Group conflict among non-human animals, from mongooses to monkeys, is well known. However, lethal conflict between previously socially affiliated animal groups outside of humans, in whom cultural ideologies can generate divisions among individuals within the same group, had not been observed before. Sandel et al. now describe the gradual dispersal of a group of Ngogo chimpanzees over many years, ending with two socially isolated groups, one of which carried out multiple lethal attacks against the other, leading to the deaths of both adults and infants (see Brooks's Perspective). The unrelated deaths of key, interconnected individuals may have contributed to the eventual violent split. —Sacha Vignieri

SCIENCE