Anyone who’s spent an afternoon chasing after an unruly three-year-old understands that babysitting is not for the faint of heart. But it turns out, at least for male mountain gorillas, joining the babysitter’s club has some major fringe benefits. Ed Yong at The Atlantic reports that male gorillas who participate the most in babysitting duties sire more than five times the offspring as male gorillas who avoid child care.
It turns out that humans and gorillas are the only great apes in which males form strong social bonds with their young. In fact, male gorillas are often quite snuggly, letting infant and juvenile gorillas cuddle, play and just hang out in their nests.
In a 2015 paper, biological anthropologist Stacy Rosenbaum of Northwestern University began studying this unusual babysitting behavior among male gorillas. The Atlantic’s Yong reports that while she expected that most of the grooming, playing and feeding would occur between offspring and their biological fathers, that turned out not to be the case. The gorillas looked after the young no matter who fathered them and gave no special attention to their offspring. That, it turns out, is extremely rare among animals, since most caregiving fathers choose to expend energy and resources on their own offspring.
In a new study published in the journal Scientific Reports that built on her previous work, Rosenbaum and her team analyzed hundreds of hours of gorilla footage in Rwanda collected by the Diane Fossey Gorilla Fund in the early 2000s. The team looked at the genetic paternity data for 23 adult males and 109 offspring. The data showed that males that hung out with juveniles the most had 5.5 times more babies than those who showed minimal interest in the little ones.
“We traditionally have believed that male caretaking is reliant on a specific social structure, monogamy, because it helps ensure that males are taking care of their own kids,” Rosenbaum says in a press release. “Our data suggest that there is an alternative pathway by which evolution can generate this behavior, even when males may not know who their offspring are.”
The researchers controlled the data for things that could also influence the number of babies male gorillas had, like rank within the group, age and mating opportunities. However, the kid-lovers—even those at the bottom of the social register—still came out on top in terms of siring young.
So why did the babysitters have better luck passing along their genes? The researchers speculate that that taking care of juveniles is an attractive trait to the female gorillas, and seemed to be more important than size, strength or social status. It’s possible that some trait linked to babysitting is preferred by female gorillas. Or, as Yong reports, it’s possible that the females find the babysitting itself sexy.
The researchers would now like to look at hormonal influences on babysitting. Previous studies have shown that in human males, testosterone declines when men become fathers, and that may lead them to focus more on child care. The team would like to know if this happens when gorillas begin babysitting. A reduction in testosterone might mean they have trouble physically competing with other males, but might gain some other advantage. It could also show that testosterone doesn’t go down, which would indicate that high testosterone levels and parenting are not mutually exclusive.
The study also sheds some light on how human fatherhood got its start. It’s possible that similar factors influenced early human species and groups, which were similarly polygamous.
“Our results really speak to a route by which the kind of fathering behavior we see in modern humans might have gotten a toehold amongst our extinct relatives,” Rosenbaum tells Abbey Interrante at Newsweek. “Human fathering is unquestionably costly—men invest a lot in their children, in cultures across the globe. But this research shows a path that selection might have taken to help establish social bonds between males and infants, among our fossil relatives.”