Isabukuru, a silverback mountain gorilla that lived in Rwanda until his death last year, was known for being exceptionally affectionate toward the infants in his group. “One infant, named Mushya, was his favorite,” says Stacy Rosenbaum, an anthropologist at Northwestern University. Isabukuru “would routinely pick Mushya up, groom him, and try to get him to interact, when Mushya clearly wanted to be doing something else, like playing with his age-mates.”
Though Isabukuru’s fondness for infants was especially striking, such behaviors are fairly common among the mountain gorillas of Rwanda’s Volcanoes National Park, the same group that Dian Fossey studied. The males, whether silverbacks or subordinates, will cuddle infants, play with them, welcome them into their nests, and just plain hang out with them. “I often describe it as babysitting,” Rosenbaum says.
“They’re incredibly tolerant,” she adds. Contrary to the common portrait of male gorillas as aggressive, chest-thumping animals, “they can be very gentle, or even loving. Even males that aren’t interested will let infants climb on their back, or sit under them while eating. They’ll let the infants do things to them that they wouldn’t let even a subadult get away with. It’s certainly not the stereotypical image you have of male mountain gorillas.”
Many mountain-gorilla groups include several adult males; some have as many as nine. The silverback might sire the majority of infants, but subordinates reproduce, too. So when Rosenbaum first noticed males babysitting infants, she naturally figured that they must be looking after their own babies. She was wrong. “They really don’t seem to have any preference for their own offspring,” she says.
That’s surprising. It takes time and energy to look after babies, and evolutionary theory predicts that males would only bother doing so for their own offspring, which carry some of their genes. For that reason, paternal care should be far more common in monogamous species, where males can be sure that babies are actually theirs. It should also be relatively rare in polygamous species like gorillas, where a male’s paternity isn’t guaranteed. Clearly, that’s not the case. So why do male gorillas look after babies that aren’t theirs?
To find out, Rosenbaum and her colleagues sifted through hundreds of hours of behavioral observations that workers from the Dian Fossey Gorilla Fund collected in the early 2000s. Based on this data, the team showed that males that spent the most time with infants ended up with 5.5 times as many offspring of their own as those that were least interested.
It was a huge difference. “Usually when we’re talking about reproductive strategies, we’re talking about tiny margins—things that increase your success just a fraction,” says Cat Hobaiter, a primatologist from the University of St. Andrews. “A fivefold increase is incredible.”
It was a huge difference, and one that held even when the team accounted for the males’ varying ages and ranks. Even when they discounted the silverbacks and looked only at beta-males or those of a lower rank, they found the same trend. “It flies in the face of what we expect,” Rosenbaum says.
Fossey wrote briefly about the bonds between male and infant gorillas in the 1970s, as did her student Kelly Stewart much later. But few others tugged on that intellectual thread. “I think people just thought that these relationships were cute but had no evolutionary consequences, and weren’t something worthy of study,” Rosenbaum says. Instead, researchers focused more on how gorilla males vie for mates—an intense competition that explains why males are so much bigger than females. But clearly, size and strength aren’t everything. As Rosenbaum shows, babysitting prowess is tied to reproductive success.
It’s possible that the males that have already sired the most offspring are also more likely to pay attention to infants—but Rosenbaum thinks that this explanation can’t be the whole story. After all, some of the males in the study were very young and had barely started fathering their own babies. And yet their attentiveness to other infants predicted their future reproductive success.
The more likely explanation is that females are preferentially mating with the males that engage most with the group’s infants. They might be attracted to personality traits that, coincidentally, make males more likely to babysit. Or—and this is perhaps the most interesting possibility—it could be that the babysitting is attractive in itself. By mating with males that are most attentive to infants, female gorillas give their own offspring a better chance in life.
Evolutionary biologists have a long history of neglecting the behavior of female animals, perhaps due to patriarchal attitudes that downplayed females’ agency and neglected their ability to act as a potent evolutionary force. Research like Rosenbaum’s shows what scientists have missed as a result.
“It’s an excellent study,” says Elise Huchard from the University of Montpellier, who studies the social lives of primates. “It provides a landmark to understand the evolution of paternal care in mammals, and opens up fascinating questions.” Could females, for example, influence the identity of the next harem leader by specifically mating with the males that show the best babysitting skills? These questions are also relevant to us. Like gorillas, humans initially lived in polygamous communities, and many societies still do. We’re exactly the type of primate that, according to traditional evolutionary theory, is unlikely to show paternal care. In fact, the opposite is true: Across cultures, fathers have a hand in looking after children. So the patterns that Rosenbaum saw in gorillas might help us understand our own evolution.
“The scenario we see in gorillas may have been important in humans as an initial toehold that eventually led to the evolution of our more elaborate form of paternal care,” Rosenbaum says. “Here’s a route by which we could have gotten started.”