Baby Boy or Girl? Mammals Can 'Choose'

Birth sex ratios can be altered by social and environmental factors, research suggests. (Image credit: zurijeta |

Mammals can skew the male-female ratio of their offspring in order to maximize their reproductive success, new research finds.

The study, published today (July 10) in the journal PLOS ONE, confirms a long-held theory that animals can influence the sex of their young in response to environmental conditions and other factors. The results come from about 90 years' worth of records for 40,000 mammals, ranging from primates to rhinoceroses, at the San Diego Zoo.

Females that produced the most males went on to have up to 2.7 times the number of grandchildren from those sons as those who had even numbers of male and female offspring. [The 10 Strangest Pregnancies in the Animal Kingdom]

"When mothers produced predominantly male offspring, those male offspring outcompeted their peers," said study co-author Joseph Garner, an ethologist at Stanford University.

Parents that produced more females also tended to produce more offspring from those daughters than those with an even gender split, though the effect was less pronounced.

Still, that doesn't mean mammals are consciously choosing to have a male or a female. Instead, environmental and social cues may be subtly influencing their physiology so that they produce offspring with the best chances of passing on their genes.

Long-held theory

Since the 1960s, evolutionary biologists have suspected that animals can influence the sex of their offspring to maximize their offspring's reproductive success. For instance, high-status males typically have more children, so wealthy parents may benefit from having mostly males. In humans, at least, this seems to be the case.

"Billionaires are much more likely to have sons than daughters," Garner told LiveScience.

(Anecdotally, former presidential nominee and billionaire Mitt Romney has five sons.)

In contrast, during times of famine, girls are better bets, because they take less energy to gestate and nurse, Garner said.

But evidence to prove the theory was surprisingly scant.

Meticulous records

To test the theory, researchers needed to show that mammal grandparents who produce sex-biased offspring have more grandchildren than those with more equal sex ratios.

That requires tracking three generations, and if even one individual is lost, then the whole data set becomes useless, Garner said. Keeping such a meticulous record of ancestry in the wild is next to impossible.

So Garner and his colleagues analyzed over 90 years' worth of San Diego Zoo records on birth and parentage for 198 species of mammals (some 40,000 individuals), looking at everything from primates to rhinoceros to horses.

They eliminated lineages with missing information. At the end, they had data on roughly 1,600 grandmothers and 700 grandfathers.

Evolutionary advantage

Across all mammals, grandmothers who produced more male than female offspring had up to 2.5 times more grandchildren than those who produced equal numbers of males and females. A similar advantage went to grandfathers who produced more males.

Having more females than males also led to better reproductive success, although the effect was smaller. Garner hypothesizes that the smaller effect is largely due to specific matriarchal species, for instance primates such as rhesus macaques. High-status females of these matriarchal species can increase their odds of passing on their genes by having more female children. [8 Humanlike Behaviors of Primates]

Still, exactly how mammals manipulate the sex of their offspring remains a mystery. Higher levels of glucose or fatty acids in the bloodstream may make conditions slightly more toxic for female embryos, or higher testosterone levels may make eggs more receptive to male versus female sperm, Garner speculated.

Either way, in this battle of the sexes, males likely aren't in control. In fact, the female body probably determines the offspring sex, Garner added.

"The sperm aren't deciding the sex of the offspring. The sperm are these pawns in this great Machiavellian game that's being played out over generations," Garner said.

Of course, the sex determination isn't a conscious decision by an animal. Instead, the female's body is likely responding to environmental cues.

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Tia Ghose
Managing Editor

Tia is the managing editor and was previously a senior writer for Live Science. Her work has appeared in Scientific American, and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.