Cantsbee the gorilla was great with kids, which is why it struck gorilla researcher Stacy Rosenbaum as odd when he suddenly started grunting aggressively at his usual gaggle of baby gorilla sidekicks. Cantsbee seemed oddly annoyed by them tagging along behind.

“Cantsbee was always incredibly gentle,” says Rosenbaum. “He was never aggressive towards infants or humans. So the infants looked startled, not sure what to do.” Eventually they got the hint and moved off into the bushes to go around the grumpy Cantsbee, who then began displaying aggressively at Rosenbaum, too.

“I thought he was just having a really bad day,” she recalls. “But then I realized he was sitting next to a snare. It sent chills down my spine—I can’t say for sure, but it seems like he was protecting not just the infants, but me, too.”

Not all male gorillas are as gentle as Cantsbee, and not all of them are followed around by a swarm of adoring kids. But it seems that Cantsbee knew what he was doing: the better a male gorilla is with babies, the luckier he is with the ladies. Rosenbaum and her colleagues have drawn on decades of data to explore the relationship between gorilla babysitting and mating success. Their results, published this week in Nature Scientific Reports, also suggest that female mate choice plays an important role in gorilla society.

Good gorilla dads

Rosenbaum spent a long time skeptical of the glaring signal she was seeing in the data. “It seemed too good to be true,” she says. The finding taps into important questions on why male parenting evolves in mammals: with limited time and resources, investing their effort in mating rather than parenting is an optimal strategy for males in a lot of different species. But male parenting does happen. In some species, males figure out who their own offspring are and look after them, giving their own genes an advantage.

That doesn’t explain gorillas like Cantsbee, though, who happily watch out for kids that aren’t their own—could it be that males who do this are more successful in some way? To figure that out, Rosenbaum and her colleagues needed paternity data, which involves collecting poop. “We have to get at least three samples from every individual,” she explains. “Then we do a bunch of back-checking in the lab. Did the samples definitely come from the same gorilla? Are we one hundred percent sure that mother-infant pair are who we think they are?”

When they compared the paternity data to behavioral data that showed how much the male gorillas interacted with babies, the results were pretty startling: the better babysitters were having way more babies themselves. “At the time, we said, 'Let’s just wait until we have more paternity data and see what happens,'” Rosenbaum said. When a second round of paternity data came back, and the result got stronger, “I thought, nah, it can’t be! So we sat for another round.” Only after adding in a third batch of paternity data was she convinced enough to publish.

That caution comes through in the paper, says Jo Setchell, a primatologist at Durham University who wasn’t involved with the research. It’s a great study, she says, well designed to answer the questions being asked. Not only is the question itself interesting, but the paper also “addresses it in a very careful way,” she adds. The team is “very clear about what the results are,” and what questions they can and can’t answer.

In this new work, the size of the effect is startling. The most kid-friendly gorillas had around five times more kids than the least kid-friendly group. And the researchers threw everything they could think of at that result to see if it would stick, including asking the obvious question: what if gorillas who already have more kids are just more likely to babysit?

It “cannot be entirely ruled out,” write Rosenbaum and her colleagues. But some of their findings point against it. For one thing, it didn’t seem to matter how many of his own babies a male actually had around at the time; being kid-friendly or not seems like a pretty stable trait for each individual. For another, some of the males in their sample didn’t yet have any kids of their own in 2003 and 2004 when the behavioral data was collected—but the more kid-friendly ones had more of their own babies down the line.

That doesn’t necessarily mean that the female gorillas are keeping a watchful eye on how the males around them interact with babies, sizing up whether they’d be good dads. It’s possible that it’s really all about personality, says Rosenbaum. The gorillas that are interacting with the kids might be more chill and sociable generally, and that might be what’s attractive about them. But it does imply that female mate choice could be a really strong force in natural selection for gorillas, she adds: “We tend to think of female gorillas as these passive beings having things done to them; our data suggests that’s not the case.”

The evolution of fatherhood

A dearth of data is one of the reasons why the evolutionary roots of male parenting are still an open question. Rosenbaum hopes that this paper will help to change that by getting other scientists who study species with similar social set-ups to do the same kind of comparison between paternity and infant social contact. “It would be super-useful to have comparable analyses,” she says.

That data won’t necessarily be a cakewalk to gather. The gorilla paternity analysis is the result of years and years of effort, says Setchell. “To produce such a carefully put-together study takes decades of data collection,” she notes. But results from other species would add to our understanding of what pressures can drive the evolution of male parenting—including in humans.

Making the intellectual leap directly from gorillas to humans is never a wise move. Setchell points out how much social variability exists just within the great apes: gorilla groups are often based around just one male; male chimps form coalitions; and orangutans are solitary. But while the findings can’t translate to modern human social dynamics, they are a useful piece of the puzzle in understanding how male parenting might have evolved in our ancestors.

Humans today have incredibly involved fathers, and culture plays a central role in that. But how did that get started? While gorillas can tell us very little about social dynamics in modern humans, they can give us clues about our past. “It’s less about what matters to us today,” says Rosenbaum, and more about “what mattered for our ancestors five million years ago.”

Nature Scientific Reports, 2018. DOI: 10.1038/s41598-018-33380-4 (About DOIs).