Can Mammalian Mothers Control the Sex of their Offspring?

San Diego Zoo

Boy or Girl? For some animals, it’s not simply a genetic flip of the coin. In fact, a new study suggests that some mammalian mothers deliberately “choose” the sex of their offspring. While scientists don’t know what physiological processes are involved in this choice, the research indicates that, somehow, sex selection for mothers is not random.

Stanford biologist Joe Garner and a team of scientists studied more than 90 years of birth records for hundreds of mammal species at the San Diego Zoo. They discovered that the ratio of male to female children is skewed to achieve the highest reproductive rate. In other words, animals with more descendants have a sex ratio that cannot be explained by random chance alone.

To test the numbers, Garner and his team required at least three generations of each mammalian species. “Biological success” occurs when grandparents have a lot of grandchildren – procreation is the name of the game in evolutionary biology. This means the first generation needs to produce offspring who will themselves reproduce. Females tend to reproduce regularly, but each female can only produce a certain number of offspring. Males, on the other hand, may or may not reproduce, but some males will reproduce with numerous females, dramatically increasing the number of offspring. Females, then, are typically a safe bet, while males represent the procreation wild card.

“Let’s imagine for a second that I am a female elephant seal, says Garner. “If I have a daughter, I can guarantee that she’s going to give me one offspring per year for as long as she lives. If I have son, chances are he will never breed. But if he happens to be one of those males that holds 10 or close to 100 females perhaps, then in just one year he’s going to give me 10 or maybe even close to 100 grandchildren. If somehow I know that my son is going to be a harem-holding male, then I should have nothing but sons. I should try and beat the system.”

And it’s not only elephant seals that are beating the system, according to Garner. “Across roughly 200 species of mammals, females are able to choose whether or not they’re going to have a boy or a girl,” he said. “This is a very strategic Machiavellian decision that they’re making.”

On average, females (grandmothers) who birthed more males had 2.7 times more grandchildren than the females whose offspring split evenly along gender lines. Males (grandfathers) who had more sons had 2.4 times more grandchildren. The more biased toward males a mother is, the more successful those males turn out to be. In some instances, though, the selection skews toward females. That’s the case with the rhesus macaque, a matriarchal species. In addition, individual mothers of a species might birth more females as reliable producers of offspring.

Humans, of course, are mammals, so presumably the sex-selection process is operating in people as well. The evidence for human sex selection is largely circumstantial. For example, in a 2013 study, scientists at the Mammal Research Institute in South Africa looked at 400 U.S. billionaires, and found they were more likely to have sons than daughters. The scientists hypothesized that sons tend to retain the family’s wealth.

While the statistics of the San Diego Zoo study are compelling, there is as yet no understanding of the mechanism for how this works in the reproductive system. One theory holds that the “gender” of the sperm have different shapes. When they move through the mucous in the reproductive tract, females may be able to selectively slow down or speed up the sperm they want to select.

Garner said that, for him, the best part of the study is that it “turns this very male-centric perspective we have on reproductive biology on its head.” He continued, “If you think about it, the females have everything to win and everything to lose. They’re the ones who’ve invested in the offspring. So they’re not just going to go along with a winner. They’re going to do the absolute best they can for their reproductive future, because they’ve invested so much in it.”