How a woman can 'chooses' the sex of their offspring

Mammals can choose the sex of their young to increase the chance of having more grandchildren, according to new research.

The study from Stanford University School of Medicine believes female mammals can determine the sex of their children by controlling different-shaped 'male' and 'female' sperm as it travels through their reproductive system.


For example, parents in good condition invest more in producing sons, whose inherited strength and bulk give them greater opportunities to produce more offspring.

Conversely, mothers in poor condition would likely play it safe, producing more daughters whose productivity is physiologically limited.

Researchers analysed 90 years' worth of breeding records for 40,000 mammals, ranging from primates to rhinos, at the San Diego Zoo to confirm a long-held theory that animals influence the sex of their young in response to environmental conditions and other factors.

'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.

The scientists assembled three-generations 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.

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

'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.'

The study builds on a theory proposed in a 1973 paper by scientists Robert Trivers and Dan Willard.

They challenged the school of thought that claimed the sex of mammals was a random event.

However, Trivers and Willard believed mammals are selfish; manipulating the sex of their offspring in order to maximise their own reproductive success.

Other hypotheses make similar predictions - that females who choose mates with particularly 'good genes', such as attractiveness, should produce so called 'sexy sons' as a result, Garner said.

Previous hypotheses had remained untested because complete three-generation were hard to obtain in the wild.

Yet Garner and his colleagues were able to analyse three generations of multiple species by using records from San Diego Zoo.
Along with animal-care supervisor Greg Vicino, Garner and his team looked at 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.

Researchers found that when female mammals such as wolves, pictured, produced mostly sons, those sons had 2.7 times more children per capita than those whose mothers bore equal numbers of male and female children.

When female mammals such as wolves, pictured, produced mostly sons, those sons had 2.7 times more children than mothers who had equal numbers of male and female children.

Major mammal species 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 were included.

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 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 isn't really known, although 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.

Garner said there may be some parallels among humans, with some studies suggesting that they may be able to adjust their sex-ratios in response to social cues.

For instance, in polygamous societies, the top-ranking wife is much more likely to have a son than the lower-ranking wife - the son holds the economic power in the family.

A study of 400 U.S. billionaires, published in 2013, found that they were more likely to have sons than daughters - presumably, the scientists hypothesised, because sons tend to retain the family's wealth.

Lack of genetic diversity can promote inbreeding-related health problems and a population's overall vulnerability to diseases and parasites. A better understanding of sex-ratio manipulation in captive animals could help lead to interventions that would help preserve the species, they conclude.