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."

Some animals can have babies without the need to mate. Colloquially called a "virgin birth," this occurs through a process called parthenogenesis, wherein a female animal creates an embryo from an unfertilized egg. The offspring tend to be only one sex — all male or all female.

When an animal exclusively reproduces this way, the process is called obligate parthenogenesis. For animals that usually reproduce sexually this process is called facultative parthenogenesis. Such optional parthenogenesis is usually very rare. 

From stick insects to sharks, here are eight animals that have virgin births

Komodo dragon 

Scientists first discovered that the Komodo dragon (Varanus komodoensis) could have virgin births in 2006. At the time, only two Komodo Dragons lived in Europe. One of these dragons, a female at the Chester Zoo in the U.K. gave birth to a clutch of 25 eggs, despite never being in any contact with males. 

Sharks

One ordinary morning at Omaha's Henry Doorly Zoo and Aquarium, zookeepers were stunned to find a 6.5 inch (16.5 centimeters) juvenile shark in a tank containing three female bonnethead sharks (Sphyrna tiburo). None of the female bonnetheads had ever been exposed to males of the species within captivity, and they bore no mating scars from different species within the tank. 

Spontaneous births like this one were once explained by the female's ability to store sperm for long periods of time after being taken from the wild. However, the female bonnetheads were collected from the wild three years prior and well before sexual maturity, so scientists ruled out this explanation. This incident genetically confirmed, for the first time that sharks undergo parthenogenesis.

Since then virgin births have been seen in zebra sharks (Stegostoma tigrinum), black tip reef sharks (Carcharhinus melanopterus) and smooth-hound sharks (Mustelus mustelus). 

California condor 

No thank you! Even when housed with males, critically endangered female California condors (Gymnogyps californianus) sometimes choose to have babies themselves.

In hopes of helping the population rebound, researchers at San Diego Zoo housed female condors with a fertile male. But apparently the male wasn't necessary: Geneticists testing DNA samples within the lab discovered two male chicks that carried two identical copies of their mother's DNA.

Stick insects 

Unlike other animals, parthenogenesis in stick insects is not rare. In fact, creatures within the genus Timema reproduce exclusively in this way — and have done so for a million years.

But having babies without sex isn't ideal: In the long run, asexual reproduction can negatively affect the animals' ability to adapt to changing environments over time. So how have Timema insects been able to survive for so long?

A study published in September in the journal Proceedings of the Royal Society B looked at eight populations of four species — Timema genevievae, T. shepardi, T. monikensis and T. douglasi. 

It turned out that sneaky sex was behind their long-term viability: two of the four species occasionally resort to sexual reproduction, creating variety in the gene pool and limiting the risk. 

Blind snake 

The brahminy blind snake (Ramphotyphlops braminus) is another species thought to only reproduce asexually. Only female individuals of this species have ever been found.  

Tardigrades 

Tardigrades are aquatic, microscopic creatures known for their ability to withstand extreme conditions. These chubby water bears can go both ways, reproducing both sexually and asexually. However, parthenogenesis is only common in lake and terrestrial habitats. 

Crocodiles  

This year scientists described the first known case of a virgin crocodile birth. After spending 16 years away from other males, a female American crocodile (Crocodylus acutus) produced a clutch of 14 eggs. 

The female crocodile arrived at the Costa Rican zoo, Parque Reptilandia, in 2002. She spent all her time after that alone in her enclosure. 

While the crocodile could produce fertilized eggs solo, the eggs weren't ultimately viable. Out of the 14, only seven developed when incubated. After incubation, only one egg had a fully formed fetus, but the baby crocodile was born dead. DNA analysis of the fetus confirmed a genetic match of over 99.9% with the female crocodile.

Mollyfish 

One of the most mind-blowing types of parthenogenesis is the sperm-dependent parthenogenesis shown by Amazon mollies (Poecilia formosa) These mollies are named after the all-female tribes in Greek mythology that only used men for reproduction, killing boys at birth. Like their namesakes, the all-female Amazon mollies depend on sperm from males to trigger the formation of a fertilized egg. But there's a catch: the males come from other related species and contribute no genetic material to the egg.

On Valentine's Day, lovers who are eager to woo their partners show their affection with traditional gifts of red roses, heart-shaped boxes of chocolates or with romantic dinners at fancy restaurants. There's usually some effort involved, but pulling off a memorable Valentine's Day is easier — and generally safer — than some of the courtship rituals performed by other animal species.

For most animals, wooing comes with heightened personal risk. A male's showy displays, while attracting a female's attention, could also attract nearby predators, and fights between male rivals can also result in a date night with a body count. In some cases, winning a cannibalistic female's affections places the male at the top of the post-coital menu.

Many of the courting behaviors practiced by animals may seem strange to us, but as peculiar and risky as they are, they work just fine for their intended audiences. Here are a few examples of unusual and extreme courtship rituals in the animal kingdom. 

Giant pandas

Pandas (Ailuropoda melanoleuca) are notoriously difficult to mate in captivity. Mating is no picnic in the wild, but for completely different reasons. In the first-ever footage of giant pandas getting intimate in the wild, filmmakers in China recorded an older male and a younger rival courting the same female, who was high above the ground in a tree.

The males had a tense standoff until the younger panda retreated. But the female wasn't ready to mate; upon descending, the female fought the older male and escaped. The two males followed her for weeks, growling at each other until one suitor dropped off and the female was ready to mate with the younger fellow.

It's possible that this prolonged male rivalry, including female "hostage"-taking, triggers female ovulation. So maybe that's why these black-and-white bears are so hard to breed in captivity, where male competition is nonexistent, according to the 2020 program "Pandas: Born to be Wild" that debuted the footage.

Giraffes

Male giraffes have to taste a lot of pee before they can do the deed. That's because the only way males (bulls) can tell if females (cows) are fertile is to determine if specific pheromones are present in her urine.

First, the bull nudges the cows and sniffs her genitalia. Sometimes it takes a few nudges, but then the cow will widen her stance and urinate into the bull's mouth. Next, the bull does a "flehmen response" by curling back its upper lip and breathing in through its nostrils, using its sensitive vomeronasal organ above the roof of the mouth to smell his potential partner's pee. Other also animals smell pee when mating, but usually the female pees on the ground for the male to sniff. In the giraffe's case, they're way too tall to do it that way.

On average, bulls have to approach 150 females before finding one who is ready to mate, a 2023 study published in the journal Animals found.

Snails

Look closely at these photos of land snail Cornu aspersum, and you'll see a small appendage close to the eyestalk. That tiny structure was propelled into the snail's head by its mate, delivering an infusion of a special mucus that prepares the snail for receiving an envelope full of sperm.

As land snails are hermaphrodites, either snail in a mating pair is capable of fertilizing the other, and both are equipped with "love darts" that they use to stab their partner — after they spend a bit of time circling around and touching each other with their muscular pseudopods.

Some snail species shoot single darts, some shoot multiple darts, and others use a single dart to repeatedly jab their mate for close to an hour, according to a 2006 study, published in the journal The American Naturalist.

Dinosaurs

Little is known about dinosaurs' mating habits, but evidence preserved in rocks in Colorado suggests that some dinosaurs practiced a ritual dance much like one performed by living birds.

Paleontologists found scrape marks — many dozens of them — in four sites that held remains of Cretaceous dinosaurs. In a 2016 study published in the journal Nature Scientific Reports, researchers explained that they saw distinct similarities between these scratches in the rock, and so-called "nest scrapes" created by certain types of male birds as part of their courtship displays.

Male birds across a number of ground nesting species — including sage grouse, puffins and various shore birds — scrape the ground in front of females, as if to demonstrate how good they would be at building a nest. They make dozens or even hundreds of scratches at a time, and usually accompany scraping with strutting, puffing themselves up, and fanning their tails.

Black widow spiders

Black widow (Latrodectus Hesperus) females are about twice as large as males, so the smaller suitors have to take some precautions when approaching a female's web, lest they be mistaken for prey and eaten before mating even gets underway.

Males stay safe by announcing their presence to the female with vigorous rump shaking.

As a male steps onto a female's web, he vibrates his abdomen, sending signals coursing along the silk strands. He advances, vibrates and pauses, advances, vibrates and pauses — a pattern distinctly different from the shorter, more irregular movements of trapped prey, researchers found in a study, published in the journal Frontiers in Zoology. The study authors also discovered that the vibrations that males produce are at a low amplitude, further distinguishing them from prey movements, which were more dynamic and percussive.   

Sea Slugs

Hermaphroditic sea slugs possess both male and female sex organs, and when pairs come together to mate, they stab each other between the eyes with a needle-like appendage called a penile stylet, delivering a cocktail of prostate fluid. This tactic was described as "just weird" by a researcher who co-authored a 2013 study about the odd behavior, published in the journal Proceedings of the Royal Society B.

Scientists are uncertain as to why exactly the slugs target this body area for stabbing, but they suspect that the hormonal injection may serve to increase the possibility of successful fertilization.

Pufferfish

"Mystery circles" on the ocean floor near Japan that measure about 7 feet (2.1 meters) in diameter were recently found to be made by a fish only 5 inches (12.7 centimeters) in length. The intricate symmetrical patterns were first noticed by divers in 1995, and in 2013 researchers described what created them: a species of pufferfish, with mating on its mind.

Males swim along the seafloor flapping their fins to sculpt the remarkably intricate ridges and valleys — a process that take seven to nine days — and then decorate them with shell fragments and sediment. After interested females are fertilized, they lay their eggs in the nest site at the center.

Though the structures are beautiful, scientists wrote in 2013 that the lines and shapes carved by the pufferfish serve to channel sediment particles, and likely don't serve an aesthetic purpose.

Jumping spiders

Body parts that reflect ultraviolet light help male jumping spiders in the Cosmophasis umbratica species catch females' eyes (all eight of them). Males lure the female spiders by striking poses that display these glowing patches prominently.

However, female C. umbratica spiders have a glowing trick of their own, possessing palps — a pair of appendages near the head — that fluoresce green in ultraviolet light, which they use to attract the males.

Both male and female spiders rely on these signals to tell who's in a mating mood, scientists discovered in a 2007 study published in the journal Science. When ultraviolet light was blocked and the spiders didn't glow, they lost interest in mating, the researchers found.

Marvelous Spatuletail

In one species of hummingbird — the marvelous spatuletail (Loddigesia mirabilis) — males attract females by whipping their lengthy tails back and forth.

And those tails are impressive, two of the four feathers measure about 6 inches (15 centimeters) in length — about twice as long as the birds' bodies — and are tipped with shiny iridescent "paddles," which the males whirl feverishly at likely mates. 

Bowerbirds

Bowerbirds are known for building elaborate structures to attract female interest, even decorating their bowers with arrays of colored objects that appear to be selected and displayed for their aesthetic appeal.

But there's more to their arrangement than meets the eye. Researchers discovered that male bowerbirds construct their bachelor pads in such a way that when the male bird stands in front of it, he appears larger and more imposing to the female viewing him from outside.

And the birds that create the most successful illusions were the most popular with the females and the most likely to mate with them, scientists wrote in a 2012 study, published in the journal Proceedings for the National Academy of Sciences.  

Mice

Male mice seeking to impress a mate sing unique high-pitched songs, vocalizing in the ultrasonic range. They produce these whistling sounds — which differ greatly from normal communication — by creating a type of feedback loop of airflow in the windpipe and larynx, according to a 2016 study published in the journal Current Biology. Scientists discovered the mechanism by shooting high-speed video of the mice's larynxes as they vocalized, capturing 100,000 frames per second.

Impressive though this technique may be, female mice are picky about which songs they like; they prefer tunes that differ from those sung by their relatives, according to an earlier study published in February 2014 in the journal PLOS One.

Red-cheeked cordon-bleu songbird

High-speed video recently revealed a mating dance performed by a species of songbird tapping their feet too quickly for the movements to be seen with the naked eye.

Blue-capped cordon-bleu songbirds (Uraeginthus cyanocephalus) — both males and females — were known to bob their heads and sing to each other during courtship, but a 2015 study published in the journal Scientific Reports was the first to capture the rapid tapping of their toes — and the birds tapped their feet faster if they were sharing the perch with a prospective mate, the scientists discovered. 

Porcupines

Male North American porcupines must go to great lengths to secure the affections of females, which only go into estrus once a year for eight to twelve hours.

Prior to ovulating, the female secretes a fragrant (to a porcupine) vaginal mucus that lures males closer. The lucky male that finds her — and manages to chase away any rivals — stimulates ovulation by drenching her in an explosive jet of urine described as "a high speed projectile" by porcupine expert Uldis Roze, author of "The North American Porcupine" (Comstock Publishing Associates, 2009) and "Porcupines: The Animal Answer Guide" (Johns Hopkins University Press, 2012).

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.

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. 

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.

A female of the world's largest squid — sometimes called the "kraken" after the mythological sea monster — that was caught off the coast of Japan apparently had just one amorous encounter in her lifetime. 

The female had sperm packets from just one male giant squid embedded in her body, which surprised researchers. Because giant squid are solitary creatures that probably run across potential mates only occasionally, scientists expected that females would opportunistically collect and store sperm from multiple males over time.

"We were almost confident that they are promiscuous," said Noritaka Hirohashi, a biologist at Shimane University in Japan. "We just wanted to know how many males are involved in copulation. So this is totally unexpected."

Related: Release the kraken! Giant squid photos 

Mysterious mating 

Hirohashi and his colleagues study reproduction and sperm biology in several squid species, but the most mysterious of all is Architeuthis dux, the giant squid. Rarely seen alive, the giant squid has a life cycle shrouded in deep ocean mystery. Video of living giant squid in their natural habitats has been captured only twice. The only thing researchers know about these mysterious creatures' mating habits is that female giant squid are sometimes found with large sperm packets known as spermatangia embedded in their muscles. Researchers writing in a 1997 paper in the journal Nature posited that male giant squid probably use their "muscular elongate penis" to inject the sperm packets into the females.

How sperm meets egg from there isn't entirely clear. It's possible that the female releases chemical cues that activate the sperm when she's ready to spawn, or perhaps she releases her eggs in such a way that they trail along the sperm packets as they leave her body. Squid females do have organs near the mouth called seminal receptacles, where some species storm sperm, and it's possible that in those species, the embedded sperm can travel over the skin to these receptacles. 

Knowing that witnessing two giant squid mating is highly unlikely, Hirohashi and his team developed a window into the process, using genetics. Examining squid specimens from fisheries and museum archives, they pinpointed some segments of the giant squid genome that would distinguish one set of squid DNA from another. Think of it like a squid paternity test: Any sperm packets found on a female can be tested to see if they came from multiple males and, if so, how many. 

The researchers are always on the lookout for sperm-spangled females. They send out flyers to local museums, fisheries and aquariums, asking them to alert the research lab if a giant squid specimen turns up. In February 2020, they got good news. 

"In this case, we found [a] Yahoo News [article] telling that the giant squid was caught," Hirohashi wrote in an email to Live Science. 

Saving sperm 

The specimen was a female, with a mantle, or main body, 5.25 feet (1.6 meters) long. It was missing a pair of tentacles and one eye but still weighed 257 pounds (116.6 kilograms). The squid had been caught in a fisher's net in Kyoto and was displayed at the Miyazu Energy Aquarium before being dissected. 

When Hirohashi's team examined the body, they found that the squid was just reaching maturity and that it had squiggly spermatangia 3.9 inches (10 centimeters) long embedded in five separate locations: three places on the squid's mantle, one by an arm and one on the head. Each location hosted at least 10 spermatangia. Some were near gashes that may have been caused by a mating male's beak. 

Genetic analysis of the spermatangia revealed that each and every one came from the same male. This was shocking to the research team; giant squid are often found bearing sperm packets, in a way that suggests that males aren't particularly picky. Spermatangia have been found on immature females, perhaps as a way for males to make their sperm available after the female matures, and even on males, perhaps because males are willing to try anything (or perhaps because they sometimes accidentally self-fertilize). All of the evidence pointed to a species that would mate first and ask questions later.

The specimen, of course, is just one female, so more research is needed to see if monogamy is the norm among giant squid females. It's possible that this female had simply only encountered one male before she was entangled in the net that ended her life, the researchers wrote in the September issue of the journal Deep Sea Research Part 1. Or perhaps it is typical for females to mate with just one male. The gashes might be part of the males' strategy for ensuring other males don't move in, perhaps by limiting a female's life span after mating so that she doesn't have time to collect more sperm. Or, the researchers speculated, the aggression and injuries could spur the females to mature and spawn so that the sperm is speedily fertilized. 

The next step is to study the spermatangia of more specimens, Hirohashi said. And researchers need to figure out how the stored sperm reaches the eggs, which are not deposited particularly close to the spermatangia. Researchers also need to figure out basically everything else about this elusive creature, including its life span, migration and habitats, he added. 

"Kids ask these questions at the aquarium, so we must answer," Hirohashi said. 

Originally published on Live Science

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.

Mice in the cabinets. Mice in the streets. Thousands upon thousands of mice in the barn, pooing so much it takes six hours to clean up their waste.

These are scenes from Queensland and New South Wales, Australia, where an out-of-control mouse infestation is making life miserable for farmers, grocers and other citizens of the eastern Australian states. One farmer interviewed by The Guardian described the rodent frenzy as "an absolute plague," more severe than anything locals have seen in decades.

Some farmers have already lost entire grain harvests to the rampaging mice, according to local media reports, while hotels have had to close because they can't keep the critters out of the rooms. Staff at a grocery store in a small town northwest of Sydney reported catching as many as 600 mice a night. So far, at least three people have visited the hospital with rodent bites, The Guardian reported.

Steve Henry, a researcher at CSIRO (Australia's national science agency) told The Guardian that the infestation is likely the result of an unusually large grain harvest, which drew more hungry mice to the area's farms earlier in the season than usual.

"They start breeding earlier and because there's lots of food and shelter in the system, they continue to breed from early spring right through into the autumn," Henry said.

Locals have responded by laying extra traps, while one farmer in nearby Queensland was just given permission to use a drone to drop poison bait on the mice from above.

Despite these efforts, Alan Brown, a farmer from the New South Wales city of Wagga Wagga, said that the plague was likely just beginning, considering the rapid pace of breeding in mice; a single breeding pair can produce a new litter every 20 days or so, birthing more than 500 offspring in a season, according to Reuters.

"A mature female can breed every three weeks, they can pump them out," Brown said. "And that's what is going on … it is building up to a massive plague."

In addition to being a nuisance and business threat, mouse plagues can also be vectors of disease, according to the Queensland Government's 1998 report on mice in Queensland.

"Perhaps the commonest [pathogens] are Salmonella bacteria that can be spread by a range of animal species (including humans)," the report said. "The bacteria are usually transmitted to people through food contaminated with infected urine or droppings, and can cause acute gastroenteritis."

Originally published on Live Science.

The "penis bone," or the baculum, is one of the most mysterious structures in mammal biology. To this day, no one really knows what it does or why it's gone missing in humans, horses, elephants and a few other species.

A 3D analysis of 82 penis bones from different animals has now found some of the most oddly-shaped bacula could very well prolong sex, induce ovulation in the female, or shovel out sperm from other romantic encounters.

The penis bone in the male honey badger, for instance, has one of the wackiest appearances. It's shaped almost like an ice-cream scoop, and this development might have evolved to fend off competition and ensure a male's offspring are actually his.

In the study, complex shapes weren't related to the size of an animal's testes, but they were weakly correlated with longer bouts of copulation and induced ovulation, which suggests these bones help to reduce sexual competition.

While prior studies have shown the width of penis bones is somehow tied to more offspring in the house mouse, we still don't know why that is. The theory that these male penis bones somehow prolong intercourse and beat out competition has been brought up before, but the results have been mixed.

Maybe because we've been overlooking one of the most important features.

"The reason natural historians are so fascinated by bacula is because they have lots of unusual features: odd ridges and grooves, strange curvature and bizarre-shaped tips," lead author and biologist Charlotte Brassey from Manchester Metropolitan University said in 2018 about the team's previous study.

"Our study is particularly novel in taking a 3D approach to understanding the evolution of the penis bone. However, all of the previous attempts to study the baculum have simplified the bone into very basic measurements of length and width, and have ignored all this important shape information."

While penis bones vary dramatically in size and shape depending on the species, the findings of Brassey and team's current analysis suggest the most 'complex' penis bones, including those with elaborate tips, hooks, scoops and urethral grooves, are commonly found among carnivores and, strangely enough, monogamous species.

Unlike other penis bones that come to a blunt and abrupt end, it's thought that animals with more elaborate tips evolved under stronger sexual competition. 

"Yet, contrary to our expectations, 'socially monogamous' species are found to possess high values for optimal baculum complexity," the authors write.

At first, this doesn't make sense; monogamous partners would surely face less sexual competition after copulation than those species who mate with multiple males.

Still, the authors explain, social monogamy is not equivalent to genetic monogamy. The African wild dog, for instance, is classified as monogamous, but there's evidence of communal breeding happening on the sly anyway.

While group-living carnivores appear to be evolving toward a more simplified, rod-like baculum, the authors found the penis bones of socially monogamous species are evolving toward a highly complex shape.

Polygamous seals and sea lions, on the other hand, face far less sexual competition because they live in harems where one male mates with multiple females. Interestingly, pinniped bacula end in a relatively simple tip, while the penis bone of wolves and dogs show deep urethral grooves and evidence of bulbus glans attachments. 

The current study, which was based on X-rays of modern museum samples, is one of the most rigorous analyses of penis bones among carnivores. Unfortunately, however, the three-dimensional nature of female genitals has been historically overlooked and is much less understood.

As such, we can't say for sure how the penile bones actually work during sex, so in the future, the team hopes to analyze these structures from within the female's reproductive tract.

Such research could allow us to better understand the function of the baculum during sex, although we might be putting too much emphasis on the bone itself. After all, analyzing one single, ossified element says little about the complexity of the glands or the cartilage that sits on top. 

"This element is currently absent from most museum specimens and hence from our analysis," the authors admit.

"Likewise, our analysis implicitly assumes baculum complexity to be an accurate proxy for penile shape complexity."

Future research also needs to incorporate soft tissue from the penis, so we can better understand how and why the baculum evolved the way it did among different species - and why some species, like ourselves, are lacking one.

"As mammals, and as apes more specifically, it is unusual that humans do not have a penis bone," says Brassey.

"By studying the role of the baculum during mating, we also hope to shine further light on why some mammals, including humans and hyenas, can be so successful at reproducing without a baculum." 

The study was published in the Proceedings of the Royal Society B.

This article was originally published by ScienceAlert. Read the original article here.

When it comes to dating in the abyssal depths of the ocean, appearance doesn't matter much. That's fortunate for anglerfish, which resemble nightmarish fanged potatoes with a little reading lamp on top. And those are just the females.

If you've never seen a male anglerfish before, you're not missing much. Measuring just a few centimeters long on average, male anglers are a mere fraction of their partners' size, and contribute a fraction of the work to their relationships. For many anglerfish species, the male's sole responsibility is to permanently latch onto an obliging mate, fuse his circulatory system with hers, then slowly allow his eyes, fins and most of his internal organs to degenerate until he becomes what biologist Stephen Jay Gould called "a penis with a heart." The male gets constant nourishment; the female gets sperm on demand. The anglerfish circle of life spins on.

It's beautiful, we know. But this unique mating ritual — which biologists call "sexual parasitism" — has long stumped researchers. How could the female angler's immune system even allow such a permanent, parasitic union to occur? Humans have a hard-enough time accepting organ transplants that don't precisely match their own tissues, so how does a female anglerfish's body accept a male's (or, in some cases, up to eight simultaneous males) so willingly? A genetic study published July 30 in the journal Science finally offers an answer: Anglerfish mating is only possible because the fish have somehow evolved away some of their most crucial immune defenses.

"For humans, the combined loss of important immune facilities observed in anglerfishes would result in fatal immunodeficiency," study co-author Thomas Boehm, director at the Max Planck Institute of Immunobiology and Epigenetics in Germany, said in a statement. "We assume that as yet unknown evolutionary forces first drive changes in the immune system, which are then exploited for the evolution of sexual parasitism."

Related: Photos: The world's freakiest-looking fish

In the new study, Boehm and his colleagues analyzed the genomes of 10 different species of anglerfish, including species that fuse permanently during reproduction and species that fuse only temporarily. In both groups, the team found a clear absence of genes crucial to the fish's antibody response — that is, how effectively the fish's immune system is able to find and identify foreign invaders.

For anglerfish that fuse permanently during mating, even more immunological hardware was missing. In addition to lacking even more genes related to antibodies, the perma-fusers also lacked genes responsible for encoding killer T cells, which normally attack infected cells or foreign tissues, the researchers said. Overall, it seemed that evolution had totally deleted the adaptive immune system — the part of the immune response that identifies and attacks specific foreign invaders — from these sexually parasitic fishes.

Strangely, anglerfish seem to have had no problem adapting to the deep sea — an ecosystem with no shortage of parasitic microbes — despite their missing immune machinery. It's likely, the researchers wrote in the study, that anglerfish compensate for their lack of adaptive immunity with a beefed-up innate immune system. In other words, they must have some pre-existing, nonspecific defenses that protect them from a wide variety of pathogens without interrupting their invasive mating process.

It's still unclear what those inborn defenses might be — but, whatever they are, they only make anglerfish an even more unique outlier among the world's vertebrates. It may be hard to believe, but it looks like anglerfish are even weirder than we thought.

Originally published on Live Science.

Coconut crabs, Earth's biggest land crabs, are internet famous from images in which they dwarf trash bins and tear birds limb from limb.

But when these crabs aren't devouring seabirds, they're chatting to each other in vibrating clicks, and scientists recently discovered that the crabs' weird clicking calls are unexpectedly diverse. 

In fact, their crabby chatter contains a range of signals that could represent complex levels of communication (for a crab, at least), according to a new study. 

Related: Photos: Ancient crab is the strangest you've ever seen

Weighing up to 9 lbs. (4 kilograms) and with a leg span of more than 3 feet (1 meter), coconut crabs (Birgus latro) are gargantuan crustaceans and the largest terrestrial invertebrates in the world. These cousins of hermit crabs once inhabited islands across the Indo-Pacific area, but people harvested coconut crabs to extinction in many of their former habitats, scientists wrote in the study, published in the December issue of the journal Zoology.

Previously, the researchers found that the crabs produced "tapping-like sounds," but they were unsure how and why the animals made those noises. For the new study, the scientists captured X-ray movies of the clicking crabs to uncover the source of their acoustic prowess; they also recorded digital audio of the crabs during interactions between males and females, to see if the clicking was linked to mating behavior.

In experiments, male and female coconut crabs clicked before, during and after mating — and the sounds that they made were different at each stage. X-rays revealed that the crabs were communicating by vibrating thin appendages known as scaphognathites, which draw air into the crabs' lungs. When the structures vibrate, they flutter against hard plates in the crabs' gill channels to generate a tapping sound. By changing the structure's vibration speed, the crabs could produce multiple sounds that varied in frequency and intervals, according to the study.

The only other crustacean that produces sound with its scaphognathites is the aquatic crayfish (Procambarus clarkii), and coconut crabs are now the only land crustaceans known to exhibit this behavior, the researchers reported.

Trash can titans?

Long before coconut crabs caught scientists' attention with their clicking, they were renowned for their girth. More than a decade ago, the internet audience was transfixed (and terrified) by a much-circulated photo of a truly monstrous coconut crab that appeared to be the size of a trash can. However, the scale in the photo was misleading, and the crab — though large — was probably not quite as big as it looked, biologist Michael Bok wrote in January 2010 on his blog Arthropoda.

An outdoor trash can (such as the one in the crab photo) typically measures about 4.25 feet (1.3 m) tall, which led viewers to think that the crab was about that length. But the bin in the photo is likely much smaller than average, making the crab look bigger by comparison, Bok explained.

Coconut crab from r/WTF

Even if coconut crabs aren't as long as a trash can, they're still formidable creatures with a pinch more powerful than that of any other crustacean — and even stronger than most animals' bites, Live Science previously reported.

In fact, researchers documented a coconut crab snatching a large seabird from its nest, breaking its wings and ripping it to pieces, Science Alert reported in 2016. Gruesome footage captured by Mark Laidre, an assistant professor in the Department of Biological Sciences at Dartmouth College in New Hampshire, showed a stealthy crab using its pincers to cripple and subdue a red-footed booby (Sula sula) in the Chagos Archipelago in the Indian Ocean.

Though the experiments in the new study only recorded interactions between amorous male and female crabs, their clicking conversation could extend beyond mating encounters, the scientists wrote. However, more tests will be necessary under a variety of conditions in order to decode the extent of coconut crabs' "language," according to the study.

• In images: The amazing world of Antarctic yeti crabs
• Image gallery: Tiny crustaceans found in fossil reef
• A gallery of creatures that molt

Originally published on Live Science.

Evolutionary scientists have been thinking about same-sex sexual behavior all wrong.

That's the implication of a new study on same-sex behavior in animals. Instead of asking why animals engage in same-sex behavior (SSB), researchers should be asking, "Why not?" the authors said. 

If they're right, same-sex sex may not have evolved independently in different animals for adpative reasons. Instead, same-sex sex may have emerged very early in time and could persist simply because engaging in it doesn't cost animals much, evolutionarily speaking.

"Usually, when evolutionary biologists see a trait that's really widespread across evolutionary lineages, we at least consider the idea that the trait is ancestral and was preserved in all those lineages," said Julia Monk, a doctoral candidate at Yale University, who co-authored the new research. "So why hadn't people considered that hypothesis for SSB?"

Related: Alternative Lifestyles in the Wild

In evolutionary science, same-sex sexual behavior has long been viewed as a conundrum: Why would animals spend time and energy doing something sexual that won't pass along their genes to the next generation? And yet, same-sex sexual behavior has been observed in at least 1,500 species, ranging from lowly squash bugs to humans.

(To avoid anthropomorphizing, the researchers don't use the terms "homosexual," "heterosexual," "gay" or "straight" to refer to animal behavior.)

"We can't assign sexuality to animals — we're trying our best to learn about them by observing their behaviors," Monk told Live Science. "And those behaviors shouldn't be mapped onto human cultural and societal contexts."

The assumption that there must be an evolutionary reason for all this same-sex sex has led researchers to search for possible benefits to same-sex behavior. For example, in humans, researchers have found that having a gay son or brother seems to be associated with a woman having more offspring in total. Other studies have posited that same-sex sexual behavior is a side effect of other genes that have reproductive benefits. 

In evolutionary biology, the ability of an animal to reproduce given its environment is called fitness. It's entirely possible that in some species, same-sex sex could have fitness benefits, Monk and her colleagues wrote in their paper, published Nov. 18 in the journal Nature Ecology & Evolution. But these evolutionary benefits may not be required for same-sex sexual behavior to exist. 

Imagine, instead, that the earliest sexually reproducing animals simply tried to mate with any and all members of their species — regardless of sex. This might have been a logical pathway for evolution, because all the bells and whistles that distinguish males from females are energetically costly to evolve. So any effort expended on mating with the same sex would be compensated for by not spending energy evolving and maintaining distinctive secondary sex characteristics, like differing colors, scents and behaviors. Those sex-distinguishing traits may have all come later in the evolutionary chain, the authors argued.

In this formulation, same-sex and different-sex sexual behavior would have started out on an equal footing, early in animal evolution. This could explain why same-sex sex is so common throughout the animal kingdom: It didn't evolve multiple times independently, but was instead part of the fabric of animal evolution from the start.

The new hypothesis undercuts old assumptions about same-sex behaviors, said Caitlin McDonough, a doctoral candidate at Syracuse University and a study co-author. Much of the research done on these sexual behaviors assumes that same-sex sex is costly for animals and that different-sex sex is not costly, she said.

"You really need to go through those assumptions and test the costs and benefits of both behaviors in a system," McDonough said.

If same-sex behaviors go back to the roots of animal evolution, the fact that these behaviors are so common today makes sense, Monk said.

"If you assume a trait like SSB is a new development and has high costs, it's going to be really hard to understand how it could become more and more common from those low initial frequencies," she said. "It would have to have really large fitness benefits, or be otherwise impervious to natural selection, for that outcome to be probable. 

"On the other hand, if you assume a trait is ancestral and was originally common, and it has low costs, it's much more likely that it would remain widespread to this day, even if it doesn't seem to contribute much to fitness."

One piece of evidence supporting this hypothesis is that some echinoderms, including sea stars and sea urchins, engage in same-sex sexual behavior. Echinoderms evolved early in the history of life, likely in the Precambrian period more than 541 million years ago.

But other evidence is slim, largely because scientists haven't systematically studied same-sex sexual behavior in animals. Most observations have been accidental, and biologists have often viewed sex between two animals of the same sex as irrelevant or improper to note, Monk said. Sometimes, researchers automatically assume that same-sex behavior isn't really about sex but instead is about dominance or bonding. And often, if two animals are observed having sex, they're assumed to be male and female without any confirmatory evidence, McDonough said.

"The science that we do is really informed and influenced by cultural biases," she said.

Thinking of same-sex sexual behavior as a standard part of the animal repertoire would change how researchers approach the study of the evolution of these behaviors. The next step, Monk said, would be to gather more data on the prevalence of same-sex behavior in animals. Then, researchers could compare species from across the tree of life to determine if all linages show same-sex behavior. If so, it would strengthen the argument that same-sex sexuality was part of life for the ancestors of all of today's sexually reproducing animals. 

• 20 weird dog and cat behaviors explained by science
• 8 animals that show their love in painful ways 
• There's no such thing as a 'gay gene,' massive study concludes

Originally published on Live Science.

This is the third time that scientists have found an unfertilized egg-laying fish with developing embryos in her belly, the researchers reported in a paper published yesterday (Feb. 20) in the journal Nature Scientific Reports. But it's only the first time that those embryos were birthed and developed into healthy adults

"Although this almost accidental find revealed a vanishingly rare phenomenon, it might help us to understand a really important change that has happened throughout the tree of life," Andrew MacColl, an evolutionary biologist and part of the team at the University of Nottingham in England that made the discovery, said in a statement. "Most animals lay eggs, but some (including almost all mammals, but few fish) retain their eggs inside and give birth to live young. Although this appears to be a difficult thing to achieve in evolution, this one little fish seems to have got there almost by itself." [Photos: The Freakiest-Looking Fish]

Researchers aren't sure exactly how Mary, who is now dead, ended up pregnant this way, given that sticklebacks like her don't have sex with each other. One possibility was that she had cloned herself, and another was that she was a hermaphrodite and so had fertilized her own eggs. But genetic analysis revealed that her babies had two separate parents.

Their best guess is that Mary wandered into a nest where a male had recently ejected a bunch of sperm to fertilize normal, loose eggs that were already there. Somehow, a bit of that sperm must have traveled up Mary's egg tube and fertilized the un-laid eggs inside her.

While the resulting embryos were healthy, it probably wasn't a very good experience for Mary. When researchers found her (as part of existing research into egg-bound sticklebacks in Scotland), she was so swollen with her young that she was "close to death," they reported.

The researchers euthanized Mary with drugs before "destroying her brain" according to animal welfare regulations. Then, they opened her up and extracted her eggs, 56 of which survived till adulthood in a laboratory aquarium. Twenty are still alive three years later, according to the researchers.

Part of what makes the find so remarkable, the researchers said, is that under normal circumstances stickleback fathers play a necessary role in egg development beyond providing sperm. After taking over a nest of eggs, the fathers fan them with their fins as the eggs develop, stimulating healthy development processes. Something about the environment inside Mary seems to have done that job just as well. [The 7 Biggest Mysteries of the Human Body]

One of the two previous examples of fish found in this condition was also a stickleback in the 1950s, though its embryos were not delivered and little is known about how they formed. According to the statement, these researchers are now "actively looking" for more sticklebacks in similar situations during their expeditions to the waters of Scotland.

If they find more examples, it's a big deal. Various animals, including several fish, are known to have independently developed the ability to give birth to live young. Perhaps, this is a window into how that evolutionary leap happens.

• Gallery: Magnificent mantis shrimp
• In photos: Spooky deep-Sea creatures
• 8 animals that show their love in painful ways 

Originally published on Live Science.

Popularized by the character Timon in the animated Disney film "Lion King," meerkats are often viewed as meek animals constantly on the lookout for danger. But within their small groups, these creatures are anything but meek — including when it comes to their mating behaviors.

Meerkats (Suricata suricatta) live in complex, hierarchical social groups or "mobs" consisting of two to 50 individuals. These groups are ruled by a dominant male and female, called the alpha pair, that have exclusive breeding rights. The group also contains subordinate females who are typically closely related to the dominant female; subordinate males who are usually the offspring of the alpha pair; and one or more unrelated immigrant males.

Meerkats reach sexual maturity at 1 year old, and males willingly leave their group permanently at around 2 years old to attempt to join or take over another group. Adult subordinate females, on the other hand, are often forcefully (and sometimes violently) evicted by the dominant female — they'll sometimes remain on the group's territory, sleeping and foraging alone or with other evicted females until the dominant female's aggression towards them subsides. [Strange Love: 11 Animals with Truly Weird Courtship Rituals]

Some females don't return to the group and instead form new groups, sometimes becoming the alpha female, said Tim Clutton-Brock, a zoologist at the University of Cambridge in the United Kingdom. A female may alternatively become alpha by inheriting the position in her natal group upon the death of the previous alpha (by being the largest and most powerful subordinate).

"And it's very common that the breeding males in groups are the ones that have fought and evicted other males," Clutton-Brock told Live Science.

Dominant females don't have much choice in their mate, given that the dominant male will do all he can to prevent her from mating with immigrant males in the group. And the dominant male will usually only mate with the dominant female, because the other sexually mature females in his group are his offspring.

Similar to humans, meerkats are able to breed year-round. But in their native home range in southern Africa, mating often coincides with periods of high rainfall, Clutton-Brock said, adding that "the rainfall is unpredictable but it's lowest between April and July."

Given that mating typically occurs underground, it's unclear if meerkats engage in many courtship rituals. In some cases, the dominant male may fight with the dominant female and grip her by her nape until she subdues and allows him to mount her from behind.

Subordinate females may occasionally mate with the immigrant males from within the group or outside of the group. But this sneaky behavior comes at a price — dominant females routinely kill subordinate females' pups and evict the wrongdoers (sometimes while the subordinate female is pregnant, forcing her to abort).

As payment for their misdeeds, subordinate females that lose their litters or return to the group after being evicted act as wet nurses for the dominant female's pups.

The dominant female has up to four litters a year and will often remain in her position until her death. When that happens, the remaining females will fight for the ruling seat, with the biggest female often coming out ahead. The ultimate winner will experience spikes in testosterone and estrogen, as well as a rapid increase in weight and skeletal size, in the three months after taking over, Clutton-Brock said.

Interestingly, recent research shows that subordinate meerkat sisters in a group actively compete with each other through eating. In experiments, Clutton-Brock and his colleagues fed younger sisters lots of food to increase their size. Older sisters responded by eating more on their own to grow bigger, helping them keep their place in the queue for the crown and the right to breed.

Follow Joseph Castro on Twitter. Original article on Live Science.

The aptly-name red-eared slider is an easily recognizable semiaquatic turtle that's popular in the global pet trade. These animals are considered one of the top invasive species of the world, and it's really no wonder when you consider their mating habits.  

Red-eared sliders (Trachemys scripta elegans), which have a red stripe around their ears and are known to quickly "slide" off objects into the water, are native to the southern United States and northern Mexico. But thanks to the global pet trade industry, they're the most widespread turtle species, having been introduced to dozens of countries, said Greg Pauly, a herpetologist at the Natural History Museum of Los Angeles County.

Though the reptiles are desired as pets for their initial small size and low maintenance, some pet owners face a rude awakening: The turtles get bigger. "The turtle may have only required a 2-gallon tank when you bought it, but after a few years it needs a 100-gallon tank or a backyard pond," Pauly told Live Science. "They're also kind of messy and kind of stinky. So, after a few years [of growth], they become a big commitment." [In Photos: The World's Freakiest Looking Animals]

Some owners take the easy way out by releasing their red-eared sliders into the wild, helping the turtles spread to the urban ponds, streams and reservoirs of every continent save for Antarctica.

In these habitats, male red-eared sliders will engage in courtship behaviors as long as it's warm enough for them to swim around. "They are pretty single-minded in focus," Pauly said, adding that the age at which the animals begin mating depends on their location. In the warmest of climates, males may reach sexual maturity at 2 year old and females 3 to 4 years old; the animals become sexually active a few years later when in colder climates.

Compared with females, male red-eared sliders have extra-long claws on their forefeet. But rather than use these claws as weapons, "Nightmare on Elm Street" style, males use their claws more like jazz hands to woo females.

When a male finds a female, he will swim up close to her, bring his forelimbs forward, and wave or rapidly vibrate his claws in front of her face. He may also use his claws to "tickle" the female's face, Pauly said.

If the female is uninterested, she'll try to avoid the male and swim around him or past him. But often, she won't get very far without running into another male. "The females are harassed constantly by all these males that are courting them," Pauly said.

Sometimes, the female will decide she's had enough and will pull her head down into her shell. But males, especially the older ones, don't always take "no" for an answer and may bite at the large fold of skin around the female's skin that's still sticking out, possibly wounding her.

It's not clear how females choose mates, if they have any choice at all.

But if the female is receptive, she'll allow the male to climb onto her back while the pair are still in the water. The male will use his forelimbs to hold onto her shell and his backlimbs to brace himself as his tries to position his cloaca (waste and reproductive orifice) as close to hers as possible.

Male red-eared sliders, like other turtles, have a large penis to body size ratio. During mating, the cloaca everts and engorges through hydrostatic pressure to become a penis that's 30 to 40 percent of the length of the turtle's body. Unlike the mammalian penis, which has a tube to transfer sperm, the turtle's penis has a channel-like groove that the sperm moves down.

Much of the male's exceedingly long penis goes into the female during copulation, which lasts up to 15 minutes.

After mating, females may store the male's sperm for an extended period of time before deciding to use it to fertilize her eggs. In fact, she could mate again and use the sperm of multiple males for a single clutch. 

"For turtles, red-eared sliders are surprisingly prolific," Pauly said. That is, a female will lay up to 30 eggs in a terrestrial nest. And if the conditions are right, she may lay up to five or six clutches in a single year.

Follow Joseph Castro on Twitter. Follow us @livescience, Facebook & Google+.

The western toad is seemingly unremarkable, lacking conspicuous characteristics that set it apart from other toads. But what it lacks in physical appearance it more than makes up in peculiar mating behaviors.

The western toad (Anaxyrus boreas) is a wide-ranging species that can be found throughout the western United States (including Alaska), Canada and Baja California, Mexico. "If you've seen a toad in the western states, it's a good chance it was a western toad," said herpetologist Greg Pauly at the Natural History Museum of Los Angeles County.

The animals typically breed anywhere between late winterand early summer, depending on location. Similar to many other species of frogs and toads, western toads are so-called explosive breeders, in which a large congregation of animals mate in a very short period of time, typically after heavy rainfall.

Western toads prefer to breed in shallow, calm aquatic habitats, including river backwaters, geothermal springs and ponds. Males migrate to these breeding sites first and find a good spot to wait for females to arrive, particularly places where the water moves slowly and there is emergent vegetation for egg attachment.

Unlike most other frog species, male western toads don't have the vocal sacs necessary to make advertisement calls to claim territory, attract mates and signal their physical fitness. Instead of calling females to them, males hop through the breeding site in active search of mates.

But the males aren't very good at detecting female western toads. "They basically jump on everything that seems reasonably toad-like," Pauly told Live Science. The horny toads will try to latch onto everything from debris like beer cans and tennis balls, to researchers' boots and hands, to salamanders and other frog species, to clumps of mud. The vast majority of the time, however, the male toads' victims are other male western toads, which quickly give off a warning chirp to be released.

Every once in a while, the male western toad will find a female western toad. When he hops onto her back, he wraps his front arms around her and grips the front of her torso in a position called amplexusor the "love grab," Pauly said. Specialized nuptial pads on his forefeet increase friction and allow him to hold on tight.

Sometimes, more than one male will find and hop onto a female, resulting in a "mating ball," with each male wrestling for access to the potential mate. Females can drown underneath these mating balls if enough males join the party.

Females don't just accept whoever comes to them first. If a female doesn't like the male on her back for whatever reason, she may refuse to release her eggs and will even try to dislodge him by crawling under an exposed root (females don't have "release" chirps like males), Pauly said.

When ready to breed (typically a couple of hours to a couple of days after amplexus begins, with the male stuck to the female's back the entire time), the female will simultaneously release two strands of eggs, one from each ovary. As she releases her eggs — between a few thousand and 18,000 of them — she'll move through the shoreline and string the eggs to vegetation, blades of grass, and sticks, among other things. The male will release his sperm as the female releases her eggs.

Once all the fertilized eggs are out in the environment, the mating pair will part ways, leaving their developing babies to fend for themselves.

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Given the helical, horn-like tusks protruding from the front of narwhals' heads, it's no wonder why the whales are dubbed the "unicorns of the sea." And these Arctic-loving animals are just as mysterious as their mythical counterparts, especially when it comes to their mating behaviors.

Narwhals (Monodon monoceros) are medium-sized whales that are closely related to the beluga whale, which is part of the same taxonomic family. The whales spend their lives in the Arctic waters of Canada, Greenland, Norway and Russia.

"Narwhals are highly migratory and this migration is closely linked to the formation of the sea ice," says narwhal researcher Kristin Laidre of the Polar Science Center at the University of Washington. "Their summering grounds become frozen over in autumn, so they make a long migration to offshore areas where they spend the winter." [Drone Footage Solves Mystery Purpose of Narwhal’s ‘Unicorn’ Tusk | Video]

The cetaceans mate at these wintering grounds some 200 miles (322 km) offshore. But the finer details of what goes on during this mating season are largely unknown due to the difficulties of observing the animals in their natural habitat, which is covered in dense ice that has just a few percent of open water, Laidre told Live Science.

Female narwhals reach sexual maturity around 8 or 9 years old while males begin mating anywhere between 12 and 17 years old. It's unclear how exactly the whales attract mates or what's involved in sexual selection or courtship.

Unlike large baleen whales, narwhals don't produce songs. They do use echolocation — which recent research shows the whales can focus into a highly precise beam, Laidre said — and make a range of whistles, clicks, grunts and squeaks, which vary between seasons. But researchers have yet to identify any noises specific to mating or courtship, she added.

Comparatively, males' conspicuous tusks appear to be involved in breeding. These tusks, which protrude 6.7 to 9.8 feet (2 to 3 meters) out of the upper jaw, are very rarely found on females, and some males grow two tusks. In 2014, scientists discovered that males with longer tusks have more massive testes, suggesting the appendages help males attract mates. "The tusk is an indicator of fitness as a mate," Laidre said, adding that scientists haven't been able to genetically show that males with longer tusks are more reproductively successful because it's difficult to biopsy (take a tissue sample) from a free-ranging narwhal.

During the summer months, researchers have observed male narwhals touching and crossing tusks (though not necessarily in an aggressive manner) while making somber-sounding whistles, often times with a female between them. This behavior suggests the tusks may also be involved in establishing dominance hierarchies.

Recently, drone footage has shown males using their tusks to stun prey.

Virtually nothing is known about how copulation occurs or how long it lasts because it's never been observed, Laidre said. But scientists know that female narwhals have a 14-month gestation period and give birth to a single calf about every 3 years.

The oldest-known narwhal is over 100 years old and female narwhals are likely reproductively active for most of that time — researchers estimate the whales stop breeding at around 70 years old.

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Female dragonflies say no to sex by faking their deaths, new research has found.  

Once a female dragonfly has mated, all she is interested in doing is laying eggs and getting on with her life. So, when stalked by an unwelcome lover (or two), she crashes to the ground and plays dead. When the duped males eventually leave, the female flies off, according to the research, published online April 24 in the journal Ecology.

In summer 2015, Rassim Khelifa, a Ph.D. student in the Department of Evolutionary Biology and Environmental Studies at the University of Zurich, was out in the Swiss Alps, collecting eggs of the moorland hawker dragonfly (Aeshna juncea). His aim was to study the effects of climate change on dragonflies by exposing larvae hatched from these eggs to varying temperatures back in his lab. All that time spent by the pools watching female dragonflies and scouting out eggs led him to discover their death-feigning behavior. [Animal Sex: 7 Tales of Naughty Acts in the Wild]

"It was a byproduct of my research," Khelifa told Live Science.

He found that the first strategy that females use to go unnoticed is to choose an egg-laying site that is concealed by plants. Even so, the females can be harassed by males on their way to and from the site. That's when they use the other strategy of faking their own death, he said. 

Out of a total of 35 female dragonflies that Khelifa observed, 27 females crashed to the ground or into shrubs, feigning their death, and of those, 21 succeeded in tricking the males. About 71 percent of those that crashed preferred dense vegetation cover to open ground for staging their death.

Hovering males rely on movement and color to detect females, so they were unable to spot the ones lying frozen on the ground or hidden away in the grass. All of the females that kept on flying instead were grabbed by the males midflight, Khelifa found.

The death-feigning females were fully aware of their surroundings; when Khelifa tried to catch them, 87 percent of the females got away.

Feigning death as a strategy for evading predators is seen in many animals, including dragonflies. Khelifa thinks that the moorland hawker females use the same approach to avoid sexual conflict with males. Mating with one male can fertilize all of the eggs of a female dragonfly and any subsequent mating with males that tend to be aggressive can harm the female, he said.   

"When a male copulates with a female, it first extracts the sperm of the previous male," Khelifa said. "This can damage the reproductive tract of the female. So, multiple mating is not advantageous for the female."

Original article on Live Science.

Between their hard shells and strong pincers, American lobsters are built to fight and keep other creatures away. But does this combative, standoffish nature extend to mating?

There are two general groups of animals called lobsters: clawed lobsters, which live in high-latitude, cold-water regions; and spiny lobsters, which are clawless and live in warmer sub-tropical waters. Clawed lobsters and spiny lobsters are not closely related.

Clawed lobsters, including the American (Maine) and European lobsters, typically live in small, hierarchical groups, said biologist Jelle Atema, who studies American lobsters at Boston University and the Woods Hole Oceanographic Institution. Males vigorously fight each other to be the dominant male of the group, though it’s a short-lived title given that lobsters remember whom they've fought for no longer than a week.

For the most part, these contests establish which crustacean gets the best shelter — something also important for females. "Females fight just as much as males," Atema told Live Science. "We think they have a dominance hierarchy, as well."

The soap opera

Female lobsters will usually mate only after they molt, which typically occurs during the warm, summer months, particularly the middle of June. If a female were to mate with her hard shell on and then molt sometime afterward, she may lose stored sperm, which she keeps in a receptacle at the bottom of her thorax, or she could even lose fertilized eggs, which she holds underneath her tail. 

How often a female will molt — and be receptive to mating — depends on her size, with the smallest of mature females molting every year and the largest every several years, Atema said.

As the female nears molting time, she will search for a suitable partner. While sitting in their rock shelters, males will use the swimmerets (appendages resembling small fins) under their abdomens to create powerful currents that shoot out into the environment. These currents are loaded with chemical cues that attract females looking to mate.

Females appear to know who is the dominant male in a group, possibly by his odor or physical size, Atema said. If a female impatiently decides to go into the shelter of a subordinate male (because the dominant male is already shacked up with another female, for example), the dominant male may eventually come by and kick the subordinate male out of his home, possibly resulting in the female losing out on her mating chances or from being protected while she molts.

"It's sort of like a soap opera," Atema said.

Lobsters, by nature, are aggressive and territorial, but females have a crafty weapon up their shells: pheromones, which not only reduce male aggression but also reflexively close males' claws.

After wafting her pheromone-laced urine into the dominant male's shelter, the female will eventually work up the courage to enter his domain. In the male's shelter, the pair will playfully "box" or tap each other's claws, and then hang out peacefully until the female is ready to molt, possibly a few days later.

A tender affair

Mating among lobsters is a tender, human-like affair.

"When it's time to molt, the female does something very remarkable," Atema said. "She will go up to the male and place her claws on his so-called shoulders — next to the eyeballs on top of the carapace — and then take her claws back." This gentle act tells the male that she is ready to molt and mate.

The female will then lie on her side, shrink her soft body away from her exoskeletonshell, and then slip out of her shell, a process that takes about 15 minutes. Meanwhile, the male stands over her, touching her with his antennae and smelling her.

Once "undressed," the female will lie down right-side up with her abdomen and claws stretched out. The male will carefully begin to mount her and, using his walking legs and mouth parts, will turn the female over so that she is splayed out on her back. He'll then insert his first pair of spinnerets into the female's seminal receptacle cavity for a few second to pass his sperm to her.

After mating, the female will absorb water to grow in size and prepare for her new, larger shell to harden over. She'll spend the next week or so in the male's den while her shell hardens. Once armored, the female leaves the male's home, making way for another female to take her turn.

Many months later, the female will push thousands of eggs out of her ovaries and through the sperm receptacle, where they're fertilized. She'll carry these eggs, glued to the bottom of her tail, for the next 9 to 11 months.

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For some people, the mere idea of a bed bug infestation can be shudder-inducing. But if bed begs' penchant for feeding on your blood while you sleep isn't enough to get your skin crawling, know that the sexual behaviors of these tiny, parasitic insects are even more cringe-worthy.

Though bed bugs have been making headlines since the early 2000s, they've been pestering people for thousands of years. "They were originally pests of bats in caves and when humans moved in, we kicked the bats out," said entomologist Dini Miller, a professor of urban pest management at Virginia Tech. Bed bugs started to feed on people, who transported the parasites out into other environments.

Today, bed bugs jump from home to home the same way they've done in the distant past: by hitching a ride on people's things, such as clothes and bags. Just one breeding pair — or even a single female that's already mated — can create a whole new infestation. [Bed Bugs: The Life of a Mini-Monster (Infographic)]

Bed bugs breed year-round, but there seems to be some seasonal variation. "We do know that populations seem to double and triple in summer months," Miller told Live Science, adding that high moisture levels due to humidity may be involved in these population spikes. "Or maybe they just feel sexier."

While in a house or apartment, the insects tend to hide together in shelters called harborages, such as in the cracks and crevices of furniture, wallpaper and box springs. They'll become active at night if they sense people or animals breathing. "When [carbon dioxide] increases in the atmosphere, that lets bed bugs know that food has arrived," Miller said. "It's like smelling bacon at a distance." The bugs will wander around in search of a blood meal, and if they come within 3 feet of such a meal, they can zero in on body heat.

Bed bugs will gorge on blood for up to 10 minutes, filling themselves up with enough blood to last for 3 to 7 days. Feeding puts the insects — especially mature males — in the mood to mate. Once back at the harborage, males will try to mate with mature females, other males, and even immature bed bugs (nymphs), Miller said.

Mating among bed bugs is an unromantic — if not horrific — affair.

Bed bugs have no courtship rituals. What they have, instead, is a type of mating behavior called traumatic insemination. That is, a male will simply climb onto a female, stab her in the side of her body with his hypodermic penis, and release his sperm into her body cavity. Over the next several hours, the sperm will migrate to the female's ovaries.

Interestingly, females have evolved a counter-adaptation to better handle traumatic insemination: a kind of secondary genital structured called a spermalege, which contains elastic proteins and is located in an area of the abdomen that males most often penetrate. These proteins, called resilins, make it so that the spermalege is easier for males to puncture, resulting in less body damage and fluid loss for the female. Males don't have a spermalege; rather, they release an alarm pheromone (acting as a mating stop sign) when another male tries to mate with them.  

Still, traumatic insemination wounds the female, leaving scars. Because of this painful and dangerous mating behavior, a female will leave the harborage and seek shelter elsewhere after being stabbed by several males, Miller said.

Mating with more than one male is not beneficial to the female anyway, as a single male can provide her with enough sperm to lay several fertilized eggs daily for up to 10 days. What's more, females that mate only once — and are not subjected to repeated stabbings — produce up to 25 percent more eggs than those that mate repeatedly, Miller said.

After laying all of her eggs, the female will need to feed and mate before being able to reproduce again. Depending on who's around, the female may even mate with her own offspring. Though, in contrast with most other animals, inbreeding in bed bugs does not appear to be genetically detrimental. 

Original article on Live Science.

Endemic to the Americas, hummingbirds are defined by their small stature and dart-like movements. But when it comes to mating, do these aerial experts keep things equally quick or do they instead take the slow and steady approach?

Given hummingbirds' propensity to congregate around flowers and artificial feeders, one might expect that they're gregarious creatures, but this is far from the truth. In fact, hummingbirds are generally solitary, territorial birds, and most of the time they can be quite aggressive to one another, regardless of sex. "Hummingbirds are mean to everybody," said Kristiina Hurme, a behavioral ecologist at the University of Connecticut.

Related: Animal sex: How birds do it

Hummingbirds typically limit their social interactions to feeding and mating. The breeding season varies between species, but it often coincides with the rain, which causes a spike in the abundance of insects, said Alejandro Rico-Guevara, an ecologist and evolutionary biologist with the University of Connecticut and University of California-Berkeley. This rich source of protein is needed for plumage molting and egg production, as well as feeding chicks. [Photos of Hummingbirds from Around the World]

How hummingbirds go about mating also varies. "Every single species has a special ritual or mating display," Rico-Guevara told Live Science. "And they are super complicated."

Forest-dwelling hermit hummingbirds (those of the subfamily Phaethornithinae) often adhere to a so-called lek mating system, in which males gather in an open area to try to woo females, which visit the males one-by-one. The males, which maintain small territories, start by chirping. This sound is seemingly simple to human ears, but it actually contains layers of complexity when slowed down with computers. "[Hummingbirds] may not only be able to see in high speed but also listen in high speed," Rico-Guevara said.

An impressed female will perch near a singing male, prompting him to perform tricks to further entice her. These alluring moves may include making sounds with his bill, flying around the female, flying side-to-side in front of her, and displaying his tail and his feathers.

Fights among competing males aren't uncommon. Many species have even evolved a kind of beak weaponry — serrated tips that resemble teeth — that aid them in these fights. During these battles, males will try to stab and bite each other, pluck off each other's feathers, and engage in a kind of aerial fencing, Rico-Guevara said.

But, Hurme added, it's not clear if these fights result in serious or fatal injuries because of the speed at which they occur and the fact that the fighting pair will often fly away.

Many other types of hummingbirds don't use a lek mating system. Instead, males will set up individual home ranges that they defend against other hummingbirds. In some cases, a male will leave his home range to find females, but more often than not it's the other way around, with a female wandering into a male's territory after hearing him sing, said Christopher Clark, a biologist at the University of California-Riverside.

When a female arrives, the male will display for her and show her how well he flies. A common move is called the shuttle display, in which the male will hover in front of his potential mate and slowly move side-to-side and up and down, Clark told Live Science.

One of the more spectacular displays is called the courtship dive, a move that only a small fraction of hummingbirds performs (those in the bee hummingbird clade). This dive involves flying high above the perched female and then quickly dive-bombing past her. When at the lowest point of the dive, the male will spread and close his tail feathers, allowing air to flow through them, which causes them to flutter and produce a sound.

"Each species has a unique tail shape with a unique sound," Clark said, adding that there's strong indirect evidence that the sounds are important to females, though it's unclear what type of tail chirps that females prefer most. It's also unknown which physical features (coloration and body size, for example) females find most attractive.

Once a female chooses a mate, she will allow him to climb onto her back; the male will then align his cloaca (waste and reproductive orifice) with hers and transfer sperm. It's a quick affair lasting just a matter of seconds.

Deed done, the pair will go their separate ways, Hurme said, adding that "the males provide no parental carewhatsoever."

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Think of it like online dating, but for primates: A Dutch zoo is using a series of photographs on a screen to help one orangutan kick off the mating process.

Breeding programs often involve international partnerships and long-distance travel of potential mates, so the Apenheul primate park in Apeldoorn, Netherlands, recently launched a four-year experiment dubbed "Tinder for orangutans." Rather than hope for the best once a male arrives at the zoo, researchers are first presenting pictures of the potential mates to a female orangutan to see how she reacts.

"Often, animals have to be taken back to the zoo they came from without mating," Thomas Bionda, a behavioural biologist at the zoo, told Dutch broadcaster NOS. "Things don’t always go well when a male and a female first meet." [Animal Sex: 7 Tales of Naughty Acts in the Wild]

Beyond hopefully increasing the chances of a successful match, the researchers will study how female orangutans make their mating choices. Bionda said the research is part of a broader program to better understand the role of emotions in animal relationships.

At Apenheul, an 11-year-old female orangutan named Samboja will be shown photos of male orangutans on a touchscreen tablet. The researchers hope that Samboja will respond strongly to one of the photos, signaling her preferred mate.

However, it's not as simple as asking Samboja to swipe right or left. The orangutan's responses are less gentle, and the first tablet successfully tested was reinforced with a steel frame, Evy van Berlo, an evolutionary psychologist, told local paper Tubantia.

Once the researchers have a strong enough screen, Bionda told NOS that the scientists can investigate whether Samboja's response to a male's appearance is enough to build attraction and lead to successful mating between the animals.

"This is completely digital, of course," Bionda said. “Usually, smell plays an important role too. But with the orangutans, it will be what you see is what you get."

Original article on Live Science.

According to the Chinese Zodiac, 2017 is the year of the rooster, an animal that signifies hard work, diligence, and confidence, among other things. But in the wild, do roosters exude these qualities when looking for love?

The chickens found in coops and farms throughout the world were domesticated thousands of years ago from the red junglefowl, a tropical bird found in various parts of Asia, with some hybridization (crossbreeding) with some closely related species, particularly the gray junglefowl. Though chickens are considered a subspecies of the red junglefowl and the two animals look similar, they have quite different social and reproductive behaviors.

On the Island of Kauai in Hawaii, however, live thousands of feral chickens — once-domesticated birds that have reverted to a wild state — that provide a unique look into how domestic animals and their genes respond to the natural environment. Recent research shows that these birds are hybrids of the red junglefowl-like chickens that Polynesians brought to Hawaii and the more modern domesticated chickens introduced to Hawaii by European and U.S. settlers. It's thought that hurricanes that hit the island in 1982 and 1992 released chickens from people's backyards and into the forests, where they met and bred with the remnants of the Polynesian junglefowls (Kauai lacks imported predators like mongooses, which wiped out the ancient birds from the other Hawaiian Islands).

"The feral chickens on Kauai, based on casual observations, behaviorally span a continuum from chicken-like to the more classic red junglefowl," said Eben Gering, an evolutionary ecologist at Michigan State University, who is studying Kauai's wild chickens.

Mixed behaviors

In their native forests, red junglefowl roosters live in set home ranges that they protect from other junglefowls. These males will typically have multiple females they watch over, as well as sometimes one or two subordinate males. The forests and undeveloped areas of Kauai reveal a similar social structure, with small groups consisting of one or two males and a few females.

But like their domestic cousins, the chickens in more urbanized areas of Kauai appear to be far more tolerant of other chickens (though they do occasionally show aggression to each other). These chickens come out in huge mobs whenever food is available, such as if someone throws leftovers on the ground. "It's hard to impress how many chickens can appear from nowhere," Gering told Live Science.

Red junglefowls are strongly seasonal breeders; domestic chickens, on the other hand, mate and lay eggs year-round. Kauai's chickens appear to follow a mixed breeding pattern — while they do breed throughout the year, their breeding behaviors have seasonal peaks. Over time, Gering said, the feral chickens may eventually adopt a more red junglefowl-like breeding behavior, as they divert their energetic investments from rapid growth and reproduction to improved immunity and physiology (traits that would allow them to better survive in the wild).

Gering and his research colleagues haven't studied the feral chicken's courtship and mating behaviors in-depth, but they have made some observations.

"We do see the same behaviors that poultry farmers see in their flocks," Gering said, adding that males have ritualized displays and behaviors that are involved with mate attraction and monopolization.

A show of bravado

One common courtship behavior is called "tidbitting," in which a male will pick up and drop a piece of food (or pretend he has a piece of food) to get the interest of a female, while also making "cute" sounds, Gering said. The male may also do a "waltz" — he'll perform a kind of back-and-forth strut while tapping one wing against the ground.

A wing-flapping display — wherein the male plucks at his neck feathers to improve appearance, and then leans back, puffs up his wings, and crows — may follow, though males often use this smooth move immediately after mating. Females are able to eject spermthat they're not excited about (such as sperm from subordinate males), so the wing-flapping display may help convince her to keep her mate's sperm or reject mating attempts from other males. 

It's unclear which qualities feral females favor most in their mates, but Gering is hoping to study this soon. In both domesticated chickens and red junglefowls, at least, females pay close attention to males' eye color, neck wattle, body size, and most importantly, comb size and brightness.

Compared with all of the strutting, tidbitting, crowing and bravado, copulation in the feral chickens is a rather unexciting event. The male will simply climb onto the female's back while she crouches down, grab at the back of her neck to help him hold on, and align his cloaca (waste and reproductive orifice) with hers to pass on his sperm. "And it lasts all of two seconds," Gering said. Females, seemingly unfazed by the event, will go right back to eating afterward.

Interestingly, many modern breeds of domesticated chickens, unlike their wild ancestors, don't brood, or sit on the eggs to incubate and protect them. But the Kauai population has reclaimed this act, helping their eggs better survive in the wild, and Gering's research suggests the behavior is tied to the expression of certain red junglefowl genes.

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Too much monkey business? A young male Japanese macaque — a tailless monkey with distinctive cheek pouches — was recently observed mounting and performing sexual actions on the backs of female sika deer on Japan's Yakushima island.

The monkey clambered onto the deer and thrust his pelvis repeatedly, in typical copulatory fashion, and chased away other monkeys that approached the deer, researchers described in a new study.

Although sexual monkeyshines between different species are relatively common in the animal kingdom, this behavior is most frequently seen between species that are closely related, or between distant species that are in captivity, according to the study authors. This male macaque was part of a group of males on the island that were of lower social standing, and his affinity for hoofed partners might stem from restricted access to available females, they said. [Animal Sex: 7 Tales of Naughty Acts in the Wild]

On the island, macaques, also known as snow monkeys, live side by side with sika deer. The deer feast on fruit that the monkeys drop from trees, while the monkeys groom the deer and are even known to ride them, much as people ride horses, scientists have found.  

However, this is the first known instance of a monkey behaving sexually with a deer, and only the second case of sexual interaction between two species that aren't closely related — the first was in 2008, when researchers observed an Antarctic fur seal trying to have sex with king penguins.

Let's get it on

The unusual activity between the deer and the monkey was first spotted by study co-author and photographer Alexandre Bonnefoy, who was observing a group of macaques on Yakushima. On Nov. 6, 2015, Bonnefoy noticed that an adult male macaque was trying to copulate with two different female deer, guarding them against advances from other males and charging aggressively at curious macaques to drive them away.

Bonnefoy's photographs and video showed the monkey climbing atop a female deer, initiating copulatory movements and hopping off after about 10 seconds. Penetration did not appear to occur, but the deer licked herself clean after the monkey dismounted, suggesting that he had ejaculated on her back, the study authors wrote.

A second encounter was less successful for the amorous primate. This time, the female deer he chose was a less willing partner, bucking and dodging to shake the monkey off.

All in the family — or not

There are several possible explanations for why a macaque would approach a deer with sexual intent, according to study co-author Cédric Sueur, an associate professor in the Department of Ecology, Physiology and Ethology at the University of Strasbourg in France.

Sometimes, interspecies sex happens because animals simply don't recognize their partner as belonging to another species — but that generally applies to closely related species, and is a highly improbable scenario for a pairing of animals as physically different as a monkey and a deer, Sueur told Live Science in an email.

"The most realistic hypothesis would be the 'mate deprivation hypothesis,'" Sueur said, "which states that males with limited access to females are more likely to display this behavior."

As young male macaques reach puberty, they are frequently excluded from social groups by the dominant male and may have limited contact with females until they find a new group to join. This can take time, making them more likely to exhibit sexual behavior toward females outside their species, Sueur said.

"As a consequence to not have access to females, these peripheral males could socially learn to have sexual interaction with sika deer in order to decrease their sexual frustration," Sueur told Live Science.

However, further observations would be required for scientists to determine how interspecies sexual activity could affect the breeding fitness of macaque males in the long term, and for researchers to better understand how the behavior emerged in the first place, he added.

The findings were published online Jan. 10 in the journal Primates.

Original article on Live Science.

The armadillo is one of the few mammals covered in defensive body armor — in fact, armadillo means "little armored one" in Spanish. Their protective plates shield them from the attacks of potential predators, but are armadillos just as guarded with their hearts?  

There are 21 species of armadillos across the globe. The nine-banded armadillo is the only armadillo species found in the United States, and it's also the state small mammal of Texas. This species is named for the bands of plates on its back; they have an average of nine bands, but the actual number varies among individuals of the species.  

Nine-banded armadillos are mostly solitary creatures and usually come together only during the breeding season, which lasts from early summer through autumn, said Colleen McDonough, a behavioral ecologist at Valdosta State University in Georgia who studies nine-banded armadillos. Adults set up small home ranges that may overlap with those of other adults, and they may have low levels of interactions in these overlapping areas, she said. [Animal Sex: 7 Tales of Naughty Acts in the Wild]

"They may be feeding near one another but not with one another," McDonough told Live Science. "They have a pretty good olfactory system, so they likely know who is around and who should be around."

The mating behaviors of nine-banded armadillos are not well known, but McDonough has made some careful observations of the animals during the breeding season. During the breeding season, males use their powerful noses to find a female with which to "pair." It's most common to see males pair with females whose home ranges overlap most with their own, McDonough said.

Pairing behavior is a kind of courtship ritual in which the male tries to remain in close proximity to the female (within a few meters) at all times. If the female is unreceptive, however, she'll do all she can to get away, including kicking at him with her hind legs, according to McDonough. But females are generally much more tolerant of males during the breeding season.

If another male comes up to a couple, the paired male will rush off to meet him; in some cases, he will simply chase the newcomer off, but other times, a duel can ensue. These battles can get vicious, with the males attacking each other with their formidable claws.

"There was this one time where I did see one male usurp the other [paired] male," McDonough said.

Aside from remaining close by his chosen mate and protecting her, the male will engage in other smooth moves, such as touching the back end of her carapace with his front paws, sniffing her genitalia, and bringing his body into physical contact with hers. The female will occasionally lift her tail (exposing her genitals) and wag it from side to side. The pair may also forage near each other and make low "chuck" sounds to each other that can be heard only within a 15-foot (5 meters) radius. [The 7 Weirdest Animal Penises]

Overall, these behaviors ramp up and intensify as the pair gets closer to mating, with the male touching the female more and becoming more focused on her genital area, McDonough said.       To mate, the female lifts her tail, and the male mounts her from behind.

Interestingly, nine-banded armadillos have delayed implantation. In other words, depending on when the female mated, her eggs may not implant into her uterus for development for several months; this delay allows her to time the birth of her quadruplets with the coming of spring.

In one unusual case, McDonough said, pregnant females were transported from the United States to England for leprosy studies. "The females were alone for at least a year and a half," she said. "But then, they gave birth."

Original article on Live Science.

In late September 2016, the U.S. Fish and Wildlife Service (USFWS) added seven of Hawaii's yellow-faced bee species to the Endangered Species List — the first time any bee has been declared endangered. What do we know about their sex lives and could this information be the key to saving these rare bees?

In Hawaii, there are more than 60 species of yellow-faced bee (genus Hylaeus), a solitary type of bee that lives in a wide range of habitats, from coastal areas to high-elevation forests. These bees are the primary pollinators of a common Hawaii shrub called naupaka, which blooms half-flowers and is the focus of a Hawaiian story about star-crossed lovers who are fated to be forever separated.

Though scientists have long been aware of the bees and their importance, "there's virtually nothing known about the mating behaviors of yellow-faced bees," said Sheldon Plentovich, the Pacific Islands Coastal Program Coordinator for USFWS.

That is to say, there have been no rigorous scientific studies on the bees' reproductive behaviors, so everything scientists currently understand about this topic is based on anecdotal observations.

Ambiguous mating game

Yellow-faced bees are active throughout the year, though they do seem to have peaks and lulls between seasons. "There are a couple months in the winter where their activity declines," said Jason Graham, a University of Hawaii entomologist whose research focuses on the species H. anthracinus. "You can still find adults, but not nearly as many."

Unlike with honeybees, which form large colonies ruled by a single reproductive queen, all yellow-faced bee adults are reproductively active. Additionally, while honeybees mate in air, yellow-faced bees mate on a substrate (the surface of something), such a plant leaf or flower.

The mating game begins with a female bee sitting on a plant, generally naupaka or heliotrope, where she may be foraging for nectar or pollen. Up to 10 male bees — which have eponymous yellow markings on their faces, as opposed to the pure black faces of females — will begin hovering in a kind of cloud and flying in erratic, circular motions 6 inches to a foot above the female.

A male from the hovering group will then touch down on the female for less than a second before flying back up to the group; other males will follow suit. It's unclear the purpose of this behavior, but the males may be trying to see if the female is receptive to mating, Graham said, adding that the female "doesn't really pay them any mind."

Eventually, one male will land on the female's back and stay for a while, tapping his antennae against her attennae and the side of her head. Again, it's unclear what function this behavior serves.

After about a minute, and if the female doesn't fly away, the male will reposition himself, moving back a little to curl his abdomen under the female's, allowing him to insert his aedeagus (reproductive organ) into her vagina for insemination. Graham has seen copulations last anywhere between 5 seconds and 6 minutes, while Plentovich has observed the act lasting for up to 20 minutes.

In some cases, a second male may land on the copulating pair before being shaken off. "It's almost like a frenzy in some situations," Plentovich said. Males in the 'cloud' may also try to mount each other in air — Graham suspects this mounting is a form of male-male competition — or "bash" against each other while on a plant, according to Plentovich.

Artificial nests

After mating, the male will fly away, while the female will go about her business. 

Females prefer to build their nests in holes in coral rocks washed ashore or in the hollow stems of naupaka or other coastal plants. The nest cells are made of packed pollen and a kind of waterproof cellophane material the female produces — each nest cell contains just a single egg.

"That is one of the reasons why solitary bees tend to be more threatened than social bees, which lay tons of eggs," Graham said.

To help protect yellow-faced bee nests from invasive ants, which are playing a major role in the bees' dwindling numbers, Graham developed artificial nests. Each nest is a wooden box with pre-drilled holes, which is hung from a branch via a cord that's covered in a sticky material to prevent ants from getting to the eggs.

The bees are successfully using the artificial nests in their natural habitat and the researchers hope to someday use them to reestablish yellow-faced bee populations in areas they once lived. 

Original article on Live Science.

Cockroaches are known for their superior survival skills, but it seems these bugs have another evolutionary advantage when it comes to the mating game: Male roaches can grow bigger testicles, if need be, to woo a mate.

A new study finds that Madagascar hissing cockroaches can adopt different strategies to win females, either evolving as a "lover" with bigger testes or a "fighter" with larger horns.

Roaches compete for females in various ways, with two main approaches being to defend a female by force, or to sneak past larger males to mate. Based on these two strategies, the researchers think that males from two species of giant cockroaches from Madagascar evolved different physical characteristics based on their tactics for winning a female. [Gallery: Out-of-This-World Images of Insects]

"These cockroaches are acting like red deer in the rut, competing for females by combat, but if they don't have the size and strength to win fights outright, they can try and sneak mates," study lead author Kate Durrant, a behavioral ecologist at the University of Nottingham in the United Kingdom, said in a statement. "A male cockroach seems to be adapted to be either a lover or a fighter, and what's interesting is that they do this before they become fully adult, at the final moult."

Durrant and her colleagues used 3D-scanning technology to precisely measure males from two different types of giant hissing cockroach species: the flat-horned cockroach, Aeluropoda insignis, and the wide-horned cockroach, Gromphadorhina oblongonota. The high-resolution X-ray computed tomography (CT) scans allowed the researchers to determine each roach's body length, body volume and the size of their "horns."

These body measurements were then compared to the cockroaches' aggression levels when fighting rival males, and the size of their testes — a sign that the roaches have more of an investment in mating.

The researchers found that the flat-horned cockroach, which is small, with short horns, was nonaggressive and had large testes. These "lover" roaches were more likely to avoid fighting by mating with females behind the backs of larger males. Wide-horned cockroaches, in contrast, were larger and heavily armored with big horns, but with smaller testes — "fighter" roaches.

"These observations support sperm competition theoretical predictions but require field observations of natural mating systems in these relatively poorly known organisms for confirmation," the researchers conclude in the study.

The trade-off between lover and fighter strategies is also seen within each cockroach species, the researchers reported in the study. In other words, individual males with smaller horns typically had larger testes to compensate, the scientists said.

The study was published online Nov. 7 in the journal Scientific Reports.

Original article on Live Science.

Similar in size, shape and coloration to large wolf spider species, the lesser-known dark fishing spider would no doubt give anybody with arachnophobia a decent scare. But the appearance of these eight-legged arthropods pales in comparison to their frightful and cannibalistic mating behaviors.

Dark fishing spiders (Dolomedes tenebrosus), as you'd expect from their name, are nocturnal and live near water. "They're around, but you have to go look for them at night, with a headlight," said Steven Schwartz, a behavioral ecologist and fishing-spider researcher at Gonzaga University in Washington.

Dark fishing spiders reproduce in the summer — males mature the same season they're born, while females take another year or two to mature.

After maturing, a male will spin a small "sperm web," onto which he'll deposit his sperm. Next, he'll "charge" his two pedipalps or palps (small appendages near the mouth) by using them to suck up the sperm. [Goliath Birdeater: Images of a Colossal Spider]

He'll then go off in search of a mate.

Finding and choosing mates

In their natural habitat, male dark fishing spiders can find females — which weigh about 14 times more than males do — quite easily by homing in on silk the females leave behind. (This species, however, does not weave webs.) In fact, Schwartz told Live Science, in tests, 50 percent of captured males were able to find a female within one hour of being released back into the wild. "And one male even contacted five females in an hour," he said.

If given the choice, males will prefer to mate with a virgin female. Sometimes, they will go so far as to hang around females that are about to mature.

This choosiness appears to be linked with their perception of available mates. That is, when presented only with a single female in the lab, a male will try to mate with her, regardless of whether she's a virgin.

But that's only half the story.

Female dark fishing spiders eat their mates after sex. In lab experiments, Schwartz tested whether males would still be willing to mate with a female 30 minutes after she'd mated (and eaten) and three days after she'd mated. In both cases, males had no problem trying their luck — they'd even try to mate with the female while she was eating her first mate.

Given the female's overwhelming size and penchant for cannibalism, you'd expect males to be a bit wary when approaching their potential mates. [Video — Mate, Mutilate, Die: The Curious Life of Male Fishing Spiders]

But it appears that pre-copulatory cannibalism is not common and may be an artifact of the laboratory setting. Schwartz has observed males dazed and running around frantically while under the bright lab lights, causing the female to launch into an attack.

Dying on purpose

When a male makes contact with a female he wants to mate with, he will tap and stroke her legs and abdomen. He will then climb onto her back and bob his abdomen up and down to send courtship vibrations to the female; the female will respond in kind.

"There are signals going back and forth between them — both chemical and vibratory," Schwartz said.

This whole song and dance can last 1.5 hours or longer. To mate, the male will insert one of his palps into the female's genital opening; then, he will inflate the "hematodochal bulb" within his palp, pushing his sperm into her body.

Then, he spontaneously dies, providing a nice meal for his mate.

Unlike with other types of spiders, the bulbs of male dark fishing spiders don't contract after sex, and it's likely that their eventual death is related to this kind of genital mutilation, Schwartz explained.

In a study published online Oct. 24, 2016, in the journal Current Biology, Schwartz and his colleagues sought to understand what evolutionary benefit there could be to the male's self-sacrifice. Turns out, female dark fishing spiders that were allowed to consume their mates produced more offspring that were larger and had higher rates of survival than those that didn't get that post-copulatory snack.

Stranger still, females that were instead given crickets that were similar in size to their dead mates didn't receive those reproductive benefits, suggesting there's something special about the males.

Schwartz is now working on unraveling this mystery.

"We want to do some chemical analyses," he said. "Do we see things that pop up in the males that we don't see in females and crickets — something weird, something we can go after?"

Original article on Live Science.

Nautiluses are physically distinct among cephalopods — the family of marine mollusks that also includes octopuses, squid and cuttlefish — in that their squishy bodies are protected by a hard, spiraled, fully developed shell. But are the mating behaviors of these ancient animals just as unique as their appearance?

Today, there are six living species of nautiluses across two genera, all of which are found in Indo-Pacific waters near the ocean bottom or alongside deep-sea corals. "That's the tricky part to studying them," said Gregory Barord, a marine biologist with the conservation organization Save the Nautilus. "Most of our understanding of nautiluses is based on captive observations because they normally live [at depths of] four-five-six hundred meters."

With that said, Barord added, it appears that nautiluses don't have specific mating seasons. Rather, similar to humans, they can mate at any time of the year after they reach sexual maturity at typically 10 to 12 years old, he told Live Science. [In Photos: Strange Purple Orb Found in Pacific Ocean]

Underwater camera recordings suggest that nautiluses are relatively solitary creatures and not found in groups in their natural habitat, unless they're feeding or mating. In fact, "females are kind of repelled by other females," Barord said.

Driven by scent

Often called "living fossils" because they've survived relatively unchanged for hundreds of millions of years, nautiluses have "primitive" eyes, among other things. But, Barord noted, they have the largest olfactory organs among cephalopods and likely use their powerful sense of smell to find food and potential mates.

Nautiluses are opportunistic feeders and baited-camera research appears to suggest the animal's sense of smell is highly tuned. But once the mollusks congregate on to the bait, they try to mate with each other. [In Photos: See an Octopus Cannibal in Action]

"The weird thing about them is that whenever you get them together, they mate or at least try to mate," Barord said. This eagerness may suggest that mating opportunities in the wild are difficult to come by.

Numerous studies have reported an odd observation about nautilus populations: Males vastly outnumber females, which appear to make up just 20 to 30 percent of populations. But sampling techniques may be to blame — since the animas live so deep, researchers often base their population estimates of nautiluses on trap rates. Female nautiluses appear to avoid other females (based on laboratory studies), so it may just be the case that they stay away from baited traps when they smell other females around, Barord suggested.

Whatever the case, it's not unheard of for a mature female to be hounded by more than one male at a time. In some cases, four nautiluses will be "attached" to one another as they try to mate, but it's unclear what kind of competition is occurring between the males. "It's hard to say what is going on," Barord said. "Are they are all transferring spermatophores [sperm packets] or are they trying to push the first male away?"

What does seem clear, however, is that unlike other cephalopods, which are often known for their dazzling courtship behaviors, nautiluses don't appear to try to woo each other before getting down to business — possibly because the animals invest a lot in scent rather than sight.

A lengthy, mysterious ordeal

Nautiluses have about 90 suckerless tentacles. Male nautiluses also have a composite erectile organ called a spadix, which is made up of four modified tentacles and looks just like "one big tentacle," Barord said.

A male-female pair will mate face-to-face and the male will use his spadix to transfer a spermatophore into an area near the mouth of the female.

Nautiluses typically mate for several hours before going their separate ways, but, like many other aspects of the nautilus's life and behavior, it's unknown what's really going on during that time.

"Is there some type of mate bonding going on there that helps them know who they are in the future?" Barord said. "Does it actually take that long to transfer the spermatophore? Or does it only take a minute and then the male guards the female for 2 to 3 hours? It's one of those things we don't know."

Original article on Live Science.

As both a national treasure of China and the symbol of the conservation-oriented World Wildlife Fund, giant pandas are known by the world over — particularly for their formally low breeding success in captivity. But just what's involved in panda-bear mating, both in and out of the wild?

Giant pandas are solitary bears that live in rather fixed home ranges. They generally come together only during the mating season, which occurs between mid-February and mid-May. "Most females go into estrus mid-March through mid-April," said Meghan Martin-Wintle, an applied ecologist and director of PDX Wildlife, a nonprofit conservation and research organization based in Portland, Oregon.

The China natives communicate with one another through vocalizations and scent markings, and these behaviors ramp up about two weeks prior to females entering estrus, Martin-Wintle told Live Science. At this time, females will go to the outskirts of their ranges and rub their anal area on trees and rocks to deposit smelly secretions from their anogenital glands. [How to Flirt in Panda: Bears' Squeaks Decoded]

Males home in on females' auditory and olfactory cues, and three to four of them will congregate on a single female. However, females ovulate and are receptive to mating only a few days a year. "They have just this one day that is the best and they have a day or two on either side," Martin-Wintle said.

Posturing and positioning

As females near ovulation, they will hang out in trees while males compete for dominance on the ground.

"It's a lot of posturing — they do these barks, like a dog, and they do these growls, " Martin-Wintle explained. And though it may be hard to imagine, male giant pandas do get into physical altercations. "They do lunges and they will come together and do swats and even try to grab onto the nape of each other's necks."

Dominance is usually established pretty quickly. The champion male will then stay close to the female and charge at any intruders, until the female comes down from the tree ready to mate.

Given that researchers rarely observe panda bears mating in the wild, it's unclear what's involved in sexual selection or if females always mate with the dominant male in her home range, Martin-Wintle said. But females in captivity, at the very least, are known to sometimes completely reject males.

For giant pandas, mating is a difficult affair, in part, because male giant pandas have one of the smallest penises relative to body size in the animal kingdom, Martin-Wintle said. A willing female must get into the lordosis posture, in which she lowers her front end, arches her back down and raises her tail end up, putting her vaginal cavity in the right position for the male to enter her from behind. "And once he obtains the position, she has to shove on to him," she said. [The Weirdest Penises in the Animal Kingdom]

After orgasm, both male and female giant pandas bleat, or let out a goat-like cry. The male will then do a "roll back," in which he sits on his butt and lower pelvic area and pulls the female so that she's sitting on him — and holds on as long as he possibly can. It's unknown why the male does this.

After mating, the male will stick around and possibly mate with the female a few more times, until she's no longer ovulating.

Sex in captivity

When captive breeding programs for giant pandas first got started in the 1950s through 1970s, the projects had a very low success rate. There were a few reasons for this failure, Martin-Wintle said.

For instance, scientists didn't understand the bears' communication system or how important it was for mating. The animals were kept separate and only brought together when it was time to mate, so mating pairs weren't able to exchange scent and vocal cues as they normally would in the wild.

Additionally, "we weren't really successful at getting the mothers to raise their cubs, and we weren't good at hand-raisingthem either," Martin-Wintle said, adding that one problem was that cubs must be physically stimulated to defecate.

Today, panda bear captive breeding programs have much higher success rates, but they could still be better. Though these programs make sure to pair adult giant pandas that are genetically separated, they don't take into consideration behavior and mate choice. "We are removing many of the things we know about Darwin and sexual selection," Martin-Wintle said.

In a study published late 2015, Martin-Wintle and her colleagues found that pandas paired with mates they preferred were much more likely to successfully have sex, produce cubs and adequately care for their young. The researchers are now looking at whether allowing males to compete for a female also improves reproductive success, possibly by increasing testosterone and sexual motivation. 

Original article on Live Science.

Live Science is hosting a book giveaway!

"Wild Sex: The Science Behind Mating in the Animal Kingdom" (Pegasus Books, 2016) is a new book by biologist and writer Carin Bondar, who is also the current lead presenter for Discovery World's "Brave New World with Stephen Hawking" and host of the web series "Wild Sex" for Earth Touch Online. She has appeared on Discovery Channel's "Outrageous Acts of Science," and has hosted online and television programs created by National Geographic Wild, Scientific American, and the Science Channel.

On Tuesday (Sept. 20) at 1:30 p.m. ET, Carin Bondar joins us at Live Science to talk about her new book in a Facebook Live event, during which we'll be giving away three signed copies of "Wild Sex" to three lucky viewers. [Tune in on Live Science's Facebook page]

As the Cole Porter love song "Let's Do It" tells us, "Birds do it, bees do it." But the specifics of how they actually, well, do it — and not just birds and bees, but every animal species — can be weirder, more strenuous, and far more complicated than you could ever imagine. Writer Carin Bondar offers a glimpse between the sheets of this particular aspect of animal behavior, in her new book, "Wild Sex," introducing fascinating examples of mating practices. Some may seem a little weird, but ultimately no matter how bizarre the sex, the goal is the same — successful procreation and continued survival of the species.

Details about the giveaway are below. And don't forget to follow Live Science on Facebook and Twitter for updates on this contest and other live video events!

Wild Sex: The Science Behind Mating in the Animal Kingdom Giveaway

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Detachable penises. Genital plugs. Sexual cannibalism.

Dating and mating in the animal kingdom aren't just complicated — they can be fraught with violence and danger. Even so, they're the only game in town. Every species must reproduce, and there are many paths to successful reproduction, though those paths may sometimes be as convoluted as the corkscrew genitals of a mallard duck.

The prospect of exploring mating positions in frogs, porcupines' use of sex toys or hermaphroditic sea slugs' penis spines might seem daunting to some, but not so for biologist and writer Carin Bondar. She leads the way into the intriguing world of animal sexual practices in her new book, "Wild Sex: The Science Behind Mating in the Animal Kingdom" (Pegasus Books, 2016). [Top 10 Swingers of the Animal Kingdom]

From finding a mate, to procreating, to dealing with the successful outcome of mating — offspring — "Wild Sex" investigates the often-harsh realities of sexual behaviors practiced by animals large and small.

Bondar is the current lead presenter for Discovery World's "Brave New World with Stephen Hawking" and host of the web series "Wild Sex" for Earth Touch News Network. She has appeared on Discovery Channel's "Outrageous Acts of Science" and has hosted online and television programs created by National Geographic Wild, Scientific American and the Science Channel.

Recently, Bondar spoke with Live Science about the unusual sexual behaviors and equipment used by humans' closest relatives and most distant cousins on the tree of life, and what people's interest in animal sex reveals about them. [Live Science Book Giveaway: 'Wild Sex' by Carin Bondar]

(This Q&A has been edited for length and clarity.) Live Science: What was it about sex in the animal kingdom that first attracted your attention as a biologist?

Bondar: Ironically, it happened when I was at home enjoying the young products of my own sexual activity — my children. I started blogging, and though I didn't just focus on sexual topics, I noticed engagement was so much higher when the topic was sex — and the crazier, the better! So I rolled with it and went more in that direction. And then I got a call from Earth Touch [News Network] to write and host a series about animal sex, and six weeks later, I was on a plane to South Africa. It just swept me up in its own tidal wave.

LS: For many people, sex is primarily about pleasure. How much is pleasure a factor for animals?

Bondar: Very, very little! Unfortunately, there's a massive dichotomy between sexual cells of males and females, so males have incredibly abundant sperm, and females have very rare and expensive eggs. Generally speaking, males always want to have more sex and get as many partners as they can, and females want to be protective of their expensive treasures and choose carefully who gets to fertilize them.

From bugs to mammals and everything in between, this sets the scene for violence, conflict and war. There's no champagne, no roses. It's very much, "I've evolved these structures to torture you and hold you down so I can have sex with you."

LS: What are some of the risks that animals in certain species face during sex?

Bondar: Bedbugs and some of the nudibranch species use what is aptly termed traumatic insemination, because it literally is a stab wound to the female's body. The sperm will travel from these wounds into her ovaries, so she basically ends up with stab wounds all over her body that can get infected, which could certainly affect her well-being and general health. [Animal Sex: 7 Tales of Naughty Acts in the Wild]

Some canine species have penis bones that get massively inflated during sex, so they can't separate. That's actually an evolutionarily smart thing to do, to not let your female get away until you get sperm in there, and it's also a fail-safe for the male, who can't get out until he finishes the job. But it comes with a huge element of danger from predators.

Then, there are a lot of invertebrates that are hermaphroditic, male and female at the same time. Sometimes they can fertilize themselves, but often they need an actual partner. Taking the male role is more favorable because it's less work, so these sexual encounters tend to become extremely violent.

LS: What about spider and insect species where males are cannibalized by females during sex? That seems like even when a male succeeds at reproducing, he loses.

It seems horrific to us, but it's reproductively clever — these males can actually increase their reproductive success by dying!If the female is busy feasting on his body, she's not out getting more sperm, so his sperm can dominate.

In mammals where sperm is so plentiful, it's a vastly different story than in spider species, where males only have one or possibly two chances to copulate in their entire lives. [101 Animal Shots You'll Go Wild Over]

We see the same thing in those infamous preying mantis videos where the female rips the male's head off — his genitals are still doing the deed while she's busy eating his head and body. His sperm has lots of time to travel where it needs to go, and she's not seeking new sperm. That's kind of a tough one for us to swallow (so to speak), but it is a reproductively sound strategy.

LS: Many species appear to engage in a type of "arms race" when it comes to sex, with female behavior or biology thwarting male advances, and male strategies and anatomy trying to work around that. But don't they want the same thing — to procreate?

Bondar: They do. However, they want it on their own terms.

These arms races are ridiculous. Like in the water strider, males have evolved these crazy structures that fit around eyeballs of the females to keep them in place, and the females have these anti-grasping spring things that they can use to spring males off when they're trying to have sex with them.

We assume as biologists that animals ultimately want to maximize biological fitness, in terms of getting their genetics to the next generation. But the problem is that females don't want just any sperm. They want the best sperm. Males, on the other hand, because it's so easy for them, they can afford a few missteps.

If I'm a male, and I put my sperm out there as often as I possibly can and with whomever, some will be bad mothers or bad children. But I'll do better in the long run. For females, it's a completely different story.

Think of the human case. If I get fertilized, I'm out of reproductive commission for at least another nine months or more, as I then breast-feed, so that takes me out of the reproductive game for close to a year. If I let any man fertilize me willy-nilly (pun very much intended), I'm jeopardizing my own biological fitness, because that's a huge commitment for me. I'm doing this huge thing in my body for the embryo.

LS: I was going to ask for the most extreme example of a sexual behavior, but … there are so many. Is there one in particular that stood out to you when you were researching this book?

Bondar: There's one of those hermaphrodites I was talking about, a sea slug. As many hermaphrodites do, each one is trying to be the male. What this species in particular does is it takes its razor-sharp penis and stabs the partner directly in the forehead. And they're doing this with a degree of accuracy and precision that's unprecedented.

Biologists puzzled about this for long time, until scientists looked at the chemical makeup of what was getting deposited. And they found that there was some kind of neurochemical warfare going on. They were ejaculating into the cerebral ganglia of the receiving partner, so they manipulated the partner to become more female. Since being female is more expensive, it makes sense for them to want to be the male as much as possible.

The other one that I really like is a cannibalistic spider species. Like many other species, they have something called a mating plug, something that plugs up the female's genitals after she's been fertilized. Sometimes males put them in, and sometimes females seal themselves up.

In this species, females take the substance they'd use to seal themselves up, and they use it to ensnare males because they're hungry. They make their genitals into a spider-sized sticky trap. A male comes along, and she'll trap him. He'll die there, and she won't necessarily eat him right way. So she'll be walking around with a dead guy on her genitals until such time as she decides to eat him.

LS: What might all these examples of different animal sex practices mean for readers in terms of understanding their own sexual behavior?

Bondar: The breakdown of the book — trying to find a mate, trying to have sex with your mate and then dealing with the aftermath of the sex — that affects us just like it does every other animal.

It's funny and gruesome and horrible to talk about all these "insert part A into slot B" examples, and people like to hear about that. But the reason people are so interested in sex in other animals is [because] we're so obsessed with it in our own lives. I always like to bring it back to humans remembering that we're part of the animal kingdom, even though we often pretend that we're not.

"Wild Sex: The Science Behind Mating in the Animal Kingdom" was published Aug. 6.

Original article on Live Science.

Sometimes they're mere pests, other times they're dangerous vectors of diseases, such as malaria and the Zika virus. Whatever the case, the lives of both male and female mosquitoes revolve around mating, so just how do the buzzing insects do it?

Across the globe there are more than 3,000 species of mosquitoes. These bite-sized animals live very short lives — with males sometimes living for only about a week and females for up to about 100 days — and employ a range of strategies to get down to business.

In tropical regions of the world, the insects breed year-round, while those in temperate areas breed according to specific seasons. Given their short life spans, "mosquitoes have what we call overlapping generations," said mosquito expert Laura Harrington, an entomologist at Cornell University. "Mating can happen throughout the season but with different age cohorts."

In many species, male mosquitoes are ready to mate within their first few days of adulthood. Females, on the other hand, are generally ready to mate almost immediately after leaving their pupal casings (a pupa is an immature stage of life between larva and adult).

In one genus called Opifex, males take advantage of this rapid female maturity. "The male will actually emerge from the pupal stage and grab the trumpet or breathing tube of the female and then mate with her," Harrington told Live Science, remarking on the fact that males are able to somehow tell which pupa are female and which are male.

In other species, a male will instead zero in on a human host using carbon dioxide cues, where he waits — buzzing around in figure-eight patterns — to intercept a blood-sucking female seeking a meal (males only drink nectar).

Swarming and courtship

Many other species take a group approach to mating, in which males form mating swarms— a nearly cylindrical cloud of flying male mosquitoes. It's unknown how these swarms form, but they tend to aggregate around some visual aberration of the environment, such as above a bare spot on the ground or level with the roof of a house.

It's also unknown how females locate these swarms, though multiple senses may be involved. "It's one of the mysteries of biology," Harrington said.

When females do fly into the swarm, however, males can identify them based on the frequency of their wing beats, which is lower than the males' frequency. This frequency identification is also used in non-swarming mosquito species and mating pairs will even harmonize their wing frequencies before mating, changing not only the beating of their wings but also moving their thorax box around, Harrington said.

It's unclear if fundamental wing frequency, which is related to body size, plays a role in mate choice, but experiments show that mosquitoes that are exposed to recordings of these "mating calls" choose to harmonize more with the wing frequencies of larger adults.

Chemical cues (pheromones) may also be important for mating, but researchers haven't investigated this much, Harrington said.

Surprisingly, only a single mosquito species is known to actively engage in courtship rituals. In the South American species Sabethes cyaneus, males will wave specialized structures on their feet — which resemble fringe boots, Harrington said — to impress the female.

Getting down to business

Mating in mosquitoes is quick, sometimes lasting no more than 15 seconds, and usually takes place in the air, though it can also occur on a surface.

Male mosquitoes have pincer-like structures called claspers on their abdomens, which they use to grab on to the female. The male's reproductive organ (the aedaegus) then everts and extends into the female's vagina for insemination.

Interestingly, the male's seminal fluid contains a suite of chemicals that have a range of physiological effects on the female, such as inducing her to lay eggs or take larger blood meals, Harrington said.

Unlike male mosquitoes, which will continue to mate until they die, most female mosquitoes will mate just once. They store sperm for the rest of their lives to fertilize tons of eggs, the amount of which depends on the ambient temperature and blood (a source of energy for egg development).

"They typically produce one batch of eggs for every blood meal they take," Harrington said. "At 28 degrees C [82.4 degrees F], you could get one egg batch every seven days, and some mosquitoes can lay 200 eggs or more in a batch."

Original article on Live Science.

Editor's note: Updated on Jan. 22, 2024 to include a photo of a mosquito.

The national animal of the United States, the bald eagle is one of the most recognizable birds in the country. But are the mating behaviors of these large raptors just as conspicuous as their "bald" heads?

Bald eagles (Haliaeetus leucocephalus) can be found throughout the contiguous United States, as well as in Alaska, northern Mexico and most of Canada.

"Generally, they are very social and have an incredible ability to find other birds," said bald eagle expert Bryan Watts, director of the Center for Conservation Biology of the College of William & Mary and Virginia Commonwealth University. "The best way of attracting eagles is to have other eagles around."

In the nonbreeding season, the birds tend to congregate into huge communal roosts. Just outside these roosts are stable gathering areas, where the eagles interact socially and call to one another, particularly in the morning, Watts told Live Science.

Related: Animal sex: How birds do it

The mating and nesting season for bald eagles varies depending on their location. For instance, eagles in the southeastern states begin their nesting season in the fall, while those in the western states begin nesting near the start of the new year, according to the U.S. Fish & Wildlife Service. Location also determines whether or not the birds migrate to nest.

Whatever the case, the animals tend to get very aggressive during the breeding season and vigorously defend their territory. Both males and females engage in these sometimes-deadly battles.  

Daring courtship displays

Scientists don't know what characteristics bald eagles look for in potential mates or how the birds assess each other's reproductive potential. "We just have fragments of information," Watts said.

It's known, however, that bald eagle pairs engage in a number of courtship rituals as they build their bond.

The most famous and recognizable of these rituals is the "cartwheel courtship flight," in which two bald eagles will fly up high, lock talons and then get into a cartwheel spin as they fall toward the ground, breaking apart at the last minute. "And [in] some cases they actually hit the ground," Watts said.

It should be noted, however, that nonbreeding bald eagles may engage in this behavior antagonistically, as a kind of ritualistic battle.

A courting pair may also engage in a chase display, where they will chase each other before locking talons and doing aerial rolls.

Additionally, the birds may perform the "rollercoaster flight." During this aerial display, one of the birds will essentially act like a rollercoaster — it will fly up high, go into a steep dive, pull up high again and dive down again.

The pair will also work together to gather material to build a nest for their chicks, starting with the largest branches and slowly working in smaller pieces. Sometimes, the male will gather materials and give them to the female to construct the nest. The final piece of the nest is a lining made of grasses, corn husks, pine straw and other soft materials, and an egg cup composed of fine plant material that can help insulate the egg — the male and female may lay these pieces together, possibly as a form of pair bonding.

In all, nest building could take a couple of months, but bald eagles can also build their nests much quicker if need be (such as if the area is getting crowded with other birds). 

Together until the end

As the pair gets closer to copulating, they will ramp down their aerial displays and instead focus more on intimate behaviors, such as sitting shoulder-to-shoulder, sitting in the nest structure together, preening and calling to one another. [In Photos: Birds of Prey]

When the female is ready, she will try to solicit the male to mate with her, such as by calling to him and bowing. To mate, the male will climb on the female's back and align his cloaca (waste and reproductive orifice) with hers to transfer sperm.

The pair may copulate several times a day during the breeding season, and the male will stay with the female to help incubate the eggs and raise the eaglets.

In some cases, new bald eagle pairs may not mate their first year together, instead spending several years in a particular location, possibly to assess each other or the nesting area.

Bald eagles typically mate for life and look for other partners only if their mating partner dies — they have a "divorce rate" that's less than 5 percent, according to Watts.

Interestingly, a "marriage" between one male and one female bald eagle isn't the only possible arrangement. "We have numerous documented examples of triads," Watts said. "Those typically involve a male and two females."

These triads can produce large clutches with contributions from both females, and all three adults provide food for the brood, Watts said, adding that "those are interesting situations, but they haven't been thoroughly studied."

The sex life of a fly whose name translates to "man-eater" has netted two researchers a posthumous award from U.S. politicians.

The 2016 Golden Goose Award, which honors basic research that might seem silly but led to important breakthroughs, will go to Edward F. Knipling and Raymond C. Bushland. The U.S. Department of Agriculture funded the researchers' study of the reproductive behavior of screwworm flies (Cochliomyia hominivorax), a parasitic species that caused major problems for farmers and ranchers before Knipling and Bushland's work led to a new type of insect control in the 1950s.

"Given the recent rise of infectious diseases like the Zika virus, developing eradication programs for carrier pests is a much-needed field of scientific research," Rep. Randy Hultgren, R-Ill., a supporter of the Golden Goose Award, said in a statement. "Even though 'worms' might make some members of Congress —- as well as the public — a little squeamish or skeptical of the research we invest in, these studies by Drs. Edward F. Knipling and Raymond C. Bushland have clearly paid off." [6 Politicians Who Got the Science Wrong]

Freaky fly

Adult screwworm flies look like large houseflies with orange heads. It's their young, however, that do the real damage. Female flies lay their eggs in scratches or cuts on the hides of livestock or other animals. The maggots, or larvae, of the screwworm fly burrow into the wounds, feeding on both the dead and living flesh of the infected animal.

"Untreated screwworm infestations can be fatal," reads a fact sheet on the insect put out by the Texas A&M AgriLife Extension. "The larvae will continue to feed on the animal and will eventually eat the host alive."

Even creepier, the maggots can infect humans, too. In 2008, an emergency-room doctor won a photo competition held by the journal Pediatric Emergency Care with a gruesome picture and case study of a 12-year-old girl who returned from a trip to Colombia with painful blisters on her scalp. A "blunt haircut" revealed the squirming larvae of the screwworm fly burrowed into the girl's skin. In total, doctors removed 142 larvae from her head. [Giant List of the Strangest Medical Cases Reported by Live Science]

Eradicating the screwworm

Knipling and Bushland's research made such infections — for humans and for livestock — much less common. The two worked in Texas and Florida through the 1940s and 1950s to understand the fly's biology and behavior, according to the World Food Prize organization, which honored Knipling and Bushland in 1992. (The pair also have a USDA insect research laboratory in Texas named after them.)

With annual meat and dairy losses to screwworm flies totaling more than $200 million a year in the 1950s, Knipling and Bushland became focused on disrupting the natural life cycle of the flies, according to their World Food Prize biography. They reasoned that if they could release enough sterile male flies into the wild, mating would result in fewer and fewer offspring, eradicating the fly population. The researchers then figured out that they could treat the flies with doses of radiation calibrated to render them sterile but healthy enough to mate.

The first test came on Sanibel Island, Florida, in the early 1950s. It worked. The researchers named their method the "sterile insect technique." According to the USDA, the sterile insect technique eradicated the screwworm fly in the U.S. in 1966. Mexico, Belize, Guatemala, El Salvador, Honduras and Nicaragua have been screwworm-free since the 1990s.

"Screwworm research may sound like a joke, but it isn't. It saved the livestock industry billions," Rep. Jim Cooper, D-Tenn., who invented the Golden Goose Award, said in the statement. Scientists are now looking to transfer the sterile insect technique to Aedes mosquitos, the species that transfers the Zika virus. For example, the International Atomic Energy Agency announced in March that it was granting 2.3 million euros ($2.6 million) to researchers applying the technique to Zika-carrying mosquitos.

The Golden Goose Award stands in contrast to the "Golden Fleece" Award, a congressional award that mocks purportedly wasteful science, and former Sen. Tom Coburn's "Wastebook," which highlighted National Institutes of Health grants the Oklahoma Republican judged as pointless. Researchers highlighted in Coburn's "Wastebook" consistently reported that their research was misrepresented.

Original article on Live Science.

Bombay Night Frog Calling

A male Bombay night frog (Nyctibatrachus humayuni) calls out in the monsoonal night for a mate. This species lives only in the Western Ghats of India. They mate at night, perched on vegetation above fast-flowing forest streams, making their behavior difficult to observe. However, researchers have now learned that these frogs mate in a unique way, with the male loosely draped over the female.

[Read the full story about Bombay night frogs' mating positions]

Froggy Style

The newly discovered "dorsal straddle" mating position in Bombay night frogs. Unlike in the most common mating positions, or amplexus, this arrangement does not involve the male grasping the female. Instead, he positions his feet on the vegetation on either side of her and releases his sperm before she lays her eggs. After he dismounts, the female releases her eggs and lets the sperm trickle down her back to fertilize them.

Egg-Laying Night Frog

A female Bombay night frog lays her eggs after mating. Males release their sperm on the female's back; she then arches her back repeatedly to get them to leave. Then, she lays her eggs and allows the sperm to drip down her back, externally fertilizing her brood.

This is the seventh mating position, or amplexus, ever observed among frogs. Most frogs mate by having the male grasp the female in various positions to bring sperm and eggs close to each other for external fertilization. But in the case of the Bombay night frog, there is no contact during the egg-laying portion of the process.

[Read the full story about Bombay night frogs' mating positions]

What Big Cheeks You Have

A male Bombay night frog calls in the forest in the northern Western Ghats near Humbarli village in Maharashtra state. The frogs live only in the Western Ghats and mate during the monsoon season (June to August). To document their mating habits, researchers spent 8-hour nights standing in fast-flowing forest streams with infrared cameras.

[Read the full story about Bombay night frogs' mating positions]

Mating Call

A male Bombay night frog calls from a fallen tree trunk. The frogs mate on leaves and branches above forest streams. The male drapes himself over the female, placing his feet on the vegetation below her. This is unlike most frog mating positions, known as amplexus, which most often involve the male grasping the female from behind.

Calling All Frogs

A male Bombay night frog calls in the forest. Beyond their unique mating position, these frogs behave in other unusual ways. Females also produce mating calls, a trait shared by only a couple dozen frog species worldwide. And males fight over territory during mating season.

Snake Snack

A snake scarfs down the recently-laid eggs of a Bombay night frog. Though snakes have been known to eat frog eggs in South America, this is the first documentation of snakes eating frog eggs in India.

Frog and Eggs

A male Bombay night frog calls from nearby a clutch of eggs on a leaf. Indian night frogs split from other frogs about 80 million years ago, making them an ancient group of amphibians. Researchers aren't yet sure why they evolved the newly discovered "dorsal straddle" mating position.

Bombay night frog nightlife

An infographic depicts the six known frog amplexus positions alongside the newly discovered "dorsal straddle." Researchers also discovered that female Bombay night frogs call, that males have territorial battles, and that snakes eat the frogs' eggs.

[Read the full story about Bombay night frogs' mating positions]

Between their pincers and venomous stingers, scorpions are formidable arachnids that few people would volunteer to tangle with. But for two scorpions looking to mate, "tango" is absolutely necessary. So just how do scorpions do it?

Scientists have identified about 2,200 species of scorpions across the globe, though the biology is only known for about 50 to 60 species, said scorpion expert Wilson Lourenço, emeritus research fellow at the National Museum of Natural History in Paris, France.

Given this extensive family tree, it should come as no surprise that there are a number of distinctive behaviors across species. But scorpion mating behaviors do share some general characteristics.

Mating among scorpions is associated with seasonal periods, with those animals living in temperate regions typically mating in the spring or summer and those in tropical regions mating during the rainy season.

During these sexually active periods, male scorpions use pheromones (chemical cues) to search for females, Lourenço told Live Science.

If two or more males stumble upon one another while hunting for mates, they may get into skirmishes. But this is unlikely to happen often in the wild and is generally a phenomenon related to captivity.

"Many scorpion populations are so scarce that few individuals will meet in the field," Lourenço said.

Scientists know very little about scorpion sexual selection — that is, which qualities the arachnids find attractive and how sexually active individuals choose their mates. In some populations in which males come in various sizes, Lourenço said, the males of a "normal" size may have more reproductive success.

Whatever the case, once a male and female find each other and decide to mate, they engage in a complex courtship ritual called a "promenade à deux."

For this dance, the male will grab the female's pincers, or pedipalp chelae, with this own. He will then lead his dancing partner around as he searches for a suitable spot to deposit his spermatophore, or sperm packet.

In some cases, the female may resist the male's smooth moves by refusing to dance, trying to push him away, or even trying to sting him. [Sting, Bite & Destroy: Nature's 10 Biggest Pests]

The male may try to appease her with a "kiss," in which he grasps her chelicerae (pincer-like mouthparts) with his own. In at least one species, Megacormus gertschi, the male may sting the female in a soft part of her body, leaving his stinger in the female for up to nine minutes; it's unknown if he's actually injecting her with venom or some other substance to try to pacify her.

Once the male deposits his spermatophore, he leads the female over it so that she can take it up into her genital opening.

In all, courtship and mating normally lasts from 2–3 to 10–15 minutes, Lourenço said, adding that researchers have documented much longer periods in captivity but these cases were due to having improper substrate conditions (males couldn't find suitable spots for their spermatophores).

Deed done, the pair halts their dance and goes their separate ways.

Sexual cannibalism does sometimes occur (with the female attacking and eating the male), but this generally only occurs in species in which cannibalism is a normal part of "population regulation," where the animals live in high densities and potentially see each other as prey, Lourenço said. This temperament is rare, and some scorpions even show some social behavior, such as sharing burrows and food.

Interestingly, some species of scorpions also reproduce via parthenogenesis, in which the ovum develops without fertilization (no sexual activity required).

The subject of numerous fables and folklore, red foxes are synonymous with sly and cunning behavior. But just how cunning are these canids when it comes to chasing tail?

Red foxes (Vulpes vulpes) are found throughout the Northern Hemisphere and Australia. Despite this huge range, the mating behaviors of different red fox populations may be the same, according to Graziella Iossa, a zoologist at the University of Lincoln in the United Kingdom.

However, the social system of these carnivores varies depending on the environment.

That is, red foxes often live in mating pairs that share a territory but forage alone. If key resources such as food, shelter and den sites are just right, they may live in small social groups with a dominant mating pair — the group shares a territory and hunts alone, but the subordinate members help the mating pair raise their cubs. [See Photos of the World's Cutest Baby Animals]

Mating among red foxes typically takes place in the spring, and adults locate one another through different communicative cues.

"Foxes find mates via vocalizations, and also utilize scent marking to communicate," Iossa told Live Science. "Although, we are unsure of any role of scent marking with regards to mate searching."

If two male red foxes, called dog foxes, incidentally meet up while searching for mates, fights can be violent and dangerous. But red foxes are territorial animals, so encounters between dog foxes aren't likely to happen very often.

"Territories are there to signal to other foxes, 'Move out, that's my space!'" Iossa said.

It's unclear how, exactly, red foxes choose their mates or what's involved in sexual selection. The animals don't engage in any courtship dances or displays, Iossa said. Research suggests that large dominant dog foxes copulate more than smaller males, but scientists don't know whether females, called vixens, have a choice in their mates, she added.

Like other canids, copulation occurs when the dog fox mounts the vixen from behind. The penis only becomes erect after the dog fox penetrates the vixen; an erectile tissue called the bulbus glandis expands while the vixen's vagina contracts, locking the penis in place. [The 7 Weirdest Animal Penises]

The dog fox will then lift a leg and swing it over the vixen's back so that the pair's hind ends are touching. This copulatory lock, or copulatory tie, can last for quite a while, sometimes up to half an hour or more, and ends when the dog fox ejaculates and his erectile tissue no longer swells.

During copulation, and throughout the two-week mating season in general, vixens may give off human-like high-pitched vocalizations called "screams."

Dominant mating pairs tend to stay together for life, and the dog fox guards his partner throughout the mating season to try to prevent any other males from mating with her. But this doesn't mean the "monogamous" animals have any code of fidelity.

In fact, dog foxes are known to frequently seek out other nearby sexual partners, who are generally already paired, Iossa said. But research suggests that large males are more likely to seek out extra partners, possibly because this unfaithfulness is energetically costly.

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With their hairy bodies and large fangs, tarantulas are one of the most recognizable and feared types of spiders in the world. Though their fearsome appearance actually belies a gentle nature — they're non-threatening to people and are popular pets throughout the world — does this temperament also extend to their partners during mating?

Tarantulas belong to the Theraphosidae family of arachnids and are the largest and longest-living spiders in the world, according to a 2013 review in the journal Arachnology. (Find out why some spiders in this family turn blue.)

These spiders, of which there are nearly a thousand species alive today, generally mate in the spring and summer; but some species are known to mate only in the winter, said Nelson Ferretti, a tarantula expert with the the National Scientific and Technical Research Council in Argentina and lead author of the review.

Once males reach adulthood, they "charge" their two pedipalps, or palps — small appendages near the front of their head — with sperm. That is, they will spin a small "sperm web," deposit sperm onto it, and take up the sperm into specialized storage structures of the palps, called palpal bulbs.

Charging their palps is energetically costly, but males can typically copulate with numerous females with a single charge, explained study co-author Fernando Pérez-Miles, an entomologist at the University of the Republic in Uruguay.

Finding and courting mates

During the mating season, charged males will go out in search of receptive females by zeroing in on their pheromones, or chemical scents, though it's unclear if only receptive females produce male-attracting pheromones.

"We have seen under laboratory conditions that males court on silk threads of virgin, young, old or copulated females," Ferretti told Live Science. "But females that are not receptive do not leave the burrow or they even expel males through aggression."

Surprisingly, if two male tarantulas come across a single female, they don't appear to show the competitive aggression toward each other that's known to be common in other animals. Ferretti has only seen a confrontation between two male tarantulas, but instead of fighting they tried to mate with each other before going about their separate ways peacefully, he said.

When a male finds a receptive female, the pair will engage in a courtship ritual.

Depending on the species, males may engage in a range of different moves, the most widespread being papal drumming (alternating taps on the female's silk threads or the ground with his padipalps) and body vibrations (high-frequency movement of the legs that also transmits seismic signals).

These behaviors likely inform the female about the quality of the male and his species, Pérez-Miles told Live Science, adding that the latter information is sometimes lost in translation (though different species aren't known to copulate).

If the female likes what she feels, she'll respond by tapping her front legs or palps to let him know she accepts him and possibly to direct him toward her burrow. In some species, the female will simply leave her burrow and head toward the male instead.

Careful copulation

To mate, the pair will orient themselves face-to-face with their bodies and front legs elevated.

In some species, the male will perform spasmodic beats on the female with his second pair of legs after coming into contact with her, a behavior thought to relax the female's fangs. In most species, males have specialized spurs that they use to clasp the female's fangs, helping them both elevate their mates to the right immobilizing position and prevent possible bites.

Males of one Brazilian species, Sickius longibulbi, take things a step further by pushing their mates all the way over and onto their backs.

Once in position, the male will insert his charged palps into the female's genital opening one to five times, before leaving to find other mates — if he's lucky enough to avoid getting eaten, which is common. [Why Some Female Spiders Eat Males Before Mating]

Importantly, sexual cannibalism in tarantulas probably has nothing to do with hunger level, and instead occurs if the male tries to approach the female without courting, enters her burrow too abruptly, or doesn't leave quickly enough after mating, Ferretti said.

Usually, however, mating leaves the female immobile for a brief time, allowing the male to get away if necessary, at least in the wild. "Sexual cannibalism is really rare in tarantulas, and probably due to captivity conditions of observation," Pérez-Miles said.

Editor's Note: This article was updated to correct the university affiliation of Fernando Pérez-Miles.

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This Valentine's Day, mischievous Cupid will be fluttering about with bow in hand, seeking out mortal hearts to pierce with his love-tinged arrows. But the winged god isn't the only one known for this kind of behavior — oddly, land snails are also known to shoot "love darts."

Land snails belong to the gastropod class, which includes thousands of snail and slug species that sport a wide range of reproductive behaviors.

Mating for these species typically occurs in the wetter months, said Joris Koene, a biologist at the University of Amsterdam in the Netherlands whose research focuses on love darts.

All land snails are hermaphrodites, meaning that the animals have both male and female reproductive organs. The snails spend their days looking for food and seeking out potential mates using chemical (scent) cues. [Photos: Bizarre Sex Lives of Hermaphrodite Sea Slugs]

"They have eyes at the tips of their tentacles, but they don't really see that well, and their hearing is also pretty much absent," Koene told Live Science. "So one way of finding partners is by locating slime trails that they leave and hopefully following in the right direction."

Courtship and darts

When two adult land snails meet up, they engage in a courtship dance, in which they circle around each other and touch each other with their tentacles. "When you look at them from the top, they look like like yin and yang," Koene said.

During this ritual, the snails look for traits that they find appealing, such as a large body size, which is suggestive of good fitness and being able to invest more energy into egg production.

For many species, the next step in the mating process is to shoot their love darts into each other.

At least nine families of land snails produce love darts, though the exact number really depends on your definition of the structure, Koene said. Some species have dart structures made of hard calcium carbonate (the same material that makes up their shells), while others have softer structures made of a fibrous substance called chitin. "I tend to not call those [chitin structures] love darts," he said.

Though it's said the snails "shoot" their love darts, the calcium carbonate structures don't actually fly through the air. Rather, the love darts are formed within a muscular sac in the body that forcefully everts when a snail comes into contact with its mate, piercing its body and hopefully reaching the hemolymph (bloodlike fluid in invertebrates).

These love darts come in all shapes and sizes — and numbers.

Many species shoot a single dart, while others may shoot multiple darts. Some snails, such as the Japanese species Euhadra subnimbosa, will repeatedly jab their mates with the same dart over and over again for nearly an hour.

In a 2006 study in the journal The American Naturalist, Joene and his colleague Satoshi Chiba found that E. subnimbosa stabs its partner, on average, a staggering 3,311 times.

Improving the odds

When the muscular sac everts, two glands attached to it squirt a kind of mucus onto the dart, which enters the female-acting snail's hemolymph. "This mucus modulates contraction in the female reproductive system," Koene said.

The mucus closes off the entrance to a certain organ in the receptive snail's body that would otherwise "digest" a mate's sperm, allowing more of the sperm to reach the sperm storage organ and increasing the male-acting snail's chance of fatherhood.

Until now, the makeup of this mucus has remained unknown. But in a study published recently in the Journal of Biological Chemistry, Koene and his colleagues identified, for the first time, one of the compounds in the mucus. "We are trying to get some idea of how these compounds evolve, but first you need to know what they are," he said.

After a snail shoots its partner with its love darts, its partner will do the same (sometimes this exchange is done simultaneously).

Next, the pair will copulate by pushing their genital openings together and simultaneously everting their "penises" into each other. They then pass a spermatophore (sperm packets) to one another, a process that can take up to 6 hours. [#JunkOff: Why Animal Genitals Are Important to Science]

After mating, the land snails will go their own ways, and possibly mate with other snails.

But a 2012 study in the journal Animal Behavior found that the love darts of E. quaesita have a secondary function: It makes the snails reluctant to mate again.

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The giant tortoises of the Galápagos Islands are infamous for being one of the animals that inspired Charles Darwin to formulate his theory of natural selection. But it may come as a surprise that despite their fame, the Galápagos tortoises have long been a bit of a mystery to scientists — at least when it comes to their sexual and social behaviors.

"One of the great misconceptions of iconic organisms is that we know much about them," said James Gibbs, a conservation biologist at the SUNY College of Environmental Science and Forestry in Syracuse, New York, who studies giant tortoises. "But tortoise social behavior is barely known, and we're just starting to figure all this out."

As Darwin noticed, there are two distinct types of Galápagos tortoises: saddleback tortoises (including the famed Lonesome George) and dome-shaped tortoises, so-named for the structure of their shells. But aside from having different shells, these reptiles also have very different behaviors. [See Images of Lonesome George, a Giant Galapagos Tortoise]

Saddleback tortoises live in dry environments where there's little to eat for 9 months of the year, Gibbs told Live Science. As their name implies, saddleback tortoises' shells rise in the front like saddles — this adaptation allows them to extend their long necks up to feed on cactus trees.

Male saddlebacks vie for control over cactus trees, but will allow as many females into their shaded domains as possible.

Domed tortoises, on the other hand, live in moist regions where food is plentiful and close to the ground, and the shape of their shells prevents them from extending their short necks up.

Because of the abundance of food, these tortoises don't control territories and instead roam around to find mates, probably using their noses as their guides, Gibbs said. "Having said that, we suspect — and it's probably true — that they may also use ultrasonic communication."

Galápagos tortoises mate throughout the year, but sexual activity typically heats up during the hot months in the beginning of the year.

When two male saddlebacks come across one another, they battle for access to cactus trees — and therefore mates. These fights, however, amount to little more than neck-stretching competitions, with the longer-necked tortoises ultimately coming out victorious (though biting is also sometimes involved).

Male dome-shape tortoises, by comparison, engage much less in competition. "There's a lot more food, but they do sometimes act like bumper cars and just bump into each other with a loud noise," Gibbs said.

Male Galápagos tortoises aren't in the business of wooing their potential mates; rather, they show their interest by harassing the much smaller females. That is to say, a male will spend up to three or four days following a female, trying to mount her, and pushing her around a bit.

"They interfere with daily activities and feeding, but we don't know if it's part of the male's strategy to wear [the female] down," Gibbs said.

Galápagos tortoises, like other reptiles, have cloaca (a single orifice for waste and reproduction) at the base of their tails. The lining of the male's cloaca folds out to form a "penis," which he needs to insert into the female's cloaca for insemination — an act that requires the female's cooperation, as she needs to extend her tail out and lift it so that the male has access.

"Tortoise mating is quite an affair to watch," Gibbs said, adding that males will thrust for 10 to 20 minutes while making loud groaning noises that can carry for miles.

After mating, the female Galápagos tortoise may store the sperm of her mate in her body for at least 7 years for later use.

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Males of the human variety may spend hours at the gym bulking up to attract the ladies, but that's nothing compared to the efforts of a new spider species from Australia.

This little brown spider sports a massive, paddlelike appendage on its legs that it flashes at females to woo mates, new research has revealed.

The new spider species, Jotus remus, can do this paddle "peekaboo" routine for hours, all to get female spiders to accept its advances. The paddle seems to be a way of separating the fertile females from those that have no interest in mating, said Jürgen Otto, the biologist who discovered the oddball spider. [Incredible Photos of Peacock Spiders]

Camping stowaway

Otto has a day job researching mites at the Australian Department of Agriculture and Water Resources in Sydney, but spends his free time hunting down gorgeous and unusual peacock spiders. (Otto maintains a YouTube channel filled with videos of the bizarre mating dances of peacock spiders.) He first discovered J. remus, while on a camping trip with his family during Christmas break in 2014. While unpacking the car after the trip, he spied an ordinary-looking brown spider sitting on his tent bag.

"At first it didn't seem really unusual. It had color, patterns and shapes I've seen before," Otto told Live Science. "But I looked closer and noticed it had these funny extensions on their third pair of legs, it seemed like a paddle."

Otto suspected the creepy crawly was a new spider species, but had no idea what the paddle was for. And finding out posed a bigger problem: He wasn't sure whether the spider lurked in the wilderness around his home or was a stowaway from his campsite at Barrington Tops National Park, about 125 miles (200 kilometers) north of Sydney.

Eventually, Otto returned to his camping location and noticed several of the spiders on a walking trail he'd visited during the trip. He suspected the darker brown arachnids were the females, so he scooped some of those up, as well as additional males and put individual males and females together with some twigs and leaves to see what they did.

Hide-and-seek

The male spider quickly hid behind a leaf as the female spider watched.

"From under the leaf he stretched out one of his legs, of the third pair," Otto told Live Science. "He exposed that paddlelike extension to her and waved it at her." The female occasionally lunged toward the male spider. The male handily evaded her.

"He seemed to have no difficulty at all escaping from these attacks; he seemed to be playing a game," Otto said.

This seemingly pointless game of hide-and-seek went on for many minutes until the male gave up. Otto tried the same thing with multiple females and male piders. Perhaps the male was trying to exhaust the females, to make them more receptive to his mating advances, Otto said he thought. But the females didn't seem to get tired, no matter how long the male persisted. Like the male spider, Otto eventually set aside the peekaboo game and pursued other spider questions.

Like a virgin

But a few months ago, some of the immature females of J. remus grew up. These spider females were "virgins" who had had no opportunity to mate with males. In these types of spiders, females can probably only mate once, so non-virgin females are of no use for males looking to pass on their genes, Otto said.

When Otto put the male spider together with the virgin female spider, the male went ahead with his paddle routine. But the female, rather than lunging at him, watched curiously. Within a few minutes, the female became very calm and still. At that point, the male spider made his move, emphatically thrusting his paddle twice.

"After those two vigorous paddle strokes, he just jumped up very quickly to the other side of the leaf and went on mating with that female," Otto said.

So the paddle dance seemed to be an elaborate way for male spiders to figure out if a prospective female is "the one," Otto said.

"The one that keeps attacking him is not the right one," Otto said. (Probably good advice for males of any species.)

Questionable behavior

While Otto has never seen a female spider eat a male, the females are superb hunters and the males of the species are significantly smaller than the females, "just the right size for him to be food," Otto said. As such, the elaborate paddle game may partly be a protective mechanism, a way for males to avoid risking danger with a female who has no interest. Still, the males are swift and never seemed to be truly threatened by the females, and the whole encounter seems almost playful, so there could be another explanation for the male spiders' sneaky moves, he added.

One question still puzzles Otto: Why do the males play so long with partners who clearly have no interest?

"If the male gets a reaction from the female telling him she is not really happy to mate with him, why does he keep on trying?" Otto said. "There's a lot of play going on that seems to be wasted energy."

Otto and his colleague David Hill, a zoologist in Greenville, South Carolina, described J. remus in a paper that was published online Jan. 7 in the journal Peckhamia.

Follow Tia Ghose on Twitterand Google+. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.

In December, reindeer spend their time carting Santa around as he delivers presents to children around the world, or so the story goes. But the famous ungulates are engaged in a very different activity in the months preceding Christmas — mating.

Though Rudolf may have been picked on by his peers, wild reindeer (Rangifer tarandus) typically live in small peaceful herds throughout most of the year. That is, until the fall breeding season, or rut, comes around.

During this time, which runs from September or October to November, competition between males heats up and every reindeer is out for himself. Depending on the reindeer sub-species (and the environment in which they live), males may opt for one of several mating strategies.

For instance, the non-migrating high Artic reindeer adopt a harem-defense strategy wherein they collect multiple females (sometimes up to a dozen or more) to mate with, and then defend them against other males. Comparatively, forest-living male reindeer focus on defending resource-rich territories — and the small groups of females that come to feed in these territories. [6 Surprising Facts About Reindeer]

Reindeer in the tundra, on the other hand, migrate in large groups during the fall, an activity that coincides with the breeding season. Because of this mobile lifestyle, males often aren't able to collect a harem and instead focus on herding and defending single females in estrous.

In some cases, however, migrating males are able to form mobile harems, but these groups are unstable and females frequently move between them.

Strategies and tactics

Within these general strategies males may adopt even more specific tactics. Research shows, for example, that males associated with mobile harems may use dominant, sub-dominant or sneaker tactics.

Dominant males will try to collect as many females as possible. They are always ready to fight other males to maintain the top spot of the social order.

Sub-dominant males hang around the harems of the dominant males, anxiously awaiting the opportunity to steal females and form their own harem — or even fight the dominant males for their mating groups. Young sneaker males try to quickly slip into harems to mate with females when the harem leader is unaware.

These tactics aren't necessarily fixed for an entire rut and can change within minutes, such as if two dominant males fight and the winner takes the loser's harem, forcing the loser to adopt a sub-dominant tactic. Fights can be quite serious at times, with some male reindeer sustaining severe injuries from their opponents' antlers.

Interestingly, up to 90 percent of mating occurs within a 10-day period called the "peak rut," in which the males' mating behaviors — including grunting, trashing bushes and chasing rivals — become highly synchronized and active. Each female is only receptive to mating for two days during this peak rut.

Choosing and courting mates

Though male reindeer try to mate with any and every female they can, it's unclear if females actively choose their mates.

But researchers have observed female reindeer going around and visiting multiple males during the couple of weeks preceding estrous — and trying to catch the attention of dominant males they fancy, suggesting that they do try to exert some level of mate choice.

Aside from these activities, reindeer don't appear to engage in many courtship behaviors.

Dominant males stay on high alert during the rut, watching their harem and testing females for estrous. To do so, a male will grunt at a female with his head held low and sniff her vaginal area while lifting his upper lips (a so-called flehmen response).

Once he finds a female in estrous, the male will ramp up his grunts and flehmens, sometimes for a whole day. The receptive female, for her part, will stay close to the dominant male and try to fend off sneaker males with her antlers.

To mate, a male will mount the female from behind, though it usually requires several tries before the female will stand still and allow him to penetrate her.

After successfully mating with a female — an act that's over in just a few seconds — the male reindeer loses all interest in his former lover and stops protecting her from other males, despite her attempts to stay near him.

Follow Joseph Castro on Twitter. Follow us @livescience, Facebook & Google+.

Despite their languid nature and bulky bodies, manatees are surprisingly graceful in their watery habitat. But do the mating behaviors of these so-called sea cows rival the elegance of synchronized swimmers, or are the mammals even more cowlike than meets the eye?

Manatees are generally solitary animals. Females (cows) are often accompanied by their individual calves, and adult manatees frequent the same seagrass beds to feed. But by and large the animals only come together to mate, said Iskande Larkin, a manatee researcher at the University of Florida.

Manatees are reproductively active throughout most of the year, though their sexual activity winds down in the winter when they're physiologically stressed, Larkin told Live Science.

It's unclear how, exactly, males find females that are in estrus. The animals do occasionally give off squeaks or squeals, but these noises are reactions to the environment rather than mating calls.

Finding mates may instead be driven by chemosensory (scent) cues, Larkin said

For one thing, research has shown that male manatees exhibit more searching behaviors when placed in pools laced with urine from females in estrus than pools containing urine from non-estrus females. Scientists have also noticed that manatees appear to have rendezvous points — key spots in the river system where they meet up — so it may be the case that females excrete an anal gland mucus on logs or other objects to draw males to them, Larkin said.

"We know that males can tell which female is in estrous and which is not," Larkin said. "If they can tell that from urine, then they can probably tell that from fecal [samples] as well."

Whatever the case, females in estrous don't only lure a single male to them — they catch the attention of entire groups. The animals will form a so-called mating herd, in which up to a dozen males will follow and huddle around a single female to mate with her.

"Males don't have tusks or claws or anything like that, but there's lots of pushing and shoving to get to the female," Larkin said.

Males closest to the female in the writhing herd are the most dominant and aggressive of the bunch, and have the best chance to mate with her. But the males on the outside of the group aren't just hanging around doing nothing.

"They try to mate with each other," Larkin said. "They're pulling 'it' out all of the time."

It's unknown how or even if the female actively chooses which male to mate with in the herd — the mating group swims around a lot and kicks up sediment, making it difficult for researchers to identify different manatees.

To mate, a male will swim beneath the female and align his body with hers, head-to-head and tail-to-tail. He'll then grasp her flippers and insert his penis into her vagina. Mating typically lasts for a few minutes, Larkin said. [The Weirdest Animal Penises]

The mating herds will last for about two to four weeks and can get quite intense, with females sometimes trying to strand themselves to get away from their suitors.

Once the herd disbands, the males will go on with their lives and look for other females.

"And the female probably takes a big sigh of relief and sleeps a lot," Larkin said.

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With their treelike height, thin legs and awkward locomotion, giraffes are the epitome of the word "gangly." But do their sexual behaviors involve smoother actions than one would assume from their appearance?

Like people, giraffes mate year-round, though there's some indication that breeding times correspond with periods of high food availability. The tall ungulates live in a so-called fission-fusion society, in which the size and the composition of herds continually shift — the social mammals use this ever-changing society to find mates.

"The best way to describe it: Giraffes live in a permanent cocktail party," said Fred Bercovitch, a biologist at Kyoto University in Japan who has studied the reproductive and social behaviors of giraffes. "At a cocktail party, you'll see there are some people who are really social butterflies and spend a little bit of time mingling with a lot of people, and others who spend more time with fewer people."

And then there are some that are on the lookout for a one-night stand. Similar to the way a person might walk around a party and chat with people to see who's interested, a male giraffe will move through a herd to find females that are receptive to mating, Bercovitch told Live Science.

This sexual hunt involves the male prodding the rear ends of different females with his head. Some females will respond by spreading their legs and urinating, sparking a so-called flehmen response from the male — he will lift his upper lip, flare his nostrils and inhale. In addition to sniffing the urine, he may even taste it, taking in additional chemical cues to see if the female is ready to mate. [Most Mammals Take 21 Seconds to Pee]

Female giraffes have a two-week estrous cycle, but are only fertile for a narrow window of fewer than four days during this period (this cycle will repeat until the female gets pregnant). Urine testing allows the male to find females in this fertile window, Bercovitch said.

Once a male finds a fertile female, he "stands behind her and looks like a statue," Bercovitch said. The female may choose to walk off, possibly to feed, but the male will follow her to stand behind her again, often with an erection the entire time.

To let her know he's ready to mate, he will occasionally tap her hind legs with his forelegs.

Though males do fight over dominance and territory by viciously swinging their heads (and hornlike "ossicones") into each other's bodies, it's unknown if males fight over mates. "Nobody has found two big male bulls fighting and the winner goes over to the female," Bercovitch said. [See Cute Pictures of Baby Giraffes]

It's unclear what qualities, if any, a female looks for in a mate. Given the narrow window available to get pregnant, a female may simply mate with any bull that has survived long enough to reach full adulthood, Bercovitch said.

Whatever the case, the female has to cooperate for mating to occur — she has to delicately balance a mate that could weigh as much as 50 percent more than her, as he mounts her from behind.

After successfully mounting a female, the male ejaculates almost immediately, typically in less than 2 seconds.

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It's tough out there in the wild, especially for male howler monkeys. A new study suggests that these primates have to make a (evolutionary) choice: deep calls or small balls?

The smaller the size of the testes in a howler monkey species, the larger the size of the animal's hyoid bone, a structure that enables the monkeys to make deep, booming calls — noises on a par with those of a tiger, though howler monkeys are only about the size of cocker spaniels.

The relative sizes of the hyoid bones and testes appear to be related to how the animal lives and reproduces, according to a new study, published today (Oct. 22) in the journal Current Biology. Howler monkey males that live in groups with more than one male invest in larger testes, whereas those that live only with females invest in a booming, attractive voice.  [See Mug-Shot Photos of Howler and Other Monkeys]

Booming Calls

Curiosity about the howler monkey's booming calls dates back to at least Charles Darwin, who suggested that the males' cries are used to attract females, which choose mates based on the depth and resonance of these calls. (Females make calls, too, so howler monkey cries are not exclusive to mating rituals.) [Video: See a Howler Monkey Hoot]

Key to these calls is the hyoid bone, a structure that looks like a horseshoe in humans but is larger and more cuplike in howler monkeys. The bone creates a sort of resonance chamber that makes howler monkey hoots and hollers really ring out.

University of Utah anthropologist Leslie Knapp and her colleagues found that, in howler monkeys, male hyoid bones are up to five times larger than female hyoids, suggesting that Darwin was right in proposing a reproductive role for the monkeys' calls. Next, the researchers measured the size of hyoid bones across 10 species of these monkeys, which live in Mexico and Central America.

Making a trade-off

The results fit a "really exciting" pattern, Knapp told Live Science. The hyoids and testes varied widely across species: The species with the largest hyoids had bones 10 times larger than those with the smallest. And the largest testes were about seven times larger than the smallest testes.

Without exception, howler monkey species with larger testes had smaller hyoids and vice versa. And the anatomical differences showed no relation to environmental factors like food availability. Instead, they were linked to the monkeys' social structure.

Species living in a "harem" structure with one male mating with several females had larger hyoids and smaller testes. Species living with multiple males and females in a group, all mating with one another, had larger testes and smaller hyoids.

It's likely that the harem-heading howler monkeys use their deep calls to attract more females, or perhaps to scare off males that might steal their mates, Knapp said. As a result, they can get away with a lower sperm load — they compete for females before the actual mating act.

For male monkeys living with other males, though, the competition comes after copulation. A greater volume of sperm may give these monkeys a fertilization advantage in a female monkey that has mated with multiple males.

It's probably impossible for howler monkeys to evolve both a large hyoid and large testes, Knapp said.

"If they could, they would," she said. "I think, because they don't, it suggests that you can have one or the other, but you can't have both."

To really nail down the notion that monkeys make this reproductive trade-off, the researchers will have to look within species to see if large hyoids or large testes confer real advantages for males.

"We kind of have answered a question that Charles Darwin pondered in the 1800s, so that is exciting for us," Knapp said. "It also opens the doors to more questions, like what is going on within species? Is there evidence that males with the very largest hyoids are the ones having the most kids?"

The study also raises questions about what humans might have in common with their distant monkey cousins. In people, men with deeper voices are considered more attractive by women than men with higher-pitched voices, studies have found, but men with deeper voices don't have better-quality semen.

Humans love to ponder whether alien life is out there, and what it might look like. So here's a burning question: Would extraterrestrials have sex?

The question isn't entirely prurient. The evolution of sex is a tricky subject. Sexual reproduction is costly. It requires finding a mate, convincing that mate to mingle DNA with you, and opening yourself up to the possibility of sexually transmitted disease or predation while you're busy wooing.

All that considered, and it might not even result in viable offspring. After all, mixing and matching a genome is a crapshoot, said Sally Otto, director of the Biodiversity Research Centre at the University of British Columbia. [7 Huge Misconceptions About Aliens]

Potential parents "know their genome works in the current environment," Otto told Live Science. "They know they survived to reproduce. And here they are, shuffling their genomes together with another individual. … You have no idea if that combination is going to survive and be fit."

And yet, sexual reproduction is very common on Earth. And given the conditions in which sex evolved, it's quite possible that aliens might get busy, too.

Swapping genes

Not all life on Earth requires sex for reproduction. Amoebas, yeast and millimeter-long freshwater hydra all manage to create offspring solo, as do many invertebrates. So do some surprisingly complex animals: Virgin births have been reported in Komodo dragons, pit vipers and sharks.

There are species, like the tiny crustacean Daphnia middendorffiana, that can only reproduce asexually. But sex appears to go way back. There are few very old lineages that are entirely asexual, Otto said. [Photos: Bizarre Sex Lives of Hermaphrodite Sea Slugs]

Amoebas, for example, date back at least a billion years, long before multicellular life evolved. For a long time, scientists thought amoebas were purely asexual. In 2011, however, researchers from the University of Massachusetts announced that they had discovered amoeba sex.

In fact, swapping genes is the norm for life on Earth. Bacteria, for example, are prokaryotes, meaning they don't have membrane-enclosed nuclei. (Eukaryotes, including both amoebas and animals, have cells with nuclei and other organelles closed in with membranes.) Bacteria don't have sex, Otto said, but they do take up new DNA by swallowing other bacteria or as a result of being infected with viruses or circular DNA molecules called plasmids. The inadvertent genetic gains can benefit bacteria by increasing genetic diversity, thus raising the chances that a new genetic sequence will confer some sort of survival benefit.

Sexual evolution

It's possible, Otto said, that deliberate sexual reproduction arose with the evolution of eukaryotic cells because all those internal membranes prevented the frequent accidental uptake of foreign DNA.

So, one question that might determine whether aliens have sex, Otto said, is what their cells look like.

"Do they evolve nuclei or other ways of protecting their DNA inside a series of membranes?" she said. If extraterrestrial life is equipped with nuclei, they might benefit from sex.

Another thing that Planet Xenon might need to prompt the evolution of sex is change.

Sex is very beneficial to organisms because the environment is rarely static, Otto explained. Offspring may have to deal with challenges that are slightly different from those of their parents' generation. As long as change is a constant, genetic variation is helpful.

If an alien planet had, for some reason, constant weather, temperature and other environmental factors, "sex would have mainly costs, but no benefits," Otto said.

Best of both worlds

Assuming alien planets aren't entirely static, extraterrestrials might try to get the best of both worlds. Some aphids (small insects that suck plant juice) clone themselves asexually when food is abundant. In fact, Otto said, these cloning aphids can have not only their babies inside them but also their babies' babies, "like a set of Russian nesting dolls."

"That really speeds up reproduction when resources are plenty," she said. 

At the end of the growing season, though, the aphids switch to sexual reproduction. This switch to sex during times of stress is a common pattern. Some water flea species spring for sex when food supplies drop or when the environment becomes hostile, according to a 1981 study in the journal The American Naturalist. Yeast simply bud off new offspring most of the time, but the yeast Candida tropicalis can also reproduce sexually, researchers reported in the journal Proceedings of the National Academy of Sciences in 2011.  

One stress that might prompt the evolution of sex on an alien planet might be alien parasites. Researchers reporting in 2011 in the journal Science found that, when given the choice, organisms pick sex over asexuality when parasites threaten, likely because sexual reproduction gives them more genetic weapons to use in the evolutionary arms race against their parasite foes.

In that study, researchers genetically modified roundworms called Caenorhabditis elegans so that some could only reproduce sexually and some only asexually. A third group was left to switch between asexual and sexual reproduction at will.

Then, the researchers exposed the worms to parasitic bacteria. They found that asexual C. elegans exposed to evolving bacteria went extinct in fewer than 20 generations. Sexual C. elegans did just fine, as did worms that could switch back and forth.

Other studies have shown similar results in yeast and other organisms that can switch from no sex to sex in tough conditions.

Hot stuff?

But even if aliens do have sex, it might not be the sort of thing people watch on pay-per-view.

In amoeba sex, for example, the cell partitions off packets of genetic material and then recombines them, either with another amoeba or with packets from other amoebas. Sexually reproducing yeast cells find each other, grow projections, merge and mate. The hermaphroditic C. elegans worm wiggles its body against another worm until it finds the vulva and then inserts needlelike structures called spicules into the opening to deliver sperm, according to WormBook, an open-access resource on C. elegans biology. [Animal Sex: 7 Tales of Naughty Acts in the Wild]

Even for fuzzier, more familiar animals, sex can get downright weird. The marsupial Antechinus, which lives in Australia and New Guinea, mates in a frenzy over about two weeks. Males often ambush females and copulate with them for up to 14 hours. The effort of their multiple marathon sex sessions takes such a toll on males that they start to bleed internally and lose all immune function. They rarely survive the breeding season. Hyena males have to mount females with care because the female clitoris is so large that it resembles a penis. And some male bats even stimulate females' genitalia with their tongues.

In other words, aliens might have sex, or they might not. But one thing's for sure: It'd be harder to invent something stranger than what already exists here on Earth.

Follow Stephanie Pappas on Twitterand Google+. Follow us @livescience, Facebook& Google+. Original article on Live Science's Life's Little Mysteries.

Like an awkward junior-high birthday party, some animal species tend to have many more males than females or vice versa, and scientists have long wondered why. Now, they've figured out a key culprit: sex chromosomes.

An animal's sex is often determined by the sex chromosomes it inherits. The new research reveals that species with X and Y sex chromosomes, including mammals, generally have female-skewed populations, whereas species with the less familiar Z and W sex chromosomes have a sex ratio tilted toward males.

The proportion of adult males to adult females in a species, known as its adult sex ratio, can vary widely in nature. For example, scientists have known that among tetrapods — that is, four-limbed animals such as mammals, birds, reptiles and amphibians — birds possess male-skewed adult sex ratios, and mammals are usually female-skewed. Extreme ratios are seen in some marsupial species, in which the males die after the mating season, sometimes leaving populations made up entirely of pregnant females. [Animal Sex: 7 Tales of Naughty Acts in the Wild]

Unbalanced adult sex ratios can have profound effects on animal behavior, research has shown. In bird species dominated by females, the males have multiple female mates, and females care for the young. In contrast, male-leaning adult sex ratios are linked with females having multiple male mates, and males raising offspring.

"Other researchers have shown that in humans, population sex ratios predict violence, rape and marital infidelity," study co-senior author Tamas Szekely, an evolutionary biologist at the University of Bath in England, told Live Science.

For instance, studies of Makushi communities in southern Guyana, where the adult sex ratios can vary due to migration, have found that when men are in the minority, they prefer flings over long-term relationships.

To shed light on what might cause unbalanced adult sex ratios, researchers focused on the chromosomes that decide sex in species. In groups such as mammals and fruit flies, males have different sex chromosomes — X and Y. However, in groups such as birds and butterflies, females have different sex chromosomes — Z and W.

The scientists analyzed 344 species of tetrapods. They found that species where females are the sex that has different sex chromosomes are significantly male-biased, with adult sex ratios slanting more than 55 percent male. In contrast, species where males are the sex that has different sex chromosomes are significantly female-biased, with adult sex ratios slanting more than 56 percent female.

The cause of these imbalances remains uncertain. One possibility is that whichever sex has two different sex chromosomes is less likely to survive to birth. Another possibility is that whatever sex has two different sex chromosomes might have a lower chance of surviving to adulthood after birth. Birds and mammals tend to have relatively balanced birth sex ratios, so the researchers suggested that in these groups, whichever sex has two different sex chromosomes may suffer a higher rate of mortality after birth.

Future research might focus on fish. "They have several different kinds of sex determination systems," Szekely said. "Hopefully, other researchers will take up this challenge."

Szekely; study co-senior author András Liker of the University of Pannonia in Veszprém, Hungary; and their colleaguesdetailed their findings in the Oct. 8 issue of the journal Nature.

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