A NEW STUDY has shown that females in the animal kingdom have even more control than we thought and can actually choose the sex of their offspring.
The study, published in PLOS ONE, has shown that mammalian species do this in order to beat the odds and produce extra grandchildren, ensuring a stronger bloodline.
“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.
Scientists analysed 90 years of breeding records from San Diego Zoo and assembled three-generation pedigrees of more than 2,300 animals.
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.
The data showed 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.
Researchers 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. The process is believed to be largely controlled by the females, Garner said.
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, 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.
Though Garner said that the exact mechanism isn’t known, one theory that holds strong is that the females is picking the sperm that will produce the sex that will serve her interests the most.
Garner said that this is done by controlling 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.
There may be some parallels among humans, Garner said, with some studies suggesting that they may be able to adjust their sex-ratios in response to social cues. For examples, in societies where a man can have more than one wife, the top-ranking wife is much more likely to have a son than the lower ranking wife.
A study of 400 US billionaires, published in 2013, also found that they were more likely to have sons than daughters — presumably, the scientists hypothesized, because sons tend to retain the family’s wealth.
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