let alone joined their husbands on military campaigns, the researchers added.

Historic records indicate courtesans were present during Alexander the Great's campaigns, the researchers told Haaretz. They provided sexual services, but were also literate and entertained their clients with poetry, dance and acting performances.

"We know that some joined generals or rulers on their campaigns — famously, the hetaira Thaïs joined Alexander on the road and he didn't like her to be far," Guy Stiebel, an archaeologist at Tel Aviv University who participated in the recent excavation, told Haaretz. 

The iron nails discovered in the roadside grave were likely credited with "magical powers," he added, such as warding off the evil eye and preventing the deceased from rising again. Nails are frequently found in ancient Greek and Roman graves, as well as in Jewish burials from the time.

The researchers hope that a more detailed analysis of the bronze mirror will reveal clues about the woman's background, as well as about the man she accompanied.

The deep sea, which encompasses waters below 660 feet (200 meters), is home to roughly a million species that have adapted to their extreme surroundings with equally extreme solutions to one of life's greatest trials: finding a mate. In the excerpt below from "Deep Sea: 10 Things You Should Know," ocean explorer Jon Copley takes a deep dive into the astonishing sex lives of animals living in the darkest corners of our planet. 

All animals face similar trials in life: searching for food, avoiding being eaten, finding a mate and rearing their offspring, and then those offspring finding a home. And just as deep-sea animals overcome the challenge of finding food in lots of different ways, the same is true for those other challenges.

Populations of deep-sea animals often become sparse where food is scarce in the deep sea, which can make it difficult to meet a member of the opposite sex for reproduction. As a result, some deep-sea animals take an opportunistic, "have-a-go" approach to mating. In several species of deep-sea squids and octopuses, for example, males try to mate with any potential partner that they meet, regardless of their sex or even their species.

Mating in squids involves the male passing a spearlike packet of sperm down a groove in one of their arms to stick on to a female's body, ready to release its contents when she produces her eggs. But male squid collected in nets from the deep sometimes have sperm spears stuck on their bodies too, in places where the jabs can't have been self-inflicted, indicating attempted mating by another male. Attempted interspecies mating has also been observed: in 1994 scientists diving in a Human-Occupied Vehicle filmed two male octopuses of different species trying to mate with each other, 2,500 meters (8,200 feet) down on the ocean floor of the eastern Pacific Ocean.

Related: Barreleye fish: The deep-sea weirdo with rotating eyes and a see-through head

As an alternative to indiscriminate mating, some deep-sea animals stay with a partner once they've met them. The sea cucumber Paroriza pallens looks like a moldy banana and spends its adult life crawling across the abyssal plains, leaving a track that can be seen in photographs of the seafloor. Sometimes the single track of a Paroriza meets another, and then the two trails continue side-by-side like a railway line. At the end of those tell-tale twin tracks there's a pair of Paroriza, now wandering across the abyssal plain together.

Paroriza sea cucumbers are hermaphrodites that develop male and female sex organs at the same time, but they can't self-fertilize. Instead, the sperm produced by each partner fertilizes the eggs produced by the other partner. Staying together means that one partner is always available to fertilize the other's eggs whenever they produce them — and the story of their encounter and subsequent fidelity is recorded in their trails on the soft mud of the abyssal plain.

When it comes to keeping a male handy for fertilizing eggs, several deep-sea animals have evolved a more extreme solution. Wood-eating clams, bone-eating "zombie" worms, and some species of anglerfishes have tiny males that attach themselves to a female once they've found her, acting as standby "accessory males" to fertilize the female's eggs when needed. In some species of bone-eating worms, for example, one female can have a harem of a dozen or more males, each about a hundred times smaller than the female, hanging on to her with microscopic hooks.

About two dozen species of anglerfish that live in the deep sea also have accessory males to varying degrees. In some species, the smaller male attaches temporarily to a female, but he can swim off and hook up with another female. In other species, however, the male fuses his mouth onto the female's body in a kiss that lasts the rest of his life. The male's blood supply joins up with hers through his lips, and he can no longer leave her or feed himself: he becomes a parasitic provider of sperm on-tap, nourished by the female through their shared circulation as she continues to feed. In some species, only one male forms this lifelong union with a female, but in others, one female can have several accessory males dangling off her at any one time.

But there's a complication in such a permanent pairing. Fish have an immune system with two main parts like ours. The "innate" immune system produces general defenses to fight off infections, while the "adaptive" immune system recognizes and attacks any "foreign" substances, including cells that are genetically different to the rest of the body. That adaptive immune system is great for tackling would-be invaders, such as disease-causing bacteria, but is a problem when sharing a blood supply with a partner. 

If we were to join our blood supply to that of another person, our adaptive immune systems would attack each other through the shared circulation, unless we were closely related genetically. It's similar to how organ transplants have to be carefully chosen and treated to reduce the risk of being rejected — so how do these deep-sea anglerfish avoid rejecting their partner in the same way?

The anglerfish species with males that attach permanently to females lack several genes that enable their adaptive immune system to recognise cells that are not their own. This means that their adaptive immune systems don't attack each other when they pair up — but it also implies that they may be less able to fight off infections than other fish. It's possible, however, that the innate immune system of those anglerfish species may compensate by producing better general defenses to fight off infection. Further research into how those anglerfish manage without a normal adaptive immune system might even reveal new ways to treat infections in humans.

Text from Deep Sea: 10 Things You Should Know. Reprinted by permission of Orion Publishing.

If you are itching to know more about what lies deep beneath the waves, you can read an interview with Jon Copley here, in which he told Live Science about new discoveries and the biggest myths about the deep sea. 

Humans have a knack for sharing intimate moments in unlikely places, as membership of the mile-high club demonstrates. So there is a significant chance that the launch of the space tourism sector may be swiftly followed by the first sex in space.

But having researched the issue, my colleagues and I believe that space tourism companies haven't adequately prepared for the consequences of people joining what we could call the "Kármán line club" (referencing the 100 kilometers-high boundary between Earth and the rest of the cosmos).

Talk of space tourism has always been in terms of the distant future. But sub-orbital space tourism — short flights with only a few minutes of spaceflight and weightlessness – already exists. Tickets range from freebies, to costing millions of dollars.

Much longer flights are just around the corner. Companies such as Elon Musk's SpaceX have well-established track records of developing spacecraft faster than the public sector. SpaceX's larger and more capable Starship spacecraft will likely operate routinely in the next few years.

When, not if

Flights have been reserved and passenger lists assembled for private flights that will loop around the Moon. Spacecraft such as Starship will have the capacity for tens of passengers, in a large cabin environment, possibly with private cabins.

Considering that space travel is no longer reserved for professional astronauts, the various motivations of space tourists and upcoming spacecraft developments, we concluded that in-space sex will probably happen within the next ten years.

The real concern is not the sexual interactions themselves, but rather if they lead to human conception in space. Early orbital space tourism flights are expected to last for days to weeks, so only the early stages of human reproduction could happen in space.

Passengers will not be allowed to board if they are already known to be pregnant, although the space tourism industry does not appear to have considered concealed or unknown pregnancies. Sometimes women don't realize they are pregnant until they go into labor.

From decades of human spaceflight, we already know weightlessness and increased levels of ionizing radiation has a profound effect on our bodies. We don’t know how this will affect the physiological processes of reproduction.

Astronauts routinely suffer muscle and bone wastage as their bodies no longer have to resist the forces of gravity. On Earth, gravity influences the distribution of body fluids, such as blood. A lack of gravity can result in increased pressure inside the skull which can make people's vision blurry and even change the brain's structure.

Limited experiments on mouse embryos, which include one that used a mini incubator on a satellite, have shown changes in embryo viability after they were exposed to space. Knowledge of the impact on human reproduction is effectively zero, but we can assume that there will be effects.

Therefore, there is an unknown potential for developmental abnormalities in human embryos conceived in space. Additionally, there could be an increased risk of ectopic pregnancy in weightlessness conditions (when the embryo attaches outside the uterus, for example in the fallopian tubes).

Even if space tourists use contraception, we can’t be sure it will be as effective outside planet Earth. There have been no studies on how contraceptives will be affected by space environments.

Taking responsibility

For the space tourism industry, there are commercial risks of litigation, reputational damage and financial loss if people conceive during spaceflight — as well as ethical and reproductive rights issues. Our research found little evidence that the sector is taking steps to mitigate these risks. There is little anecdotal evidence from behind the scenes.

There is also a darker side to consider — the risk of sexual assault in space. Imagine trying to evade the advances of a fellow passenger or staff member during spaceflight. You would be completely trapped.

The space tourism industry and other relevant parties should urgently come together to discuss these issues and formulate a strategy to protect all those involved. A simple solution could be a combination of pre-spaceflight counselling with all space tourists about the risks of human conception in space. Legal waivers absolving the space tourism operators of liability if human conception was still to occur could also be considered.

Space tourism is already happening and it seems likely that sexual interactions between some participants will occur very soon. The question is whether the sector will be prepared for the possible consequences.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Scientists have finally filled the gaps in the sequence of the human Y chromosome, 20 years after the first draft of its DNA code was published. 

The work reveals a complete catalog of the genes in the male sex chromosome — the last human chromosome to be fully sequenced — and how they're organized. 

The findings, published Wednesday (Aug. 23) in the journal Nature, may make it easier to detect gene variants on the Y chromosome and link them to specific traits, as well as to develop personalized therapies for sex-specific genetic diseases. A companion paper, also published in Nature, used new software built on this advance to sequence the Y chromosomes of more than 40 additional men worldwide. The findings show that the Y chromosome evolved to have extensive variation in its size and structure among individuals.  

"This is an amazing piece of work and a landmark study in genetics," Dianne Newbury, a professor of medical genetics and genomics at Oxford Brookes University who was not involved in the research, told Live Science in an email. "The sequencing of this unit [the Y chromosome] provides a complete picture of the whole human genome and will kickstart many studies into the sex chromosome that had not previously been possible." 

Related: 'Disappearing' Y chromosome in aging men may worsen bladder cancer, mouse study shows

The Y chromosome is one of two chromosomes, along with the X chromosome, that determine biological sex in humans. Females typically have two X chromosomes, whereas males usually have one of each type. 

The Y chromosome is important for the development of male sex characteristics; it carries the sex-determining region Y (SRY) gene, which codes for a protein that promotes the development of the testicles and blocks the development of female organs such as the uterus and fallopian tubes. It was partially sequenced in 2003 as part of the Human Genome Project, which produced the first draft human genome. However, the Y chromosome has a complex structure containing many long, repetitive sequences of DNA, and until now, less than 50% of the sequence was known.

"When we're putting a genome back together, we can't sequence a whole chromosome in one piece; we can only sequence little bits and pieces, and then we have to stitch everything back together again," Adam Phillippy, lead author of one of the new studies and a senior investigator at the National Human Genome Research Institute, told Live Science. "And just like reconstructing a puzzle, it's the repetitive parts that look the same that are always the hardest, and you save those for the end."

Using a combination of cutting-edge DNA sequencing and machine-learning technologies, as well as insight acquired from sequencing the other 23 human chromosomes, Phillippy's team were able to completely assemble the Y chromosome. 

They discovered it contains 62,460,029 base pairs of DNA — the pairs of "letters" that make up the rungs of DNA's ladder. That's 30 million more than in the most current reference genome, GRCh38, which has been continually improved over the past 20 years. The team corrected errors in GRCh38 and uncovered the complete structures of several gene families, such as DAZ and RBMY, which are involved in sperm production. They also identified 41 new protein-coding genes and provided insight into noncoding regions known as satellite DNA — which don't carry blueprints for proteins — as well as sequences that have often been mistaken for bacterial DNA in the past.

Using lessons they learned in building this new genome, called T2T-Y, the team then developed software called Verkko that can be used to efficiently assemble the full sequences of additional chromosomes. This is important because T2T-Y was constructed from only one donor's Y chromosome, so it doesn't give a complete picture of how it may vary among people. 

In a separate study, scientists used Verkko to fully assemble the Y chromosomes of 43 men from populations across Africa, the Americas, Asia and Europe who donated DNA samples to the 1000 Genomes Project — an international project to create a catalog of human genetic variation. These sequences reflected the extent of modern diversity in the Y chromosome, with the 43 chromosomes and T2T-Y sharing a common ancestor around 183,000 years ago. The sequences ranged from 45.2 million to 84.9 million base pairs in length, and taken together, they show that the Y chromosome has substantially diversified in its size and structure over this time.

In particular, the team found "surprising" variation in the repetitive regions of the chromosome, Charles Lee, senior author of the companion study and scientific director and professor at the Jackson Laboratory for Genomic Medicine, told Live Science in an email. He explained that the Y chromosome contains the largest block of condensed, noncoding DNA — known as heterochromatin — in the whole human genome. This region is made up mainly of two repetitive DNA sequences, which were organized differently among participants in the study but always appeared in the same 1-to-1 ratio, in terms of the number of times they were found in the chromosome.

"To me, this strongly suggests some as-of-yet unknown functional significance for this chromosome region," Lee said. Indeed, scientists no longer consider heterochromatin to be "junk" DNA, with recent research suggesting it can play important roles in cells, and may have even contributed to the evolution of the human brain.

In their paper, Phillippy and his team wrote that existing reference genomes will need to be updated and that appropriate labels will have to be given to any newly identified regions of the Y chromosome. Lee's team hopes to learn more about the functional impact that structural variations in the Y chromosome may have on health and disease, and their possible link to human evolution, he said. 

Kerstin Howe, head of production genomics at the Wellcome Sanger Institute who was not involved in the research, reiterated the importance of this advancement. "Assembling complete sequences of 43 Y chromosomes across space and time not only helps us to investigate sex chromosome evolution but also human evolution more generally," she told Live Science in an email. 

As for developing new therapies to treat diseases that may be linked to the Y chromosome, Phillippy said: "It's a long road from having the genome to developing, approving and rolling out some kind of therapy." Nevertheless, he is optimistic. 

"This is like the blueprint that we're looking at, and if there's complete holes in it, you might not even know where to begin," he said. "But by having them filled in, we have the complete picture."

Scientists may have uncovered the brain's "on switch" for male libido, a new study in mice suggests. 

The switch is actually more of a circuit, consisting of a group of neurons that connects multiple regions of the brain. The newly identified circuit not only helps male mice recognize females but also controls their desire to have sex with them, complete the act itself and experience pleasure as a result, the study suggests. 

The authors say the findings, published Friday (Aug. 11) in the journal Cell, may improve our understanding of how sexual behaviors are regulated differently in male and female mammals. Someday, the work could lead to the development of new drugs to treat low libido, which is estimated to affect 1 in 5 men at some point in their lives. Although maintaining a high sex drive is not essential for one's health, the sudden, unexpected loss of libido can be distressing to both individuals and their sexual partners.  

"If these centers exist in humans — and now we know where to look — it should be possible to design small molecules that can be used to regulate these circuits," lead study author Nirao Shah, a professor of psychiatry and neurobiology at Stanford University, said in a statement.   

Related: Why do guys get sleepy after sex?

In the new study, researchers looked at the brains of adult virgin male mice who hadn't seen a female mouse since they were weaned. Previous research by the group had pinpointed a group of neurons that regulate whether male mice can identify the sex of female mice on sight. These neurons connect two regions of the brain: the bed nucleus of the stria terminalis (BNST) in the amygdala, a key brain region for emotional processing, and the preoptic area of the hypothalamus (POA). Having revealed the importance of these cells, the team decided to explore exactly how they communicate.  

The team found that certain neurons in the BNST secrete a chemical known as substance P. The small molecule binds to receptors on specific POA neurons, which subsequently activate and shoot messages to regions of the brain that respectively regulate movement and the experience and anticipation of feelings of pleasure associated with sex. 

When the researchers stimulated these BNST neurons in male lab mice, the rodents' POA neurons became increasingly more active and they began having sex with the female mice after a 10-to-15-minute delay. Directly activating the mice's substance P-sensitive POA neurons boosted the rodents' sex drives so much that they attempted to mate with inanimate objects.

What's more, by stimulating the rodents' POA neurons, the researchers dramatically cut the break time male mice normally take between rounds of sex. Instead of taking about five days to recover their sex drive after ejaculation, the mice needed only a second or less to be ready to have sex again. Conversely, when the team stopped these POA neurons from working, the mice lost all interest in mating. 

Shah's team is still working to identify a similar circuit in female mice, but in the meantime, they're confident that a human equivalent of the male mouse libido circuit will be identified. "It's very likely there are similar sets of neurons in the human hypothalamus that regulate sexual reward, behavior and gratification," he said in the statement.

Shah added that any future drugs that target this circuit would work very differently to the likes of Viagra, which treats erectile dysfunction by increasing blood flow to the penis. Instead, the drugs would directly amplify or reduce the activity of a specific area in the brain that controls male sexual desire, he said. This could help treat either low or high sex drives by ramping up or turning down the circuit's activity. 

However, such drugs are a long way from reaching the market. 

"Regulating libido is enormously complex in humans with lots of social, political, ethical and other considerations that need to be addressed prior to thinking about any such approach," Shah told HealthDay.

Editor's note: This story was updated on Aug. 17, 2023 to correct the description of one of the mouse experiments.

Masturbation in primates is deeply rooted in evolution, with the behavior likely going back at least 40 million years, to the ancestor of all monkeys and apes, new research suggests. 

The study researchers found that masturbation is an ancient trait in primates and speculated that it may drive reproductive success.  

Masturbation occurs in a wide variety of animals, from rodents to reptiles, but is especially common among primates, and even more so in primates in captivity. On the surface, masturbation would seem to be at odds with passing on genes. After all, it takes time, attention and energy to masturbate that could be used for other activities that directly increase the odds of reproducing, such as actually mating or finding food. 

As a result, scientists in the past have assumed masturbation in primates was an aberrant activity produced by the stress of captivity, or simply a byproduct of a high libido, the researchers wrote in the study. However, these theories can't explain why wild primates masturbate, or why primates masturbate when willing partners are around, the study authors wrote.

To understand why primates engage in such "autosexuality," Matilda Brindle, an anthropologist at University College London, and colleagues created a huge dataset of primate masturbation using hundreds of publications, 150 questionnaires and observations from zoo keepers and primatologists. 

It represented about 38% of primate species and 79% of genera. Their findings, published June 7 in the journal Proceedings of the Royal Society B, showed that within captive primates, 74% of females and 87% masturbated. In wild populations, 35% of females and 73% of males exhibited this behavior. 

The team then used a computer model to estimate how long the behavior had been a part of the primate repertoire. They looked at masturbation habits in living species, their mating systems and the prevalence of sexually transmitted infections. They then combined that data with the evolutionary relationships between extinct and living species to create reconstructions of masturbation among ancestral species.

Related: Monkeys in Indonesia use rocks as 'sex toys'

Their findings showed primates have likely been masturbating for tens of millions of years. "We know that amongst the primates, the ancestor of all monkeys and apes after the split from tarsiers probably masturbated, meaning it was present around 40 million years ago," Brindle told Live Science in an email. The model also suggested female masturbation existed prior to the split from tarsiers — a small species of primate that has barely changed in the last 45 million years.   

But the study wasn't able to address the evolutionary reason for masturbation. One theory posits that masturbation after sex is evolutionarily advantageous because it clears pathogens from the genital tract. And in fact, masturbation was more common in species where the mating system involves males and females having multiple partners — it could help males clear out low quality sperm before mating, so the high quality sperm is passed to the female quickly, before a competitor takes his place. Masturabtion was also more common in species with a high load of sexually transmitted infections (STI), lending some weight to the pathogen hypothesis.

But without more data, the new study can't say whether high libido or an evolutionary benefit is the driving force behind the behavior.

Pathogen presence did not appear to influence female masturbation. Past data suggests that in humans, female masturbation creates a more hospitable environment for sperm, which would boost the odds of conception if they had sex around the same time. But this also has a downside: "Because sexual arousal and masturbation make the vagina less acidic (to be more hospitable to sperm), but this also makes it more vulnerable to pathogens," Brindle said. 

The team now hopes to gather more data on masturbation in primates — particularly females — to better understand the evolutionary purpose of the behavior "We aimed to collect data on masturbation frequency too, but didn’t have enough to conduct meaningful analyses," Brindle said. 

She added that frequency of masturbation will be an important thing to measure as it will shed more light on the driving factors. "I’d love to explore these hypotheses further, with the benefit of more data, and begin to tease apart which hypotheses might explain masturbation in different species and under which ecological and social circumstances."

Finding love can be difficult. But when you're bemoaning your own love difficulties, spare a thought for the giraffe. Giraffes don't go into heat like cats or dogs, don't have a breeding season, don't make mating calls and don't give visual clues that they're ready to mate. So how do giraffes find partners?

It's kind of gross, but this is how baby giraffes are made: a male giraffe — called a bull — nudges a nearby female giraffe — a cow — and sniffs her genitalia. Sometimes he has to nudge her a few times, but eventually the female giraffe widens her stance and urinates for about five seconds into the male giraffe's mouth.

The male giraffe then performs what's known as a "flehmen response" by curling back its upper lip, baring its teeth, and breathing in with its nostrils closed for several seconds. (The name comes from a German word for baring the teeth.) The flehmen response is also used by animals like horses and goats to transfer scents to the vomeronasal organ above the roof of their mouth, a very sensitive component of their sense of smell.

Animal attraction

Humans don't do flehmens; but animals do it when they want to get a good smell of something that interests them. In the case of an aroused male giraffe, he's hoping that chemicals he can detect in the female's urine — called pheromones, which can trigger a social response in some animals — will signal that she's in estrus, or fertile and ready to mate. 

When a male giraffe doesn't smell the right chemical signal in her urine, he leaves that female alone and moves on to another. But while most animals will wait until urine is on the ground before they smell it, the giraffe is too tall to do that, Lynnette Hart, a professor at the University of California Davis School of Veterinary Medicine, told Live Science. 

Hart is the lead author of a new study on the behavior of giraffes published in the journal Animals. The co-author is her husband Benjamin Hart, an emeritus professor at the same veterinary school, and they've observed these and other behaviors among giraffes during field trips to the Etosha National Park in Namibia in southwest Africa. 

The Harts were able to get especially close to dozens of giraffes gathered near water holes in the park; usually giraffes can be seen only at a distance and run away when they are approached, Lynette Hart said. "This was really unique, to be right up close."

Giraffe sex

Mate-finding can be a lot of work for male giraffes. The Harts observed bull giraffes approaching cow giraffes roughly 150 times, but they only saw one time when the approach led to consummation. Once all these steps occur, giraffes mate in the manner of most mammals by the bull mounting the cow. Sex is conducted at a precarious height above the ground and lasts only a few seconds. 

If she gets pregnant, the female giraffe will gestate for a whopping 400 days before she gives birth standing up, so that her baby giraffe is well developed enough to stand and walk when it's born.

Sexsomnia is a sleep disorder in which a person unknowingly enacts sexual behaviors whilst they're asleep. There is no cure for the condition, but medications and lifestyle changes have been shown to manage the disorder effectively.

It's thought that 7.1% of the global adult population will experience sexsomnia at some point in their lives, according to a 2018 review in the journal Cureus. It's also thought to be more common in men than women, according to a 2007 paper in the journal Social Psychiatry and Psychiatric Epidemiology, although it is unclear why.

Sexsomnia is sometimes called sleep sex, but it can involve all kinds of sexual behaviors including masturbation, moaning and orgasm. It can be distressing and embarrassing for both the person experiencing it and anyone sharing a room with them. Live Science spoke with experts to find out what is understood and what is still unknown about the disorder.

What is sexsomnia?

Sexsomnia is classed as a parasomnia, a group of disorders that includes sleep walking, sleep talking and night terrors. Parasomnias are grouped by sleep stage. Some occur during rapid-eye-movement (REM) sleep — "light" sleep in which our brain is active and dreaming — and some occur during deep, non-REM sleep.

Sexsomnia happens when the brain is suddenly, partially-aroused from deep, non-REM sleep, according to a 2010 report in the journal Clinical Neurology and Neurosurgery. The person, still technically asleep, begins to enact certain sexual behaviors. Nearly all episodes are followed by amnesia, said Dr. Rexford Muza, a sleep physician at Guy's and St Thomas' Hospital in the U.K.

During an episode, a person might fondle themselves or a bed partner, or they might make sexual noises or movements. Sexsomnia can involve sexual acts that the person has never done before or would never do when awake, Muza told Live Science.

A 2016 review led by Muza in the journal Current Opinion in Pulmonary Medicine found that women are more likely to masturbate during an episode, whilst men most frequently report initiating sexual intercourse with the person in the bed beside them. Sometimes the bed partner will be awake and consenting — though the person with sexsomnia is still asleep and won't remember in the morning. However, there have been cases in which neither party has been awake, which can have serious legal consequences.

What causes sexsomnia?

Not much is understood about the mechanisms behind sexsomnia and other non-REM parasomnias.

Sleep studies can monitor the nightly brain activity of people who experience parasomnias using an electroencephalogram (EEG). According to a 2016 study in the journal Sleep, in cases of sleepwalking and night terrors, which are also disorders of partial awakening like sexsomnia, EEG showed activity in the motor cortex and the cingulate cortex, the former being responsible for bodily movement and the latter for decision making. The rest of the brain still appeared soundly asleep.

But why would a partially awake brain enact sexual behaviors? Dr. Carlos Schenck, a professor of psychiatry at the University of Minnesota Medical School, said disorders of partial awakening tend to involve one of our basic instincts.

"Eating, sex, walking, fear — all these basic instincts and primitive behaviors can be inappropriately released during sleep," he told Live Science. Why these particular behaviors occur in some individuals and not others isn't fully understood. 

What triggers sexsomnia?

From cases of sexsomnia filmed in laboratory studies by Muza and his team, most are triggered by a bed partner moving. This stirring likely causes a partial arousal in the brain, he said, but anything that could disturb sleep — snoring, an external noise or sudden temperature change — could be a trigger.

People with obstructive sleep apnea — in which the throat muscles intermittently relax and block the airway during sleep — can get sexsomnia as a symptom, according to the Mayo Clinic, as trouble breathing causes them to make gasping or choking noises in the night, partially waking up the brain.

Stress can also be a trigger. In one 2019 study, published in the journal Frontiers in Neurology, 80% of participants with sleep-related motor behaviors found stressful situations could lead to more episodes, while a 2023 study in the journal Military Medicine, which examined four individuals, suggested the stress of being on military duty may increase service members' risk for all non-REM parasomnias.

A high libido, however, is not a trigger for sexsomnia, Schenck said. "And there's no correlation between unsatisfied sexual drive and sexsomnia," he said. 

How is sexsomnia diagnosed?

Many people with sexsomnia are only aware of the problem because someone sleeping near them has noticed it. In someone who sleeps alone, sexsomnia can go undiagnosed for quite some time. In these people, the realization can come from an injury caused by vigorous masturbation which they can't explain in the morning, Schenck said.

For sexsomnia to be diagnosed, a physician must first rule out other conditions that can present in similar ways, including epilepsy and obstructive sleep apnea, Muza told Live Science. Next, he would look at the patient's medical history, as people with sexsomnia often have a history of other parasomnias, such as sleep walking or night terrors.

Finally, a physician would run a sleep study — called a polysomnography, which records brain waves, blood oxygen levels, heart rate, breathing, and eye and leg movements — to monitor a person's nighttime activity. Though a video-polysomnography can support a diagnosis if a sexsomnia episode occurs on the night in the lab, it can't be relied upon, said Muza. 

"The problem with a sleep study is you might come in and sleep beautifully and there are no events on that night," he said. "That doesn't exclude [a diagnosis] at all if the sleep history is convincing."

How is sexsomnia treated?

There is no cure for sexsomnia, but there are ways for people to manage the condition.

"One way we treat sexsomnia is by consolidating sleep and suppressing the arousal [waking-up] response," Muza said. "So, if we can make [a patient] sleep deeper, it's less likely that they'll have a partial arousal [leading to sexsomnia]."

Prescription medications, including benzodiazepines, a class of sedative drugs that include clonazepam, can help to induce sleep, though they can also make it difficult for a person to wake up from deep sleep, Muza said, which can lead to a partial arousal. Melatonin, a natural sleep hormone, can also act as a hypnotic but without the usual grogginess or the abnormal behaviors, Muza said. However, it's important to speak to a doctor before taking any medication for a sleep disorder as they can have side effects. 

Changes to nighttime routines can also reduce arousals. Sleeping in a bigger bed or leaving more room between yourself and a partner can make it less likely that physical touch will spur on a sexsomnia episode, Muza said. "And I always tell my patients to avoid sleeping next to minors or strangers, because you've got the propensity to react that way."

Avoiding alcohol and caffeine within a few hours of going to sleep can also help, Schenck said, as alcohol and drug use has been documented as a precipitating factor for sexsomnia. He has also found that hypnosis can be effective as a treatment, and it has been found to lengthen the amount of time spent in deep, non-REM sleep, according to a 2014 study in the journal Sleep. However, more robust, large scale research is needed before this can be suggested as a viable treatment. 

Although there is no cure for sexsomnia, there is hope for patients, Muza said, because parasomnias tend to go away with time or patients develop intelligent strategies for dealing with them. Sexsomnia, he said, will not always be so distressing.

This article is for informational purposes only, and is not meant to offer medical advice.

Scientists have developed the world's first "vagina-on-a-chip," a small device that contains live human cells and replicates the cellular environment found inside the vaginal canal. 

By adding bacteria to the device, aptly named the Vagina Chip, researchers can study how different microbes affect the health of the vagina, the team reported in a new study, published Nov. 26 in the journal Microbiome. They can also test how different drugs and probiotics change the composition of the vaginal microbiome, the community of microorganisms living within the canal. 

"The vaginal microbiome plays an important role in regulating vaginal health and disease, and has a major impact on prenatal health," first author Gautam Mahajan, a former researcher at Harvard University's Wyss Institute for Biologically Inspired Engineering, said in a statement. "Our human Vagina Chip offers an attractive solution to study host-microbiome interactions and accelerate the development of potential probiotic treatments," which work by introducing beneficial bacteria into the vagina, said Mahajan, who now works at the organ-on-a-chip company Emulate, Inc. in Boston.

The Vagina Chip device measures only 1 inch (2.54 centimeters) long and contains donated cells from two women; the cells were collected from the lining of the vagina and from the connective tissue that runs beneath the lining, according to The New York Times. These two types of cells sit on either side of a permeable membrane, which replicates the 3D structure of the vaginal wall. 

Related: Vaginal bacteria can vary widely in healthy women 

The scientists allowed these cells to multiply for five days in the device before adding the sex hormone estradiol, a form of estrogen. The hormone changed which genes were switched "on" in the tissues and triggered the production of mucus, as would occur in vivo.

With their device complete, the team ran several tests with bacteria commonly found in the vagina, namely, several strains of Lactobacillus bacteria; studies suggest that these microbes make up more than 70% of a healthy vagina's microbiome, according to the Wyss Institute statement. 

The researchers found that the Lactobacillus bacteria successfully produced lactic acid in the Vagina Chip, thus driving down the pH of the tissue inside. A healthy vagina typically has a pH of 4.5 or less, meaning it's acidic, and this acidity helps prevent the growth of harmful bacteria that could cause disease. In addition, adding the Lactobacillus bacteria reduced the number of inflammatory molecules circulating in the tissues.

Having run experiments with "good" bacteria, the team did the same with "bad" bacteria, meaning bugs associated with bacterial vaginosis (BV), a common vaginal infection caused by the overgrowth of specific bacteria. These bugs include Gardnerella vaginalis, Prevotella bivia, and Atopobium vaginae, and when introduced into the Vagina Chip, all three bacteria caused the pH of the device to rise along with the number of inflammatory molecules and damaged vaginal cells.

BV increases the risk of sexually transmitted diseases, such as chlamydia and gonorrhea, both of which can negatively affect future fertility by triggering a harmful inflammation, according to the Centers for Disease Control and Prevention (CDC). If developed during pregnancy, BV also raises the risk that a baby will be born prematurely or at a low birth weight. The new vagina-on-a-chip could help scientists develop new and better treatments for BV, sexual health physician Achyuta Nori of St. George's, University of London, who was not involved with the study, told Scientific American. 

"This is an opportunity to bring women's health into the modern times, using modern technology," Nori said. The chip offers an advantage over testing therapies in lab animals like mice, whose vaginal microbiomes greatly differ from humans'; in addition, it's historically been difficult for drug developers to recruit human patients for BV treatment trials, partly due to patients' concerns over safety, experts told the Times. 

That said, organ-on-a-chip technology does have its limitations, so it cannot perfectly capture all the factors that influence the human vaginal microbiome. But the Vagina Chip is a good first step towards further research.

Sure, birds can fly, but do they have sex? Can they do it in the air? And where do they keep their reproductive organs?

To understand how birds have sex, it is necessary to move beyond preconceived ideas of how other animals have sex — particularly mammals.

"Birds and mammals differ quite a bit in their reproductive anatomy," Kevin J. McGowan, a senior course developer at Bird Academy, Cornell Lab of Ornithology in Ithaca, New York, told Live Science in an email. 

"Both have testes and ovaries, but how the eggs and sperm move around is different," he said. "Birds' reproductive tracts, their urinary tracts, and guts all empty into a single chamber that opens to the outside, the cloaca. Female birds have no uterus or vagina, the oviduct empties into the cloaca."

Related: Strange love: 13 animals with truly weird courtship rituals

Additional resources

Watch fascinating videos of bird mating dances from the Audubon society. Learn more about courtship rituals from the Royal Society for the Protection of Birds. See images of the most unique courtship dances from National Geographic. 

The nerve that enables the human clitoris to detect pleasurable touch contains thousands more nerve fibers than once estimated — about 10,000, rather than 8,000. Medical researchers discovered this by doing something that had never been done before: They actually counted the fibers.

Previously, it was widely accepted that the clitoris contained about 8,000 nerve fibers, but the origins of this number are fuzzy, lead study author Dr. Blair Peters, an assistant professor of surgery in the Oregon Health and Science University (OHSU) School of Medicine, told Live Science.

"The 8,000 number, it wasn't even an actual scientific paper," he said. The number comes from a line in a book called "The Clitoris" (Warren H. Green, Inc., 1976) by physician Dr. Thomas P. Lowry and his then-wife Thea Snyder Lowry, in which the authors briefly mentioned a study of cow clitorises and extended its findings to people. 

"It wasn't based on human data," Dr. Rachel Rubin, an assistant clinical professor of urology at Georgetown University and a urologist and sexual medicine specialist at a private practice in the D.C. area, told Live Science. The cow-derived stat has been cited many times without being fact-checked — until now.

Related: How many organs are in the human body? 

In their research, Peters and their colleagues examined the two dorsal nerves of the clitoris, which are dense bundles of nerve fibers that relay sensory signals from the clitoris to the brain. These nerves run down either side of the clitoral shaft and relay information about touch, pressure and pain, while other nerves handle functions like muscle tone and blood flow.

The sampled dorsal nerves contained between 4,926 and 5,543 nerve fibers each, or an average of 5,140 fibers. With two dorsal nerves per clitoris, that translates to about 10,280 nerve fibers that enable sensation in the pleasure-producing organ. These findings, which have not yet been peer-reviewed, were presented Oct. 27 at a joint scientific meeting of the Sexual Medicine Society of North America and the International Society for Sexual Medicine. 

What's remarkable, Peters said, is that these 10,000 fibers all plug into the clitoral glans, the visible portion of the clitoris located where the labia minora (inner lips) of the vulva meet. In comparison, the median nerve, which runs through the wrist and supplies sensation to most of the hand, contains 18,000 nerve fibers. When you compare the surface area of the clitoral glans to that of the hand, "10,000 compared to that 18,000 becomes shockingly high," he said.

Peters pursued this research, in part, to inform his work as a plastic and reconstructive surgeon specializing in gender-affirming surgeries, including gender-affirming phalloplasty, or the surgical construction of a penis from other tissues in the body. 

To create a penis capable of erogenous sensation, surgeons take tissue from an area of the body with an ample supply of nerves, typically the forearm or thigh, according to the OHSU Transgender Health Program. Once the phallus is made, those nerves are then hooked up to nerves in the pelvis, and ideally, the nerves grow together and begin relaying sensory signals to the brain.    

"I wanted to take a closer look, basically, at the nerves we connect when we make a penis," Peters told Live Science. 

Broadly, research into the basic anatomy of the vulva, which includes the clitoris, could also aid in the diagnosis and treatment of nerve injury and help surgeons navigate procedures near the genitals without causing inadvertent damage.

The new research was made possible by seven transmasculine patients who underwent phalloplasties and volunteered to donate samples of their clitoral tissue. These donated tissues were then preserved, stained blue and magnified 1,000 times under a microscope so that an image-analysis software could count individual nerve fibers.

All the patients underwent testosterone therapy prior to phalloplasty. There is some evidence that testosterone can boost nerve regeneration in the context of an injury, but in normal, healthy nerves, the hormone shouldn't change the number of fibers present, Peters said. "However, this study did not have controls without exposure to testosterone," they said, so it would be worth repeating the study with tissue samples from cisgender women who'd never had testosterone. Such samples would likely come from cadavers, rather than people undergoing surgery.

The new research highlights how little is known about the anatomy and function of the clitoris, Rubin said. This reflects historical biases in medical research that have left modern doctors with huge knowledge gaps. Rubin discussed these historic inequities in a New York Times investigation published in mid-October.     

"It is likely that no doctor has ever examined your clitoris or asked about orgasm in a medical setting," she said. "And that's not because it's not worthwhile."

Editor's note: This story was updated on Nov. 9 to acknowledge a New York Times investigation on the same subject that heavily featured Rubin. The story was originally published on Nov. 1.

It’s a question that’s been keeping (some of us) up at night: why do guys get sleepy after sex? Many men barely leave time for the bed sheets to settle before they make a beeline for the land of nod, but is there any scientific reason why one gender might be more predisposed to a post-sex snooze?

Well, there is, as it happens. Science journalist and author Melinda Wenner Moyer took a look at the possible factors that could play a part in this phenomenon.

For many women, the correlation between sex and snoring is one of those annoying facts of life: no matter when passionate encounters occur, men always seem to fall asleep immediately afterwards. Dave Zinczenko, the author of "Men, Love and Sex: The Complete User Guide For Women," explained the phenomenon to Huffington Post writer Arianna Huffington this way: "Men go to sleep because women don’t turn into a pizza."

I doubt I am ever going to become a pizza, and I’ll never have the foresight to order one beforehand. So in lieu of a cure, a better explanation will have to do. Although women sometimes feel sleepy after sex, the phenomenon does seem more pronounced in men. What is it, then, that spirals them into the land of nod?

First, the obvious reasons for sex’s somnolent sway: the act frequently takes place at night, in a bed, and is, after all, physically exhausting (often more so for the man than the woman, although this certainly varies). So when sex is over, it’s natural for a guy to feel sleepy.

Secondly, research using positron emission tomography (PET) scans has shown that in order for a person to reach orgasm, a primary requirement is to let go of "all fear and anxiety." Doing so also tends to be relaxing and might explain the tendency to snooze.

Then there is the biochemistry of the orgasm itself. Research shows that during ejaculation, men release a cocktail of brain chemicals, including norepinephrine, serotonin, oxytocin, vasopressin, nitric oxide (NO), and the hormone prolactin. The release of prolactin is linked to the feeling of sexual satisfaction, and it also mediates the "recovery time" that men are well aware of—the time a guy must wait before "giving it another go." Studies have also shown that men deficient in prolactin have faster recovery times.

Prolactin levels are naturally higher during sleep, and animals injected with the chemical become tired immediately. This suggests a strong link between prolactin and sleep, so it’s likely that the hormone’s release during orgasm causes men to feel sleepy.

(Side note: prolactin also explains why men are sleepier after intercourse than after masturbation. For unknown reasons, intercourse orgasms release four times more prolactin than masturbatory orgasms, according to a recent study.)

Oxytocin and vasopressin, two other chemicals released during orgasm, are also associated with sleep. Their release frequently accompanies that of melatonin, the primary hormone that regulates our body clocks. Oxytocin is also thought to reduce stress levels, which again could lead to relaxation and sleepiness.

What about the evolutionary reasons for post-sex sleepiness? This is trickier to explain. Evolutionarily speaking, a man’s primary goal is to produce as many offspring as possible, and sleeping doesn’t exactly help in his quest. But perhaps since he cannot immediately run off with another woman anyway — damn that recovery time! — re-energizing himself via sleep may be the best use of his time.

And although there is conflicting information as to whether women feel sleepy after sex, a woman often falls asleep with the man anyway (or uses it for some key cuddling time), which is good news for him: it means she is not off finding another mate. When the man wakes up and she’s still there, he just might be ready to go again.

It’s also possible that sleepiness is just a "side effect" associated with a more evolutionarily important reason for the release of oxytocin and vasopressin. In addition to being associated with sleep, both chemicals are also intimately involved in what is called "pair bonding," the social attachment human mates commonly share. The release of these brain chemicals during orgasm heightens feelings of bonding and trust between sexual partners, which may partially explain the link between sex and emotional attachment. This bond is favorable should the couple have a baby, as cooperative child rearing maximizes the young one’s chances for survival.

The bottom line is this: There are many potential biochemical and evolutionary reasons for post-sex sleepiness, some direct and some indirect — but no one has yet pinpointed the exact causes. One thing, however, is certain: we females better get used to it, because it doesn’t look likely to change anytime soon.

I will leave frustrated American women with one final thought: if you are upset at the ubiquity of the post-sex snoring phenomenon, remember that things could be a lot worse. A recent survey of 10,000 English men revealed that 48 percent actually fall asleep during sex.

Talk about coitus interruptus!

Originally published on Live Science on Feb. 1, 2013 and updated on Aug. 24, 2022. 

This answer was provided by Scienceline, a project of New York University's Science, Health and Environmental Reporting Program.

Four companies selling honey-based products containing unlisted active drug ingredients have received warning letters from the U.S. Food and Drug Administration (FDA). The companies promoted these products as sexual enhancers, and laboratory testing revealed that they contained active ingredients found in drugs for erectile dysfunction — namely, tadalafil (Cialis) and sildenafil (Viagra), the FDA announced Tuesday (July 12).  

Tadalafil and sildenafil may be used only under the supervision of a licensed health care professional, in part because they interact with common prescription medications known as nitrates. Nitrates are commonly taken by people with diabetes, high blood pressure, high cholesterol and heart disease and work by causing blood vessels to dilate, thus increasing the flow of blood and oxygen to the heart, according to the medical database StatPearls.

Tadalafil and sildenafil also cause blood vessels to dilate, so if someone takes these drugs at the same time as nitrates, their blood pressure may plummet to dangerously low levels. Severely low blood pressure can be life-threatening because vital organs may not receive an adequate supply of blood and oxygen, and this deficit can lead to shock, irreversible organ damage and death, according to the Mayo Clinic.

"Tainted honey-based products like these are dangerous because consumers are likely unaware of the risks associated with the hidden prescription drug ingredients in these products and how they may interact with other drugs and supplements they may take," Judy McMeekin, the FDA's associate commissioner for regulatory affairs, said in the agency's statement. 

Related: Men who took erectile-dysfunction drug had blue-tinted vision for days 

"Products marketed with unidentified ingredients may be dangerous and, in some cases, deadly to consumers," she said. "We encourage consumers to remain vigilant when shopping online or in stores to avoid purchasing products that put their health at risk, and instead seek effective, FDA-approved treatments."

The four companies that received warning letters include Thirstyrun LLC (also known as U.S. Royal Honey LLC), MKS Enterprise LLC, Shopaax.com, and 1am USA Incorporated dba Pleasure Products USA. The FDA said these companies violated federal law by allegedly selling active drug ingredients in products marketed as foods and by making unauthorized claims that their products treat disease or improve health.

Specifically, the products — including Etumax VIP Royal Honey for Him, Dose Vital Honey for Men, Secret Miracle Royal Honey for Her and X-Rated Honey for Men — were promoted as sexual enhancement products.

"The FDA has requested responses from the companies within 15 working days stating how they will address these issues or providing their reasoning and supporting information as to why they think the products are not in violation of the law," the FDA statement reads. "Failure to promptly address the violations may result in legal action, including product seizure and/or injunction."

The FDA encourages consumers to speak with a health care provider before trying any over-the-counter product marketed for sexual enhancement. Consumers can also refer to the FDA's health fraud products database to check if a product has been flagged as potentially hazardous. 

Consumers and health care providers can report adverse reactions related to specific products through the FDA's MedWatch program or the Safety Reporting Portal. 

Originally published on Live Science. 

For a type of orb-weaving spider, mating has a spectacular finale: The male catapults off a female's body at a speed too fast for a human to see with the naked eye. 

These amorous acrobatics aren't meant to impress the spiders' partners; rather, a male springs into action to escape the female's hungry mandibles, as sexual encounters for these arachnids would otherwise end with the male being eaten.

They leap for their lives by using a mechanism that has never been seen before in spiders, involving a joint in their front legs that enables them to launch their bodies dozens of centimeters in a split-second by storing kinetic energy and then suddenly releasing it, according to a new study.

Prior to mating, male spiders would secure themselves to the female's web with a silk "safety line," so that after catapulting they could climb back up to mate again. Males sometimes mated with the same female up to five times; and with the risk of being cannibalized looming in every encounter, post-sex catapulting likely evolved as a means of survival, the researchers reported.

Philoponella prominens spiders are tiny — males' bodies measure about 0.1 inches (3 millimeters) long, while females are about twice that size — and they live in colonies that can contain more than 200 spiders in a vast network of webs. When the study authors observed a colony of the orb-weavers in Wuhan, China, in 2019, they noticed that mating always ended with the males catapulting off the females so quickly "that common cameras could not record the details," said Shichang Zhang, lead author of the study and an associate professor at Hubei University's School of Live Sciences in Wuhan. That prompted the researchers to take a closer look at what was going on while the spiders were mating, Zhang told Live Science in an email.

Related: Weird and wonderful: 9 bizarre spiders 

But taking a closer look turned out to be exceptionally challenging. Because the spiders were so small and mating was over so rapidly (lasting only about 30 seconds from copulation to catapult), the study authors struggled to focus their high-speed camera's macro lens in time to film the mating act and its aftermath. In many cases, the males finished and sprang away before the camera lens was focused and ready.

"That's the most difficult part in this research," Zhang said.

The scientists collected around 600 P. prominens spiders and conducted 155 successful mating trials. In spite of the photography challenges posed by the speedy spiders, the researchers captured images with a camera shooting 1,500 frames per second, and then used software to measure the energy and speed of the male spiders' catapulting escapes. After the first mating, 97% of the males catapulted — and all of them survived. Males that failed to catapult "were captured, killed and consumed by the females," the study authors reported. When the scientists prevented 30 males from springing away by disrupting the catapult mechanism in the spiders' legs, all of those males were also devoured.

But when the spiders were able to fling themselves away, the stored energy in their legs propelled their tiny bodies at remarkable speeds: up to 2.9 feet per second (88 centimeters per second). That's the equivalent of an adult human taking a flying leap and landing one second later at a distance of nearly 1,740 feet (530 meters), Zhang said. 

"We hypothesized that the mechanism of the catapulting is that the legs are folded against the female, and then when released the hydraulic pressure causes the legs to rapidly expand," the scientists reported.

Similar mechanisms using hydraulic energy storage and release for rapid limb movement are found in other animals, such as the mantis shrimp's knockout punch and the blink-and-you'll-miss-it speedy snap of a trap-jaw ant's mandibles. "But they use the fast actions to either capture prey or escape predators," Zhang said. By comparison, the wee orb-weavers are the only known animals to use the technique as protection against sexual cannibalism, catapulting to safety so that they may live to mate another day. 

This could be a way for the males to signal their fitness as mates, as males that are physically superior to their competitors can perform multiple catapults while mating, "thereby increasing their chance of paternity," the study authors concluded.

The findings were published April 25 in the journal Current Biology.

Originally published on Live Science.

Many myths and misinformation surround the vagina. For example, you might read on the web that the hymen — the membrane that partially covers the opening of the vagina— can  only break during first intercourse. While this is common, the hymen can tear in other circumstances such as during vigorous exercise, according to The Ohio State University. 

Also, evidence confirming the existence of female ejaculation was only released in recent years, according to the journal Clinical Anatomy. Controversy continued in the medical community regarding whether it existed, despite proof — some of it dating to nearly 2,000 years ago — suggesting that the phenomenon is real.

To separate fact from fiction, we've rounded seven facts worth knowing about a woman’s vagina.

1. The vagina is just one part of a women’s private parts

Sometimes, a woman's entire genital region is referred to as the vagina. But in fact, the outer portion of a woman’s privates is actually called the vulva. That includes the inner and outer labia, the clitoris, clitoral hood and the opening to the urethra and vagina, according to the National Vulvodynia Association (NVA).

The actual vagina is an internal structure, along with the other parts of the female reproductive system including the cervix, uterus, ovaries and Fallopian tubes (which are sometimes called the oviducts).

• Related: Probiotics for vaginal health

2. Kegel exercises strengthen pelvic floor muscles

Kegel exercises can help  women who have problems reaching an orgasm, according to The Journal of Sexual Medicine, but they also strengthen the pelvic floor muscles. This can help women who have trouble holding in their urine, stool or gas.

“You can do these exercises just about anywhere,” said Dr. Courtney Leigh Barnes, a gynecologist at the University of Missouri.

"To do a kegel exercise, act as though you are going to stop peeing and hold it for a couple of seconds, or you can insert your finger into the vagina and tighten your muscles", Barnes said. "These exercises can also be used to help with pelvic organ prolapse.”

Pelvic organ prolapse is a condition in which the organs in the pelvis—like the bladder—drops and pushes against the vagina, according to the journal The Lancet.

3. Probiotic supplements could help keep the balance

Probiotics, also known as the "friendly bacteria," have become a popular treatment against the "bad" bacteria that might cause harm to the body. Some evidence suggests they help digestion and combat diarrhea and gut inflammation.

A few studies have also hinted that using probiotics could help against vaginal infections, such as yeast infections, but according to Barnes, “there isn’t enough proof to make a specific recommendation.”

“More research needs to be done to say for sure when, how much, and what type of probiotics are helpful,” she said.

Barnes said she has had patients who suffered from chronic vaginal infections who have felt better after taking probiotics.

"If I have a patient with recurrent vaginal infections, I will recommend probiotics in the form of fermented goat’s milk," she said, but it's not right for everyone.

4. Discharges women should worry about

Although the vagina is considered a self-cleaning organ, according to the NHS website, and some discharge is normal, that doesn’t mean it’s not vulnerable.

“Any vaginal discharge that seems excessive, painful, irritating or foul in odor should be evaluated by a doctor,” Barnes said.

Some women try to diagnose their own vaginal infections at home, and use over-the-counter medications. "Unfortunately, research has shown that patients aren't very accurate when it comes to self-diagnosis," Barnes said. 

She recommends that women get evaluated if there is an abnormal discharge. Although it can sometimes be as simple as a yeast infection, "other times, the infection can be more complicated, or even a sexually transmitted infection," she said.

5. Sex can keep the vagina healthy

Through the various stages of women's lives — including childbirth and breastfeeding, as well as normal aging and menopause — the body undergoes hormonal changes that could lead to vaginal dryness, according to the American College of Obstetricians and Gynecologists.

"Estrogen helps keep the vagina healthy and lubricated," Barnes said. "Once estrogen levels drop, the vagina can become dry, and sometimes even be a source of pain."

Experts say that having sex can prevent the vagina from becoming thin and tight. “Safe vaginal intercourse can help keep the vagina healthy and dilated,” Barnes said.

To help make intercourse more comfortable, she suggested using lubricants. And for women who experience extreme dryness and discomfort, using hormonal therapies in the form of pills, patches, vaginal rings or creams may be an option.  

"There are some risks to certain types of hormone therapy,” she cautioned, "so it's important for women to discuss them with their doctor before making a decision to use them."

6. Good and bad bacteria

It's normal to have bacteria in your vagina. In fact, there are some bacteria known as lactobacilli that keep the acidity of the vagina in the normal range, according to the journal Frontiers in Microbiology.  

But sometimes the balance between good and bad bacteria can be disrupted. When that happens, women could experience a discharge that smells bad, or have an itching or burning sensation, Barnes said.

But there are ways that women can keep their vaginas healthy, and keep the good bacteria present.

"I always tell my patients to avoid douching," Barnes said. "Flushing out the vagina with anything that might kill the lactobacilli can result in an overgrowth of other types of problematic bacteria."

She also recommended using soaps to clean only the hair-covered areas of skin.

"Water is sufficient for cleaning the non-hair-bearing regions," she said.

7. The clitoris is much bigger than it looks

The clitoris has 8,000 nerve endings aimed for sexual pleasure, according to the Anatomic Study of the Clitoris and the Bulbo-Clitoral Organ — that's around  the same number that are found at the end of the penis, but due to the size of the two glans, the density of nerve endings in the clitoris is about 50 times higher, the journal states. 

The clitoris, which is a small pink organ that lies underneath the clitoral hood, is a powerhouse of pleasure. Although it extends into the vagina for about 3-4 inches (9-11 cm), according to the journal Clinical Anatomy, the clitoris is considered an "external" organ. As researchers noted in a 2011 article in the journal Obstetrics and Gynecology, "the glans and body are visible, while the roots are hidden, therefore they are not "internal.'"

Additional resources

To find out what’s normal for a vagina, the NHS website answers some commonly asked questions. Alternatively, to learn about how vaginas are finally losing their stigma, you can read this article by BBC Culture.

Bibliography

"Female ejaculation: An update on anatomy, history, and controversies". Clinical Anatomy (2021). https://onlinelibrary.wiley.com/doi/abs/10.1002/ca.23654

"Effects of Sex Education and Kegel Exercises on the Sexual Function of Postmenopausal Women: A Randomized Clinical Trial". The Journal of Sexual Medicine (2017). https://www.sciencedirect.com/science/article/abs/pii/S174360951731192X

"Pelvic organ prolapse". The Lancet (2007). https://www.sciencedirect.com/science/article/abs/pii/S0140673607604620

"Lactobacilli Dominance and Vaginal pH: Why Is the Human Vaginal Microbiome Unique?". Frontiers in Microbiology (2016). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5143676/

"Anatomy of the clitoris and the female sexual response". Clinical Anatomy (2015). https://onlinelibrary.wiley.com/doi/abs/10.1002/ca.22524

Transgender is an umbrella term that describes people whose gender identity or expression does not match the sex they were assigned at birth. For example, a transgender person may identify as a woman despite having been born with male genitalia.

About 1.4 million adults in the United States identify as transgender, according to 2016 research by the Williams Institute at the University of California, Los Angeles. Broken down on a state level, that research found that 0.8% of adults in California, Georgia, Hawaii and New Mexico identify as transgender, compared with 0.3% of adults in Iowa, Montana, North Dakota South Dakota and Wyoming identify as such.

What is transgender?

Being transgender means different things to different people, according to the National Center for Transgender Equality (NCTE). "There's no one way to be transgender, and no one way for transgender people to look or feel about themselves," the organization says on its website.

A person's internal sense of being male, female or something else is their gender identity. For cisgender, or non-transgender people, their gender identity matches their sex at birth. For transgender people, the two do not match. 

Sometimes, a person's gender identity doesn't fit neatly into two choices. People who see themselves as being both male and female, neither male nor female or as falling completely outside these categories may identify as genderqueer, according to the Human Rights Campaign (HRC), an LGBTQ advocacy organization. (LGBTQ refers to the community of lesbian, gay, bisexual, transgender and queer or questioning individuals.)

How a person communicates their gender identity — through dress, behavior, voice or body characteristics — is their gender expression. A person's gender expression may or may not line up with society's expectations of masculinity or femininity, according to the HRC. The term "gender non-conforming" refers to people whose gender expression is different from conventional expectations of masculinity or femininity. However, not all gender-non-conforming people identify as transgender, and not all transgender people identify as gender non-conforming.

The public's understanding of gender identity and expression is evolving as more transgender people share their stories, according to the HRC.

The transgender flag

The transgender flag was designed in 1999 by Monica Helms, an openly transgender American woman and Navy veteran, according to nonprofit organization Point of Pride. The light blue and pink tones represent traditional colors for baby boys and girls, while the white horizontal line represents "those who are intersex, transitioning or consider themselves having a neutral or undefined gender."

According to Point of Pride, the flag is designed to be symmetrical so it appears the same whichever way up it is flown. The original flag created by Helms, and first displayed at a pride parade in Phoenix, Arizona, in 2000, is now held at the Smithsonian National Museum of American History.

Sex versus gender

Sex and gender are two different concepts. A person's sex refers to his or her biological status as either male or female. The determination of a person's sex depends primarily on various physical characteristics, including chromosomes, reproductive anatomy and sex hormones, according to the American Psychological Association (APA). 

Gender, on the other hand, is a societal construct that deals with the expected behaviors, roles and activities typically associated with the different sexes, the APA said. Gender roles, which vary across cultures, influence how people act and feel about themselves.

Sexual orientation is different from gender identity. Sexual orientation is a person's physical, emotional or romantic attraction to another person, while gender identity is about one's own sense of self, according to GLAAD, an anti-discrimination organization. Transgender people may be straight, lesbian, gay or bisexual. For example, a person born with male genitalia may transition to being female but may be attracted to females. In this case, the person may identify as lesbian even though she was born with male genitalia.

Making the transition

Trying to change a person's gender identity is no more successful than trying to change a person's sexual orientation, GLAAD said. In other words, it can't be changed. Some people may take steps to better align their sex with their gender using hormones and surgery. HRC points out, however, that many transgender people cannot afford medical treatment or have no desire to pursue surgeries.

"So-called 'gender reassignment surgery' (more commonly called 'gender-affirmation surgery' by both medical professionals and transgender individuals) usually references transgender genital surgery," said Dr. Joshua Safer, medical director of the Center for Transgender Medicine and Surgery at Boston Medical Center (BMC), who is also in the BMC Department of Endocrinology. "There are also chest-reconstruction surgeries and facial feminization surgeries, among other options."

Genital surgery is typically reserved for transgender individuals over the age of 18 who have been treated with hormones, if that is what is medically indicated, and who have lived for at least a year in the gender roles that match their gender identities, Safer said. Candidates for surgery are reviewed by a medical team that considers mental health and physical health in determining the best treatment strategy, potentially including surgery, for each person.

Related: How gender reassignment surgery works

Altering the voice so that it better matches gender identity can also be important to those transitioning. "Here, we envision a world where a transgender person feels no need to change their voice or speech — that is, they would live in a world where people accept and respect them as whatever gender they claim, regardless of how their voice sounds," said Dr. Leah Helou, a speech pathologist who leads the University of Pittsburgh Voice Center's transgender voice and communication services. 

"However, in the absence of such radical and global acceptance, many trans individuals feel that their communication style is a top priority for making their external self congruent with their inner self," Helou said. "Our goal is to serve and support those people, while advocating for broader acceptance of the transgender population."

Names and pronouns

After transitioning, transgender people often change their names — either to one that matches their gender or to something neutral. A critical step in transitioning is changing legal documents, including driver's licenses, Social Security cards, passports and credit cards. They often have to go to court to order the changes to be made — an expensive, time-consuming task, according to the NCTE.

It is considered rude to call people who have transitioned by their former name (called "dead-naming"), and it is appropriate to respectfully ask them their name and which pronouns they prefer, according to the HRC.

Most transgender people prefer to be identified with the pronoun that corresponds to the gender with which they identify, according to the HRC. A transgender woman should be called "she" or "her," if that's what she prefers. Some transgender people don't believe in binary gender appellations and prefer "they" or a non-gendered pronoun.

Discrimination

Being transgender is not a mental disorder. It cannot be "cured" with treatment. Transgender people may experience a persistent disconnect between their assigned sex and their internal sense of who they are, according to the HRC. Medical professionals refer to this disconnect as gender dysphoria because it can cause pain and distress in the lives of transgender people.

In 2012 The American Psychiatric Association announced that a new version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) would replace the term "gender identity disorder" with the more neutral term "gender dysphoria." This was followed in 2019 by the World Health Organization removing the term "gender identity disorder" from its list of mental illnesses, CBS reported. 

Research has shown that transgender people are at high risk of experiencing prejudice and mental-health problems. The 2015 National Transgender Discrimination Survey found that 60% of health care providers refuse treatment to transgender people. Additionally, the research found that 64% to 65% of the transgender people surveyed suffered physical or sexual violence at work, and 63% to 78% suffered physical or sexual violence at school. 

In the most recent iteration of this survey, taken in 2015, results revealed that even children are at risk: Those in grades K through 12 who had come out as transgender reported being verbally harassed (54%), physically attacked (24%) and sexually assaulted (13%) because of their sexual identity.

Transgender people often face discrimination even when using the bathroom. In a 2016 poll of Americans by CBS and The New York Times, 46% of respondents said those who are transgender should use the bathrooms assigned to their birth genders, while 41% said such individuals should be able use the bathroom that matches their identities. In May 2016, the U.S. Education and Justice departments stepped in to advise school districts to permit transgender students to use bathrooms and locker rooms that matched the students' gender identities. In response, several states joined in a lawsuit, stating that the federal government had overreached its authority.

Because of discrimination and other factors, the suicide rate among transgender people is high. The Suicide Prevention Resource Center reports that more than 83% of transgender people had thought about suicide and 54% had attempted it. (The National Suicide Prevention Lifeline is 1-800-273-8255.)

Help for parents

Parents who think their children may be transgender should seek the assistance of experts. "Determining if children are transgender can be a challenge and should be done with careful evaluation from a knowledgeable multidisciplinary team," Safer said.  

It is important to note that many children question their gender identities without being transgender. Safer advised parents to be respectful of the child's feelings and recognize that there will be no actual medical intervention until the child begins puberty. Even then, initial medical treatments are reversible, he said.

Related: How parents and doctors can support transgender children

Transgender history

Though transgender people have existed for hundreds and even thousands of years according to an article by the HRC, the transgender movement for social change has existed for less than a hundred years. According to Susan Stryker, a visiting professor at Yale University, laws specifically targeting transgender people began to emerge in the 19th century. "In the 1850s, a number of U.S. cities began passing municipal ordinances that made it illegal for a man or woman to appear in public "in a dress not belonging to his or her sex,"' she wrote in her book "Transgender History" (Da Capo Press, 2009).

In the early 20th century, gender reassignment, or affirmation, surgery was only being carried out in a few specialist medical centers around the world. One of the largest, in Germany, was targeted by the Nazis, according to Elliot Evans, author of "Queer Permeability" (Routledge, 2020). "Magnus Hirschfeld’s pioneering Institut für Sexualwissenschaft [Institute of Sexology, or Sexual Science] in Berlin, provided treatment for Lili Elbe (whose story was made famous by the 2015 film "The Danish Girl") and Dora Richter," Evans wrote in an article for All About History magazine. "It was all but destroyed after Hitler came to power in 1933, its archives and library publicly burned. The Institute only re-emerged – this time in Frankfurt – in 1973." 

Transgender healthcare did not fully emerge until the second half of the 20th century, though it was still very limited and in many countries gender affirmation surgery remained illegal. "In the US, the Stanford Gender Dysphoria Program in Palo Alto, California, associated with the pioneering doctor of transgender healthcare, Harry Benjamin, would not be founded until 1968," Evans wrote. "Some medical assistance for trans individuals was available in Europe: in Sweden, and in Denmark’s Copenhagen University Hospital, which provided surgical interventions for the American trans woman Christine Jorgensen in the early 1950s, after she had obtained special legal permission from the Danish Minister of Justice." 

Additional resources

• National Center for Transgender Equality
• The Journal of Homosexuality: Attempted suicide among transgender persons: The influence of gender-based discrimination and victimization
• Human Rights Campaign: Transgender FAQ

All sperm perform the same basic job: They fertilize egg cells. But in a new study, researchers have figured out that size matters, and it's largely the female that pushes sperm to be big or small.

Sperm cells come in a huge variety of sizes. For instance, the parasitoid wasp Cotesia congregata produces little swimmers that are less than one-thousandth of a centimeter long, while fruit flies make sperm with 2.3-inch (6 cm) tails that coil tightly to fit inside their tiny bodies. 

In the new study, the researchers set out to determine how sperm size varies among species and what might be driving the differences.

"We have all these studies that show evidence of natural selection pushing sperm size in various species to be either bigger or smaller, but we wanted to take more of a zoomed-out view and look for trends across species," said lead author Ariel Kahrl, a postdoctoral researcher in evolutionary biology at Stockholm University. 

Related content: Sexy swimmers: 7 facts about sperm

Kahrl and her colleagues examined data from 3,200 species and discovered a governing principle that determines sperm size in a species: Females with small reproductive tracts drive the production of bigger sperm, and the need to spread sperm far and wide shrinks sperm across evolutionary timescales.

Here's why. For the most part, animals use two modes of sexual reproduction. One group — which includes mammals, insects and birds — are internal fertilizers that carry eggs inside their bodies. External fertilizers, by contrast, eject their eggs into the environment and hope for the best. Commonly, these species live in water, like fish and sea urchins. In both modes, tons of sperm are competing in a battle royal for the prize of fertilizing the egg, but the challenges of each mode exert incredible evolutionary pressure on sperm size.

"We found that external fertilizers tend to have really small sperm because they have to make a ton of it to reach the eggs," Kahrl said. External fertilization requires ejecting a cloud of sperm, typically into water. As the sperm spread, they become diluted, so the best strategy would be to produce as many sperm as possible to maximize the chance that at least one will reach an egg. Because an animal has a limited amount of energy to use for making sperm he can't afford to make them any bigger than they absolutely have to be. 

It's a completely different situation for the internal fertilizers. "We think that for internal fertilizers, the female's reproductive tract influences the way sperm fight each other," said study co-author John Fitzpatrick, an assistant professor of biology who is also at Stockholm University. In internal fertilization, the sperm work in a tight space, so reproduction becomes less of a treasure hunt and more like a game of king of the hill. In this situation, bigger may be better for shoving other sperm out of the way, regardless of whether they came from the same father or different potential fathers. 

"Some of these species make huge sperm, and if you're making enormous sperm, you don't make that many of them," Kahrl said. "These males coil up their sperm like a ball of yarn and pass it along." 

In addition to internal and external fertilizers, the researchers examined a rarer third reproductive mode, called spermcasting. Spermcasting is like a combination of internal and external fertilization; for example, a river mussel might eject sperm into a stream, and that sperm would ride the currents until it is picked up by a stationary, filter-feeding female. 

"With spermcasting, you have this dilution effect because the sperm are ejected into the water, but when the sperm enter the female, they evolve rapidly under the same types of pressures that we see in internal fertilizers," Fitzpatrick told Live Science. The spermcasters, though, have smaller swimmers, similar in size to the sperm of external fertilizers, likely because ejecting sperm into water incentivizes making more of them, forcing them to be small. But once those sperm are taken up by the female, the biggest sperm tend to win.

Despite being internal fertilizers, humans don't have monster sperm. Instead, human sperm measures a modest 0.002 inches (0.005 cm) long, well within the range seen in external fertilizers. That's because animals with bigger bodies have reproductive tracts that allow the sperm to spread out similarly to the way external fertilizers' sperm do. 

The smaller the reproductive tract, the larger the sperm. And for a fruit fly, it's as cramped as it gets. "Fruit fly sperm is 20 times the length of the animal's body," Kahrl said.

The researchers published their findings 21 June in the journal Nature Ecology & Evolution.

Originally published on Live Science.

Female dragon mantises aren't subtle about letting males know that they're ready for sex. When it's mating time, females of this rare praying mantis species inflate a glistening, greenish gland that juts up and outward from the lower part of their abdomen. 

The structure has two branches, so it resembles a very small, moist balloon in the shape of the letter "Y," and it emits pheromones that males find irresistible. Sexually receptive females puff up the gland at night "and only when undisturbed," scientists recently reported in a new study. 

Other types of mantids emit pheromones from small bulges in their abdomens, but in those mantises the bumps protrude only slightly, if at all. The dragon mantis (Stenophylla lobivertex) is the only known mantis in which females have evolved a blow-up sex gland of this size, according to the study.

Related: Lunch on the wing: Mantises snack on birds (photos) 

Dragon mantises are found in Ecuador and Peru, and are known from only a handful of specimens. Adults measure about 1.6 inches (40 millimeters) long, and the brownish-green color and texture of their bodies resemble that of dried-up leaves.

The Y-shaped glands, however, look shiny and damp. When fully inflated, the gland measures 0.2 inches (6 mm) long, is about 0.04 (1 mm) thick at both ends and each lobe "can also be moved in a tentacle-like manner," the researchers wrote. In fact, that movement led the study author who discovered the unusual gland to suspect that the mantis had been infected by a parasite.

"When I saw the maggot-like structures peeking out from the back of the praying mantis and then withdrew, I immediately thought of parasites that eat the animal from the inside, because that is not really uncommon in insects," Frank Glaw, a reptile and amphibian expert at the Bavarian State Collection of Zoology in Munich, Germany, said in a statement.

When the scientists investigated the structures more closely, they found that the glands were filled with hemolymph, a bloodlike fluid in invertebrates. Threadlike tubes called trachea, which carry air to insects' tissues, were visible through the gland's thin membrane. 

The researchers observed mating behavior in four female dragon mantises: one in the Peruvian Amazon, and three captive females obtained from a breeder in Germany. During the observation periods, which were conducted over several months for each group, females inflated their glands and emitted scent-calls at night for about 2 to 5 hours. They only did so under cover of darkness and when they were left undisturbed. If the mantises were interrupted or distracted while waggling their shiny love sticks, "the pheromone gland was retracted instantly," the study authors wrote. 

Because the insects are so rarely glimpsed in the wild, they are thought to be scarce in their native habitats. Waving an inflatable come-hither sex gland could therefore help dragon mantises find each other more easily, which could be critical for the species' survival, lead study author Christian J. Schwarz, an entomologist at the The Ruhr-University Bochum in Germany, said in the statement.

"We suspect that Stenophylla lobivertex can release the pheromones with the protrusible organ more efficiently and in a more targeted manner than other praying mantises," Schwarz said. "This can be very important, especially for rare species with a low population density, so that males can reliably find their females."

The findings were published online April 21 in the Journal of Orthoptera Research.

Originally published on Live Science.

Young adults say pornography is their best source of information about how to have sex, new research finds.

In a nationally representative survey, a quarter of 18- to 24-year-olds listed porn as their "most helpful" source of information on how to have sex. This made porn the most-mentioned source of information, even above asking one's own partner. 

The findings suggest a need for more education on healthy sexual relationships and on what pornography is about, said study leader Emily Rothman, a professor of community health sciences at the Boston University School of Public Health.

"Pornography is created to be entertainment," Rothman told Live Science. Pornography creators are focused on what's profitable, not what's instructional, she said: "It's fantasy."

Related: 5 ways porn affects the brain

Sex education

The proliferation of free streaming porn sites has made explicit content easier than ever to access, raising concerns about what, if any, impact pornography has on adolescents and young adults, Rothman said.

"There is concern from many different corners that increasingly young people are turning to pornography for information about how to have sex," Rothman said. She and her colleagues have been working for years on porn literacy instruction for teenagers. The concept behind porn literacy is not to shame people for being interested in pornography or to police the content they watch, she said. Rather, their trainings and courses aim to put pornography into context; to help young people understand why they might see content that doesn't represent healthy, consensual relationships; and to promote communication between sexual partners.

"One of the problems we can run into with people taking their information from pornography instead of from their partner is, they could erroneously assume that certain sexual acts that they see on pornography are going to elicit a pleasurable response for their partner and try it without asking for consent," Rothman said. "And then we run into problems."

A recent content analysis, published in the journal Archives of Sexual Behavior, examined more than 4,000 videos on free streaming pornography sites. It found that between 35% and 45% contained depictions of violence, almost always toward women. While an interest in depictions of rough sex is not necessarily a problem in and of itself, Rothman said, these scenes rarely show the participants negotiating consent or taking other safety precautions, and thus shouldn't be used as an instruction manual.

The new study, published Jan. 4 in the journal Archives of Sexual Behavior, used data from the 2015 National Survey of Sexual Health and Behavior, a survey weighted to be nationally representative. In the survey, about half of participants said they hadn't received useful outside information on how to have sex. But the other half -- 357 young adults and 324 adolescents (ages 14 to 17) -- said they had.

 Sources of information 

Of those who said they had received helpful information, about a quarter of young adults put pornography at the top of the list, over sexual partners, friends, media and health care workers. Adolescents were much less likely to mention pornography, with only 8.4% turning to porn as their top source of information. Instead, the 14- to 17-year-olds were more likely to mention parents (31%) and friends (21.6%) as their top sources of information.

"The good news is, adolescents, in general, are listening to their parents," Rothman said. "It is worth it for parents to continue to have conversations with their adolescent children about sex. Your kids care about what you say."

The high proportion of young adults turning to pornography for information is more troubling, Rothman said. She and her colleagues are working to develop a Porn Literacy app for high-school sex education classes that educators can use to supplement their courses. Rothman's team has already developed specific porn literacy classes, available online and through the Boston Public Health Commission, but this information is rarely part of school-based sex education. Participants in the porn literacy classes are never shown pornography; instead, they discuss topics such as consent, body image and relationship violence. The app would expand the program's reach.

"We want to create this app that's like a self-guided tutorial that gives information about everything from sexually transmitted infections and communicating with their partner in a healthy way to myths and facts around porn so that at least young people are getting better information about what porn is and what it isn't, and doing it in a sex-positive way," Rothman said.

Originally published on Live Science.

The sex we're assigned at birth depends largely on a genetic flip of the coin: X or Y? Two X chromosomes and you (almost always) develop ovaries. An X and a Y chromosome? Testes. These packages of genetic material don't just differ in terms of the body parts they give us. With 45 genes (in comparison to around 1,000 on the X), the Y chromosome is puny. And research suggests it has shrunk over time — a proposition that some have, in turns, glumly or gleefully interpreted as predicting the demise of men.

So is the Y chromosome really dying out? And what might that mean for men?

To begin to answer these questions, we have to go back in time. "Our sex chromosomes weren't always X and Y," said Melissa Wilson, an evolutionary biologist at Arizona State University. "What determined maleness or femaleness was not specifically linked to them." 

Related: Why do men have nipples?

When the very first mammals evolved between 100 and 200 million years ago, they didn't have any sex chromosomes at all. Instead, the X and Y were just like any other set of chromosomes — identical in size with corresponding structures, Wilson said.

It's important to note that animals don't need sex chromosomes. That was true then, and it's still true now, said Jennifer Graves, a geneticist at La Trobe University in Melbourne, Australia. All of our chromosomes are a cocktail of sex-related and non-sex-related genes. The only special feature of the Y chromosome is one gene, SRY, which acts as an on-off switch for the development of testes, Graves added. In the case of alligators and turtles, an on-off switch isn't even necessary — the temperature in which embryos develop determines their sex. Our mammalian ancestors likely shared this characteristic, Graves wrote in a 2006 review on the subject, published in the journal Cell. But at some point, a plain old, non-sex chromosome in one of these ancestors developed a gene with an on-off switch like this. And that was it: you suddenly needed a Y to develop male reproductive parts.

But as soon as the Y chromosome existed, it was primed to shrink. Over time, genes develop mutations, many of which are harmful, Wilson said. Chromosomes can avoid passing on these mutations by recombining with one another. During meiosis, when our bodies produce sperm and eggs, paternal and maternal chromosomes randomly mix and match their arms. This genetic dance breaks up variants of genes — harmful and beneficial alike — and makes it more likely that only functional copies will get passed on. All the chromosomes do this: chromosome 1 from mom swaps arms with chromosome 1 from dad, and so on. The Y, however, does not have a swapping companion. Although X chromosomes can recombine with one another, Y chromosomes and X chromosomes aren't similar enough to recombine. And because you rarely have two Y chromosomes in an individual, Y can't recombine with itself. 

"If a bad mutation occurs, usually you'd be able to swap with your partner. But the Y can't do that," Wilson said. So Y chromosomes accumulated harmful mutations; over time, those mutations were weeded out by natural selection until the Y got smaller and smaller. 

Graves's research suggests that 166 million years ago, the Y chromosome had 1,669 genes — "same as the X-chromosome" at that time, she said. "So it doesn't take a great brain to realize that if the rate of loss is uniform — 10 genes per million years — and we've only got 45 left, the whole Y will disappear in 4.5 million years." 

Uniform is the key word here. More recent research suggests that the rate of degradation has slowed over time. In a 2005 study published in the journal Nature, researchers compared the human Y chromosome with that of a chimpanzee. Then in 2012, the same team of researchers sequenced the Y chromosome of a rhesus monkey, again publishing the results in Nature. The researchers found that the human Y chromosome has lost only one gene since humans and rhesus monkeys diverged evolutionarily 25 million years ago. It hasn't lost any genes since the divergence of chimpanzees 6 million years ago. These results suggest that decay has not occurred in the linear fashion that Graves originally suggested, in which 10 genes are lost per million years.

Related: Why do women have orgasms?

Loss of the Y chromosome isn't off the table — it's happened to other species, Graves pointed out. Two species of underground rodents called mole voles have independently lost their Y chromosomes. So have three endangered species of spiny rats living on several small islands in Japan.

But as those species demonstrate, the loss of the Y chromosome doesn't doom survival; both spiny rats and mole voles still have males and females. "People think that sex is sort of a very determined thing," said Rasmus Nielsen, a geneticist at the University of California, Berkeley, "That if you have a Y chromosome, then you're a man, or you don't have [a] Y chromosome, then you're female. But it doesn't work like that." 

In fact, 95% of genes that are expressed differently between males and females don't actually live on the X and Y chromosomes, Wilson said. For instance, ESR1, a gene that encodes for estrogen receptors, is found on chromosome 6. These receptors are vital for female growth and sexual development. 

"Losing the Y chromosome doesn't mean losing the male," Nielsen added. Instead, the loss of the Y chromosome would likely mean that another gene would take over the job as the main determinant of sex — the on-off switch, Graves said. "There are heaps of genes out there that will do a perfectly good job." 

But how likely is that to happen? "It's possible," Wilson said, "but not in our lifetime."

Originally published on Live Science.

Under a microscope, human sperm seem to swim like wiggling eels, tails gyrating to and fro as they seek an egg to fertilize. 

But now, new 3D microscopy and high-speed video reveal that sperm don't swim in this simple, symmetrical motion at all. Instead, they move with a rollicking spin that compensates for the fact that their tails actually beat only to one side. 

"It's almost like if you're a swimmer, but you could only wiggle your leg to one side," said study author Hermes Gadêlha, a mathematician at the University of Bristol in the U.K. "If you did this in a swimming pool and you only did this to one side, you would always swim in circles. … Nature in its wisdom came [up] with a very complex, ingenious way to go forward." 

Related: Sexy swimmers: 7 facts about sperm

Strange swimmers

The first person to observe human sperm close up was Antonie van Leeuwenhoek, a Dutch scientist known as the father of microbiology. In 1677, van Leeuwenhoek turned his newly developed microscope toward his own semen, seeing for the first time that the fluid was filled with tiny, wiggling cells. 

Under a 2D microscope, it was clear that the sperm were propelled by tails, which seemed to wiggle side-to-side as the sperm head rotated. For the next 343 years, this was the understanding of how human sperm moved. 

"[M]any scientists have postulated that there is likely to be a very important 3D element to how the sperm tail moves, but to date we have not had the technology to reliably make such measurements," said Allan Pacey, a professor of andrology at the University of Sheffield in England, who was not involved in the research. 

The new research is thus a "significant step forward," Pacey wrote in an email to Live Science. 

Gadêlha and his colleagues at the Universidad Nacional Autónoma de México started the research out of "blue-sky exploration," Gadêlha said. Using microscopy techniques that allow for imaging in three dimensions and a high-speed camera that can capture 55,000 frames per second, they recorded human sperm swimming on a microscope slide. 

"What we found was something utterly surprising, because it completely broke with our belief system," Gadêlha told Live Science. 

Related: The 7 biggest mysteries of the human body

The sperm tails weren't wiggling, whip-like, side-to-side. Instead, they could only beat in one direction. In order to wring forward motion out of this asymmetrical tail movement, the sperm head rotated with a jittery motion at the same time that the tail rotated.The head rotation and the tail are actually two separate movements controlled by two different cellular mechanisms, Gadêlha said. But when they combine, the result is something like a spinning otter or a rotating drill bit. Over the course of a 360-degree rotation, the one-side tail movement evens out, adding up to forward propulsion.

"The sperm is not even swimming, the sperm is drilling into the fluid," Gadêlha said. 

The researchers published their findings today (July 31) in the journal Science Advances.

Asymmetry and fertility

In technical terms, how the sperm moves is called precession, meaning it rotates around an axis, but that axis of rotation is changing. The planets do this in their rotational journeys around the sun, but a more familiar example might be a spinning top, which wobbles and dances about the floor as it rotates on its tip. 

"It's important to note that on their journey to the egg that sperm will swim through a much more complex environment than the drop of fluid in which they were observed for this study," Pacey said. "In the woman's body, they will have to swim in narrow channels of very sticky fluid in the cervix, walls of undulating cells in the fallopian tubes, as well have to cope with muscular contractions and fluid being pushed along (by the wafting tops of cells called cilia) in the opposite direction to where they want to go. However, if they are indeed able to drill their way forward, I can now see in much better clarity how sperm might cope with this assault course in order to reach the egg and be able to get inside it," Pacey said

Sperm motility, or ability to move, is one of the key metrics fertility doctors look at when assessing male fertility, Gadêlha said. The rolling of the sperm's head isn't currently considered in any of these metrics, but it's possible that further study could reveal certain defects that disrupt this rotation, and thus stymy the sperm's movement. 

Fertility clinics use 2D microscopy, and more work is needed to find out if 3D microscopy could benefit their analysis, Pacey said. 

"Certainly, any 3D approach would have to be quick, cheap and automated to have any clinical value," he said. "But regardless of this, this paper is certainly a step in the right direction."

Originally published in Live Science.

The reason for the female orgasm has long eluded scientists. Men need them for reproduction; women don't. So why do female orgasms exist?

Scientists studying this issue are divided, said David Puts, a biological anthropologist at Penn State University. Some scientists think female orgasms are totally purposeless. But evidence suggests that they may have once helped (and perhaps still help) us survive and reproduce.

One theory holds that women have orgasms because men have them, said Kimberly Russell, an ecologist at Rutgers University in New Jersey. Some researchers argue that female orgasms exist because as fetuses, we all start out with the same basic parts, regardless of sex. Orgasms in women, like nipples on men, just happen to stick around.

Related: What's an orgasm?

"It might be an anatomical bonus," she told Live Science. In this scenario, the orgasm didn't evolve specifically for females, and it might not serve a specific evolutionary function for them. 

But there's a problem with the argument that orgasms have no function, said Patricia Brennan, an evolutionary biologist at Mount Holyoke College in Massachusetts. It's not adaptive for our bodies to devote too much energy to traits, like nipples, that aren't beneficial. These traits tend to disappear or become less pronounced over time. That's far from the case for female orgasms, she said. According to the Kinsey Institute, female orgasms tend to last longer than male orgasms and can occur multiple times in a row — something that's rare in men. In other words, female orgasms use a lot of energy for a trait that supposedly has no function, she said.

Plus, there's nothing diminished about the anatomical structures involved in the female orgasm, Brennan noted.

The clitoris, a highly sensitive part of the female genitals that has a key role in orgasms, is homologous to the penis. Like male and female nipples, they grow from the same anatomical structure. But contrary to popular belief, Brennan told Live Science, "a clitoris is not just a mini penis."

The human clitoris has “structures that are incredibly well developed,” Brennan said. "To me, that screams selection."

There are multiple theories about how, exactly, the female orgasm helped our ancestors pass on their genes. Although women don't need to have an orgasm to conceive, some research suggests that wasn't always the case. Many female mammals, including rabbits and cats, ovulate only when they mate. Based on an analysis of how traits have been passed down through the tree of life, one study published in the Journal of Experimental Zoology found that our female ancestors probably needed orgasms in order to reproduce.

But again, this theory doesn't explain why orgasms stuck around in women, Brennan said. 

"If orgasms evolved for some adaptive reason, but they're no longer adaptive, they should have disappeared. And clearly they haven't gone away," Brennan said.

Some research suggests that orgasms still create the perfect conditions for conception — even if they're not necessary to ovulate. One study found that women who had orgasms close to when their male partner did actually "upsucked" more sperm into their bodies compared with women who had orgasms much earlier or later than their partner. Scientists have even tried to draw correlations between the number of orgasms a woman has and the number of children she has. But the evidence for these hypotheses is shaky and doesn't draw a direct causal link between orgasms and conception, Puts told Live Science. 

Plus, these theories leave a major question unanswered, Russell said. What if the orgasm has nothing to do with reproduction? What if, instead, it evolved only for pleasure?

Sex doesn't have to feel good for reproduction to take place, Russell said. "We know this from looking at animals! Sex can be very uncomfortable and still gets done," she said. But culturally, the idea that sex might be for more than just babies is somewhat of a taboo topic, Russell said.

Sex that feels good for both males and females has an important social role, Russell said. It relieves stress and helps partners bond. Ancestral humans might have engaged in sex to create more cohesive groups, smoothing over conflict and cementing their social network. We see these behaviors in other primates, like bonobos, who might use sex to help dispel a fight over a tasty piece of fruit or even a clan rivalry, the BBC reports. It follows from this argument that evolutionarily, female orgasms might have acted as a kind of social glue. 

That pleasure alone is enough to make a trait adaptive goes against popular conceptions of why sex, and orgasms, exist. But for Brennan, it makes perfect sense. "To experience pleasure — that seems evolutionarily like a good idea," she said. 

• Why do guys get sleepy after sex?
• What is the 'birds and the bees'?
• Why do we have sex? 

Originally published on Live Science.

There isn't much that's sexy about lichen, the slow-growing fuzz found on rocks, walls and tree bark. But apparently not everyone agrees. Experts in New Zealand are warning men not to consume a type of lichen that's being touted as a "natural Viagra" and alternative treatment for erectile dysfunction (ED), according to news reports.

This type of lichen is known scientifically as Xanthoparmelia scabrosa, but it has been dubbed "sexy pavement lichen" due to its similarities with ED drugs, according to the New Zealand-based news outlet Newsroom. The lichen grows naturally on roads and pavements and is abundant in New Zealand. It contains a compound similar to the active ingredient in Viagra and other ED drugs, which is part of a class of drugs known as PDE5 inhibitors, Newsroom reported.

Thankfully, there have been no reports of anyone actually licking lichen off the pavement in the hopes of treating erectile dysfunction. But powder from ground up Xanthoparmelia scabrosa is being marketed as an ED treatment — for example, the Chinese e-commerce site Alibaba lists hundreds of varieties of lichen powder available for purchase, according to Newsweek.

But consuming Xanthoparmelia scabrosa in any form could be dangerous. The chemicals in the lichen could be toxic,  Allison Knight, a lichenologist at the University of Otago in New Zealand, told the Newsroom. What's more, in urban areas, this lichen can contain high levels of heavy metals, including copper, lead and zinc.

"I always say in my talks that I don't recommend going out and licking the footpath," Knight said.

In addition, if you buy Xanthoparmelia scabrosa powder, there's a chance it may not even contain natural lichen.

One analysis of a Xanthoparmelia scabrosa product found that it was 80% Viagra and 20% grass clippings, Newsroom reported.

Knight noted that growing vast amounts of this lichen for use in alternative medicine products would be difficult.

“Most lichens grow very slowly, just a few [millimeters] per year, so it would be scarcely sustainable to harvest them," Knight said.

•  Viagra Goes Generic: 5 Interesting Facts About the 'Little Blue Pill 
•  7 Surprising Reasons for Erectile Dysfunction 
•  Gallery: Oldest Living Things in the World 

 Originally published on Live Science.