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.

Read: Why are gorillas committing mob violence?

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.