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.