Keeping an eye on the kids has been a task often carried out by females across the animal kingdom. Among non-human mammals, the theory has been that evolution played a part — that it was more important for the males to focus on mating over parenting since the benefits were arguably greater.
Mountain gorillas, however, behave differently. Organized in social groups that often contain multiple males, they will often take care of and interact with infants that are not their own, in essence helping to raise all of the group's young.
Scientists were curious as to why this behavior was happening and what it might say about our own evolution as humans.
A dad to many, a father to none (yet)
To determine if they were onto something regarding mountain gorilla behavior, the researchers looked at hundreds of hours of observations collected by the Dian Fossey Gorilla Fund, based out Rwanda, between 2003 and 2004. Specifically, the researchers calculated the total percentage of focal follow time that was exhibited between males and infants younger than 3.5 years old. This "follow time" was comprised of both resting physical contact and grooming activities.
What the researchers found was that the males who spent the most time with infants sired more children of their own, sometimes as many as 5.5 times as many offspring as those males who didn't exhibit much interest in the young members of the group.
Such an increase is huge. "Usually when we’re talking about reproductive strategies, we're talking about tiny margins — things that increase your success just a fraction," Cat Hobaiter, a primatologist from the University of St. Andrews, told The Atlantic. "A five-fold increase is incredible."
"Males are spending a lot of time with groups of kids — and those who groom and rest more with them end up having more reproductive opportunities," said Kuzawa continued. "One likely interpretation is that females are choosing to mate with males based upon these interactions."
This trend continued even after researchers accounted for the differences among the males' ranks in the group and their ages. Even among the beta males, the researchers found the same uptick in offspring.
"We've known for a long time that male mountain gorillas compete with one another to gain access to females and mating opportunities," Christopher Kuzawa, professor of anthropology at Northwestern University, said in a statement, "but these new data suggest that they may have a more diverse strategy. Even after multiple controls for dominance ranks, age and the number of reproductive chances they get, males who have these bonds with kids are much more successful."
The researchers published their findings in the journal Scientific Reports.
Paternal care and hormones
The confirmation of this behavior in gorillas may point to an alternative path for how fathering behaviors evolved among our early ancestors.
"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," said Stacy Rosenbaum, lead author of the study and a post-doctoral fellow in anthropology at Northwestern. "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."
In addition to the reproductive benefits and the possible evolutionary ones, it could also indicate biological changes, something the researchers will focus on next.
"In human males, testosterone declines as men become fathers, and this is believed to help focus their attention on the needs of the newborn," Kuzawa said. "Might gorillas that are particularly engaged in infant interaction experience similar declines in testosterone? Because this would probably impede their ability to compete with other males, evidence that testosterone goes down would be a clear indication that they must be gaining some real benefit — such as attracting mates. Alternatively, if it does not go down, this suggests that high testosterone and caretaking behavior don't have to be mutually exclusive in mountain gorillas."
And the latter concept would indicate that there's something pretty "manly" about raising kids, even if they're not yours.