Basie’s killers were chimps he grew up with. While chimps are known to attack male strangers on the outskirts of their territory, what’s remarkable here is that a previously unified community, called Ngogo, fragmented and went to war with itself.
“While rare, these events show that chimps can change from being friendly to becoming enemies,” says Duke University primatologist Anne Pusey, who was not involved in the study.
It’s impossible to say exactly why splits happen, but researchers have some hunches. In the new study, scientists analyzed more than two decades of “social-network data”—detailed daily logs of who is grooming, feeding, mating or otherwise interacting with whom. These data help determine when a social gulf began to emerge, and what events directly preceded it.
Before the two sides went to war, they first stopped associating with each other— likely because of the deaths of key individuals that had moved easily among neighborhoods. “If you’re not engaging in those daily practices holding everything together, the social glue starts to fall apart,” says Catherine Hobaiter, a primatologist at the University of St. Andrews in Scotland, who was not involved the study.
Study co-authors noted that five older males and one adult female died of a likely respiratory illness in 2014, the year before the two sides started avoiding each other. “Those males were very central,” says Negrey. “To have lost them in a very short time was very significant because they had strong ties throughout the community,” in what the researchers call different “neighborhoods.”