A Stanford researcher and his colleagues have produced a surprising new study which shows that mammals can effectively “choose” the sex of their offspring and they’re doing it for their own selfish reasons – so they can have more grandchildren.
The researchers used data on more than 2,300 animals at the San Diego Zoo to track three generations for nearly 200 mammal species. Their numbers confirm what evolutionary biologists have theorized for decades – that females are strategically evaluating their options and making decisions that will serve their own reproductive interests, said Joseph Garner, PhD, associate professor of comparative medicine at Stanford and senior author of the study. Garner told me:
You can think of this as being girl power at work in the animal kingdom. We like to think of reproduction as being all about the males competing for females, 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.
What the researchers found is that grandparents could strategically choose to give birth to sons, if the sons would be of high enough quality to give them more grandchildren. In fact, they found that when females did have mostly sons, those sons had 2.7 times more children per capita than those who had equal numbers of male and female offspring. The same was true for male grandparents, with the study showing that when grandfathers produced mostly sons, those sons on average had 2.4 times more children per capita. But again, it’s most likely the females controlling the process, Garner said.
How they do it remains a bit of a mystery, he said, though one theory is that females control the sperm as it moves through the reproductive tract, selectively slowing down or speeding up the sperm they want to select.
Garner noted that there have been studies showing some similar patterns among humans. For instance, one found that billionaires are more likely to have sons than daughters because sons tend to retain the family’s wealth, or so the theory goes.
To read more on this tantalizing subject, see the study, which appears online today in PLOS ONE.
Photo by EvaSwensen