Males who grow up in an environment where they must compete for mates are more likely to breed boys than girls, according to research on mice carried out at the University of Western Australia and published in the journal Evolution Letters.
When mammals mate, it’s the father that determines the baby’s sex (unbeknownst to King Henry VIII). His X chromosome, combined with the mother’s, will produce female offspring while a Y chromosome creates a male.
However, there is growing evidence that sex allocation, previously thought to be adaptive and unmalleable, may respond to various environmental and social conditions.
“Consequently,” Renée Firman and co-authors write, “genetic sex determination is no longer viewed as the all-powerful constraint on sex allocation that it was once considered to be.”
It’s also recognised that males may have more control over their offspring’s gender than previously thought. Therefore, Firman and colleagues set out to test how upbringing might sway the outcome.
“We knew from our previous research on mice that male exposure to different social conditions during development leads to changes in fertility,” she says.
In that research, Firman’s group wanted to learn what would give males an advantage in a “sperm competition”.
They found that when wild male house mice (Mus musculus domesticus) were housed with other males, generating a strong perception that they had to compete for female mates, they produced more, better quality sperm than when growing up with low competition.
In the new study, the researchers created similar conditions, exposing the mice to environments with high male or high female density. They included a natural experiment investigating the impact of siblings’ gender ratios in utero on future sex allocations.
Once the male house mice reached sexual maturity, they analysed their proportions of X and Y chromosomes using a specialised genetic assay and tested other male traits that give a competitive mating advantage including the size of their bodies and testes.
They found that higher male density in both the natural and controlled environments, pre- and postnatally, produced more Y chromosomes as well as signs of the other male traits tested.
This is the first study to show that the sperm sex ratio is sensitive to a father’s social environment, says co-author Paco Garcia-Gonzalez, from the Donana Biological Research in Spain.
Firman acknowledges, however, that the result seems counter-intuitive in an evolutionary sense. “The ‘local mate competition’ theory actually predicts that under high male densities parents should produce daughters,” she says. “So, in essence, what we found in our study goes against this theory.”
But, she adds, “because fertilisation and the development of the young occurs within the female, we expect that offspring sex is largely under maternal control”.
In fact, in a similar experiment yet to be published, females reared in environments with high male density produced litters with higher female ratios.
“This result reflects the optimal fitness scenario for mothers because the production of daughters, which are guaranteed of high mate availability, reduces mate competition for their sons.”
As far as the current study goes, Firman notes it would be interesting to see if human males have similar responses to competitive social environments, “perhaps by looking at sperm sex ratios in relation to the number of brothers a male has”.
She adds that the findings could have important implications for other fields interested in offspring gender selection, such as human research into preventing sex-related genetic disorders, wildlife conservation and livestock industries.