Inshuti, a lone silverback, couldn't accept that he wasn't welcome. When he approached the Beetsme gorilla group, its males made it clear that they didn't want him around. Inshuti followed them nonetheless, and that seemed OK—at first.

But then the screaming started. Observers from the Karisoke Research Center in Rwanda watched as Inshuti sped away, pursued by three males from the Beetsme group. As the males pinned him to the ground, the rest of the group (including females and juveniles) caught up with them and attacked Inshuti as a mob. The alpha male bit into Inshuti and shook him like a wolf shakes its prey.

“It was hands-down the most surprising and disturbing thing that I have ever seen in my years in the forest,” says Stacy Rosenbaum, who researches social behavior in gorillas. Seeing an entire group attack in coordination was totally unheard of—and this is a gorilla population that has been under close observation since Dian Fossey started studying them in the 1960s. If the gorillas had been doing this kind of thing in the preceding 40 years, someone would have noticed.

Inshuti escaped with extensive injuries when the attack suddenly and mysteriously stopped short. Why he inspired such aggression is not clear, but he has the dubious honor of being the first recorded victim of gorilla-on-gorilla mob violence.

Multi-male groups mean opportunities to gang up

Although gorillas generally have the reputation of being “gentle giants,” Rosenbaum says this reputation is questionable: “This is kind of the iconic species for infanticide!” But unlike humans and our closer cousins the chimps, gorillas haven’t been known for what’s called “coalitionary aggression:” the tendency to team up with other individuals to beat the crap out of another member of the same species.

Adult male gorillas certainly don’t always get on with each other. While many conflicts between them are limited to aggressive displays like chest-beating (i.e., all bark and no bite), they do occasionally fight and kill each other one-on-one. Chimpanzees, on the other hand, share cooperative aggressive behavior with humans: they coordinate with other members of their social groups to conduct lethal attacks and raids.

One of the explanations for this difference highlights the role of risk. In a one-on-one fight, the risk to each individual is considerable: neither male has an obvious advantage, and a strong likelihood exists that if you get involved in a fight like this, you’ll end up severely injured or dead. If you have three friends at your back, though, the individual with whom you’re picking a fight is the one who’s likely to end up much worse off. The animals in the team all individually have a much lower level of risk, and attacks allow them to get rid of the threat and competition of outsiders. It’s an evolutionary win-win.

This means that animals like chimps and humans, who live in groups with multiple males, can team up and engage in coalitionary violence. Gorillas have tended to live in “harem”-like structures with one male, multiple females, and their offspring. Coalitions weren't really an option.

That changed in the 1990s. For some reason, multi-male groups started emerging, in some cases nearly reaching male-female parity. It’s probably no coincidence that coalitionary aggression—starting with the attack on Inshuti in 2004, and two further incidents in 2010 and 2013—began to pop up around the same time.

The appearance of coalitionary aggression is an exciting possibility for primatology research. “On the one hand, it’s not very nice,” says Alexander Georgiev, who studies monkey and ape reproduction and wasn't involved with this gorilla research. “But on the other hand, it shows the possibility that this is not really a fixed species trait.” The flexibility of the behavior is what’s so interesting, he explains: it shows that coalitionary violence is not a matter of “either you have it or you don’t,” but rather depends on circumstance.

Everyone’s still pretty confused

There are still plenty of questions to be answered about the attacks. To begin with, why have some gorillas started living in multi-male groups? One possibility, says Rosenbaum, is population density. Until relatively recently, poaching of gorillas kept their numbers devastatingly low, but the population in the Beetsme group’s forest has recovered to an extent. Still, the population doesn’t seem as though it has reached an unsustainable size, Rosenbaum adds. In general, food competition isn’t a major feature in gorillas’ lives: “They basically live in a giant salad bowl,” says Georgiev.

Maybe the denser population makes it harder for males to disperse and find an area where they can begin their own groups, Rosenbaum suggests. Maybe human behavior in the forest has caused enough disruption to change the gorillas’ behavior. So far, though, none of the explanations is terribly convincing, she admits. “We’re still very much at the point where we’re tossing ideas out there and kicking them around, but we don’t have one convincing explanation.”

Another question centers around Inshuti himself, who was beaten up again in 2013. “What the heck is wrong with Inshuti?” asks Rosenbaum. Is he just so odd that other gorillas want to be more aggressive toward him?

While not much is known about the gorilla who was killed in the 2010 attack, Inshuti was well-known to the researchers at Karisoke. “He’s a notorious animal,” says Rosenbaum. While it’s not obvious that other gorillas consider him a total weirdo, she explains, he seems more persistent than many other lone silverbacks. “It could be, potentially, that the [other gorillas] know that he means it in a way that other lone silverbacks don’t, and if they don’t do something, he’s just going to keep coming back.”

Let’s not take this too far

Although the mob violence paralleled the changes in gorilla society, no definite link has been found. The finding is “an important contribution to the literature on inter-group aggression,” says primate researcher Thad Bartlett, but “I think it is difficult to draw any strong conclusions about the evolution of coalitionary killing or the fitness benefits to the participants in these cases.” That is, we’ve only seen this happen three times; we don’t know yet whether this is a behavior that natural selection would favor.

Even though natural selection is Rosenbaum’s proposed explanation, she agrees. Raising the question of whether a behavior might have evolutionary consequences is important, she says, especially when that question involves the death of animals in their prime. But it’s “entirely possible that it has no evolutionary consequences whatsoever, that the death of a few more lone males won’t mean anything from a genetic perspective, and that the behavior will disappear as fast as it appeared.” More observation is needed.

And while it’s fascinating to think about what a better understanding of coalitionary aggression might mean for humans, we should not take the comparisons too far. The rise of coalitionary aggression in gorilla groups with male-female parity doesn’t mean that human polygamy would bring about peace on earth. Just because something happens in other primates, doesn’t mean that it’s unavoidable in humans, says Georgiev. If anything, he adds, it’s a reminder of how much human culture has allowed us to reduce violence dramatically over the course of our history as a species.

“We have culture wrapped up in all of this,” says Rosenbaum. “The cultural mechanisms that we’ve got in place to keep our worst impulses in check actually work remarkably well. Humans are an unbelievably peaceful species when you consider the massive social groups that we live in. I think we sometimes don’t give ourselves enough credit for that.”

Nature Scientific Reports, 2016. DOI: 10.1038/srep37018 (About DOIs).