Mammals "Choose" Sex of Offspring: Girl Power in the Animal Kingdom

Is it a boy or is it a girl? The answer may be simpler than you might expect. Scientists have discovered that mammals have the ability to "choose" the sex of their offspring in order to beat the odds and produce extra grandchildren.

You wouldn't think that mammals would have a "choice" when it comes to the sex of their children. Yet it turns out that environmental factors may play a key role. Researchers analyzed 90 years of breeding records from the San Diego Zoo, revealing that mammals seem to rely on some unknown, physiological mechanism to manipulate the sex ratios of their offspring.

In 1973, researchers proposed a theory that challenged the idea that sex determination in mammals was random. They hypothesized that mammals are "selfish" creatures, manipulating the sex of their offspring in order to maximize their own reproductive success. Parents in good condition, for example, would invest more in producing sons whose inherited strength and bulk could help them better compete in the mating market and give them greater opportunities to produce more offspring. Parents in poor condition, in contrast, would be more likely to produce daughters in order to play it "safe."

In order to better examine this possible phenomenon, the researchers assembled three-generation pedigrees of more than 2,300 animals. They found that, surprisingly, grandmothers and grandfathers were able to strategically choose to give birth to sons, if those sons would be high-quality and, in return, reward them with more grandchildren.  In fact, they found that when females produced mostly sons, the sons had 2.7 times more children per capita than those whose mothers bore equal numbers of male and female offspring.

"You can think of this as being girl power at work in the animal kingdom," said Joseph Garner, associate professor of comparative medicine, in a news release. "We like to think of reproduction as being all about the males competing for females, with females dutifully picking the winner. But in reality females have much more invested than males, and they are making highly strategic decisions about their reproduction based on the environment, their condition and the quality of their mate. Amazingly, the female is somehow picking the sperm that will produce the sex that will serve her interests the most: The sperm are really just pawns in a game that plays out over generations."

Currently, researchers aren't exactly sure how parents manipulate the sex of their offspring. Yet there are some theories. It's possible that females can control the "male" and "female" sperm, which have different shapes, as they move through the mucous in the reproductive tract. It's possible that the to-be mothers can selectively slow down or speed up sperm that they want to select.

The findings could be important to help in breeding efforts in zoos. Understanding sex-ratio manipulation could allow zoo keepers to better preserve species through interventions.

The findings are published in the journal PLOS One.