Tracing zoo birth records through three generations, the researchers found evidence for a decades-old theory of evolutionary biology: Mammals rely on some unknown physiological mechanism to manipulate the sex ratios of their young. (Also read why some female animals store semen.)
The end result of this mysterious phenomenon is more grandchildren, and hence more of the original parents’ genes in the gene pool, according to the study, published July 10 in PLOS ONE.
“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,” lead author Joseph Garner of Stanford University said in a statement.
Scientists still don’t know how female mammals’ bodies influence the gender of their unborn babies, or exactly what factors might tip the scales in favor of more boys or more girls. (Also see “Heat Triggers Sex Change in Lizards by ‘Turning Off’ Key Gene.“)
This new study only demonstrates that biased sex ratios—not the 50/50 boy/girl ratio that one would expect by chance—happen in mammals.
So what are some of these mammals manipulating the sex of their babies?
1) Vaal Rhebok
The vaal rhebok is a species of antelope native to southern Africa. It may not look scary, but the males of this species have a violent reputation.
Herds are made up of several females and one dominant male who mates with all of them. That male will defend his harem from solitary males looking to get lucky with snorting, stamping, and chasing, and if those don’t work, serious fighting that often ends with the death of one of the contestants. (Also see “Male Antelope Scare Females Into Staying for Sex.”)
For these antelope, it’s a safe bet to have a daughter, who will become a harem member and reproduce each breeding season. But having a son is risky—he may never mate at all.
However, females that give birth to a harem-holding male hit the genetic jackpot in terms of number of grandchildren.
2) Francois’ Langur
This medium-size Asian primate is quite the looker, with black silky hair, distinguished white sideburns from its ears to the corners of its cheeks, and a tall and pointy crest of black hair on its head.
Langurs live in matriarchal groups led by the females. A social hierarchy exists for the females, although they all share in the responsibility of raising baby langurs. Groups usually only have one adult male, and young males set off in search of groups of their own when they reach sexual maturity.
Although the study found having more sons than daughters resulted in more grandchildren than having an equal number of sons and daughters, a similar but smaller effect was seen in the opposite direction. (Read: “How a Tiny Critter Has Seven (Yes, Seven) Sexes.”)
Giving birth to more daughters also led to more grandchildren than having a balanced sex ratio. This could be due to matriarchal species like Francois’ langurs: Females high up on the social totem pole can increase their odds of passing on their genes by having more daughters.
3) Human Beings
Although not studied in the new research, there’s some tantalizing evidence that this phenomenon applies to people as well.
For instance, the top-ranking wives in polygamous societies are more likely to have sons than the lower-ranking wives, according to an analysis of 19th-century Mormons. And a 2009 survey of 400 U.S. billionaires found they were more likely to have sons than daughters.
Why sons are favored are unknown, but it may be related to social factors: In the case of polygamous societies, sons hold the economic power in the family. As for the billionaires, the researchers hypothesize having more sons helps keep wealth within the family.
And, of course, those mothers with the means to do so can now choose the sex of their babies through in vitro fertilization.
More sons could translate to more grandchildren for many mammals, but scientists have yet to comment on the stresses associated with raising a passel of boys.