Some mammals can 'choose' sex of offspring: study

Some mammalian species can 'choose' the sex of their offspring in order to produce extra grandchildren, scientists say.
The study led by a University of Stanford researcher is able to prove for the first time a fundamental theory of evolutionary biology: 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, senior author of the study, published in PLOS ONE.

The scientists assembled three-generation pedigrees of more than 2,300 animals and found that grandmothers and grandfathers were able to strategically choose to give birth to sons, if those sons would be high-quality and in turn reward them with more grandchildren.
Researchers turned to the San Diego Zoo, enlisting the help of animal-care supervisor Greg Vicino in combing through decades of records on more than 38,000 animals from 678 species.
The researchers ended up with a pool of 1,627 granddams (female grandparents) and 703 grandsires (male grandparents) for whom they had a complete record of three generations.

Major mammal groups were represented, including primates; carnivores, such as lions, bears and wolves; cloven-hoofed animals, such as cows, buffalo and deer; and odd-toed grazing animals, such as horses and rhinos.

They 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," he said.
The same was true of grandsires, with the researchers showing that when grandfathers produced mostly sons, those sons on average had 2.4 times more children per capita.

Garner said the mechanism through which parents manipulate the sex of their offspring isn't really known, though one theory holds that females can control the "male" and "female" sperm, which have different shapes, as they move through the mucous in the reproductive tract, selectively slowing down or speeding up the sperm they want to select.