Mammals Choose Sex Of Their Babies To Promote Life
July 11, 2013

Mammals Choose Sex Of Their Babies To Promote Life

Lee Rannals for - Your Universe Online

Mammals are able to "choose" the sex of their offspring in order to beat the odds and produce extra grandchildren, according to a study published in the journal PLoS ONE.

Researchers analyzed 90 years of breeding records from the San Diego Zoo, helping to prove for the first time what has been a fundamental theory of evolutionary biology. Scientists have theorized before that mammals rely on some unknown physiologic mechanism to manipulate the sex ratios of their offspring as part of a highly adaptive evolutionary strategy.

"This is one of the holy grails of modern evolutionary biology -- finding the data which definitively show that when females choose the sex of their offspring, they are doing so strategically to produce more grandchildren," said Joseph Garner, PhD, associate professor of comparative medicine at Stanford University School of Medicine and senior author of the study.

Scientists assembled three-generation pedigrees of over 2,300 animals and found that grandmothers and grandfathers were able to strategically choose to give birth to sons, assuming those sons would be high-quality and in turn would help reward them with more grandchildren. According to Garner, this process is largely controlled by the females.

"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," he said. "You can think of this as being girl power at work in the animal kingdom."

He said, "We think of reproduction as being all about the males competing for females, with females dutifully picking the winner." However, the researchers have learned that females "are making highly strategic decisions about their reproduction based on the environment, their condition and the quality of their mate."

Scientists Robert Trivers and Dan Willard, founders of the field of evolutionary sociobiology, first proposed a theory in 1973 that challenged the conventional wisdom that sex determination in mammals is random, with parents having a 50-50 shot at having a male or a female. Trivers and Willard hypothesized that mammals manipulate the sex of their offspring in order to maximize their own reproductive success.

In 1984, T.H. Clutton-Brock of the University of Cambridge found that dominant mothers of wild red deer produced significantly more sons than deer who held a subordinate position within the herd.

"This paper was a huge leap forward, providing the first suggestion that the idea might work in mammals," Garner said. "But because it relied on data from only two generations, it couldn't show whether females that produced more sons also gained more grandchildren from those sons."

He added that this key prediction of the hypothesis has remained untested, because complete three-generation pedigrees are hard to obtain in the wild.

The researchers of the latest study were able to advance the study by reconstructing three-generation pedigrees of multiple species. The project required years of work to reconstruct the pedigrees and breeding histories of the animals. They ended up with a pool of 1,627 female grandparents and 703 male grandparents for whom they had a complete record of three generations.

The team found that when females produced mostly sons, those sons had 2.7 times more children per capita than those whose mothers bore equal numbers of male and female offspring.

"The question is, within each species, among females who had more sons, did those sons do better in terms of producing more grandchildren per capita? And the answer is yes," Garner said. "Females are choosing and being very Machiavellian about it. They're doing it for their own benefit. A grandfather producing more male offspring also has more success. But that could be entirely determined by the female."

The researchers concluded that a better understanding of sex-ratio manipulation in captive animals could help lead to interventions that would help preserve the species.