A man's spit may indicate what kind of father and husband he is. In polygamous societies, men with high levels of testosterone in their saliva are more likely to take several wives and give their children less attention, compared to those with less of the sex hormone coursing through their bodies.
The new study of rural Senegalese villagers adds to previous work underscoring testosterone's critical role in a mating and parenting.
High testosterone levels have been linked to increased sexual activity, infidelity and marital conflict. However, after men become fathers, their bodies typically pump out less of the hormone.
"This is good for us, so we can adapt to social challenges very quickly," says Alexandra Alvergne, an anthropologist at the University of Montpellier, France, and the University of Sheffield, UK, who led the new study.
To find out whether testosterone's connection to mating and parenting also applies in societies where men may have several wives, Alvergne measured testosterone levels in 21 polygynous fathers as well as 32 monogamous dads and 28 unmarried men without children, all living in Senegalese villages. She also asked the men's wives how much time and money their husband devoted to his family.
No matter how many wives they had, fathers had lower testosterone levels than single men, on average, Alvergne and her colleagues found. Among fathers, those with more testosterone tended to invest less time in their wives and children. And polygynous men under the age of 50 produced more testosterone than monogamous men, on average.
Older men with more than one wife made less of the sex hormone than other men. While older men may make less testosterone, they typically enjoy more prestige in their villages, which could make it easier to find multiple wives, Alvergne suggests.
"I think the evidence is piling up" that testosterone affects mating and parenting in humans, says Peter Ellison, an anthropologist at Harvard University.
Genetics plays some role in determining how much testosterone men produce, but culture, fatherhood and other factors tinker with its levels over a lifetime.
"If you have a young child – a young baby – that you're at times responsible for, it would be really good to lower your testosterone. Not only are you less likely to forget the child and pursue some other mating opportunity but your temper may be lowered," Ellison says.
While it isn't clear exactly how fatherhood tempers testosterone levels, Ellison and others believe that paternal behaviour feeds into the endocrine system to crank down its natural levels.
In cultures where men aren't expected to be outstanding fathers and are constantly on the lookout for potential mates, testosterone levels tend to stay high, Ellison says. "It's not that polygynists somehow have different genes that make them polygynists."