Such “mob violence,” as Rosenbaum puts it, is especially surprising because it took place near Rwanda’s Karisoke Research Center, the institute established by the prominent primatologist Dian Fossey in 1967. The gorillas there are some of the most well-studied primates in the world, and yet no one had seen anything like the 2004 attack before.

It wasn’t a one-off incident, either. In June 2010, the Karisoke staff saw another mixed-sex group of 42 gorillas beating, biting, and dragging a lone male for 18 minutes. The victim escaped, but died from his injuries. In May 2013, another group of nine gorillas was seen attacking two victims—again including poor Inshuti.

These acts are somewhat similar to those documented in chimps, where groups of males will deliberately kill individuals from rival groups, attacking them in teams and beating them to death. One such campaign, which lasted for four years, has been described as chimp warfare.

But while the Karisoke gorillas were attacking in groups, they didn’t stalk their victims like chimps do. “We didn’t have any evidence of premeditation,” says Rosenbaum. Instead, it seems that Inshuti and the other targets just happened to be in the wrong place at the wrong time. Also, in chimps, it’s only the males that wage war. Among the gorillas, females and juveniles also took part in the beatings.

That was the most surprising part of the discovery, says Martha Robbins, a primatologist at the Max Planck Institute for Evolutionary Anthropology who has studied these gorillas. “I find it extremely unlikely that an adult female with a small baby attached would start biting a silverback, who is twice her size,” she says. And yet that’s exactly what Rosenbaum saw. “They were actual participants: hair-pulling, biting, kicking.”

“You might think this is the kind of thing animals do all the time, but they rarely do,” says Joan Silk, from Arizona State University. This suggests that for most species, the costs of such violence outweigh the benefits. And in a minority, the balance shifts.

Outnumbering your opponents, for example, greatly reduces the cost of violence; you're less likely to suffer retaliatory injuries if you attack together. In chimpanzees societies, groups frequently split apart and reform, which creates many chances for coalitions to come across lone individuals. Such encounters are rare for gorillas, which typically live in large stable groups with several females and a single dominant male. But since the 1990s, the Karisoke gorillas have been slowly forming larger groups, containing three or more adult males each.

No one knows why. “There’s been some speculation about the role that humans might have played, or climate change, or that the population is denser now, but none of those are convincing explanations,” says Rosenbaum. Whatever the reason, these large groups created an imbalance of power, where individual bachelors like Inshuti would suddenly find themselves greatly outnumbered. “If you only have one attacker, it’s dangerous,” says Rosenbaum. “Gorilla males have huge teeth and are extremely strong. But once you get enough males in the group, the cost to individual participants is low.”