The world’s largest known group of chimpanzees recently burst into a lethal conflict. Much like in a civil war, the group fractured into two. Then one faction began killing former group mates on the other side, researchers write in the journal Science. It’s an exceedingly rare event: scientists estimate that chimpanzee communities split, on average, every 500 years.
The paper is “a tour de force,” says Joseph Feldblum, an evolutionary anthropologist affiliated with Duke University, who was not involved in the study.
The research relied on three decades of data, starting in 1995, from Kibale National Park in Uganda, home to the Ngogo chimpanzees—a community of about 200 individuals. Social relationships here clump around two primary groups, named the Central and Western clusters. For decades they formed a single community in which chimps shared territory, changed their typical cluster affiliations and sometimes mated across clusters—until 2015, says the study’s lead author, Aaron Sandel, a primatologist at the University of Texas at Austin.
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On June 24 of that year, Sandel was observing Ngogo chimpanzees when a Western party approached a Central one. Typically the two would mingle and then split. But this time the Western chimps quieted when they heard their Central group mates. Then they ran away, and the Central bunch chased after them.
“Nothing like that had been observed before,” Sandel says. Even his colleague John Mitani, a University of Michigan behavioral ecologist who had studied chimps for two decades, didn’t know what to make of the scuffle. And Mitani is “rarely surprised by what they do,” Sandel adds.
The clusters began to separate geographically and socially. By 2017 the two groups occupied entirely distinct territories and patrolled their borders against outsiders. The following year deadly violence ensued.
A group of about 200 chimpanzees has split into two factions in Uganda’s Kibale National Park.
Beata Whitehead/Getty Images
Between 2018 and 2024 the researchers witnessed Western adults kill seven males and 17 infants from the Central group, the paper reports. An additional 14 adolescent or adult Central males disappeared during that time, and their bodies were never recovered—these males hadn’t shown signs of illness, so at least some of them also might have fallen prey to the Western group’s aggression. Sandel’s team has documented at least two more attacks on Central males since the data analysis for this study ended in 2024.
The authors say that the group split because its social relationships broke down. Sandel suspects those ties were strained by an unusually large group size, competition over food and reproduction, changes in which individuals were alpha males, and diseases killing adults that had been key bridges between clusters.
A similar fracture happened in Gombe National Park in Tanzania in the 1970s. There the late primatologist Jane Goodall observed that several chimpanzees had split off into a new group, and former group mates later killed many of them. Similar events preceded that break: the clustering of social relationships, a rise in reproductive competition, new alpha males and the deaths of several community-bridging males. But because humans frequently gave Gombe chimps bananas, some primatologists doubted whether the group fission was a natural behavior.
“[Ngogo] is the first time that you could say definitively that the civil war is actually happening,” Sandel says.
Now he wants to understand whether social relationships drive conflicts in humans as they do for our closest living relatives. One prominent theory is that war stems from cultural differences such as ethnicity, language and religion. Peace interventions, such as cultural diplomacy, focus on bridging these divides. But if, like for chimpanzees, managing conflicts boils down to maintaining cordial and inclusive social networks, then these efforts may miss the most important point, Sandel says: it’s not just about understanding the cultures of other groups but also about nurturing friendships that connect them.
“What we have to do is maintain interpersonal relationships,” he says. “In our own daily lives with the people that we interact with, if we can reunite—even in the face of conflict—then I think that’s a recipe for maintaining peace.”
