Between 1998 and 2009, John Mitani witnessed 18 murders firsthand, and found circumstantial evidence for three more. But no police were ever called, for these killers were all chimpanzees, from the Ngogo community in Uganda’s Kibale National Park.

Chimpanzees are highly intelligent animals, capable of great acts of empathy, technological sophistication, culture and cooperation. But they can also be murderers. Groups of chimps, mostly male, will mount lengthy aggressive campaigns against individuals from other groups, attacking them en masse and beating them to death. Their reasons for such killings have long been a source of debate among zoologists, but the aftermath of the Ngogo murders reveals an important clue. After the chimps picked off their neighbours, they eventually