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Even though modern day science has the ability to help couples choose the sex of their unborn child, this practice is illegal. By choosing sexes, population sex demographics could be skewed and the effects of that change are unpredictable. In mammalian animals, however, a new study has found the first real evidence that animals have the biological and adaptive ability to choose the sex of their unborn offspring. This finding suggests that mammals are capable of picking out the right mate that would increase the chances of reproducing male offsprings. 

"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 and senior author of the study, according to the press release.

The researchers from Stanford University School of Medicine worked with animal care supervisor, Greg Vicino from the San Diego Zoo to look through 38,000 animals from 670 species. After sifting through the data, the researchers focused on 1637 grandmothers and 703 grandfathers. They produced three-generational pedigrees of over 2,300 animals from a wide range of 198 species.

The researchers found that some grandparents appeared to have the ability to increase their chances of having male offspring through an unknown physiologic mechanism that is allowing them to change the sex ratios of their offspring. This mechanism can be studied by analyzing how dominant females tend to pick dominant and healthier mates, which then increased their chances of producing male offspring.

"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," Garner explained.

The researchers found that within their sample set, the females who were able to reproduce more males were 2.7 times more likely to have grandchildren. Having grandchildren allows the genetic line of dominant animals to continue.

"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."

The study was published in PLOS ONE.