Two hundred chimpanzees in Uganda are embroiled in a ‘civil war’

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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 their former group mates on the other side, researchers write today in the journal Science. It’s an exceedingly rare event: scientists estimate that chimpanzee communities split, on average, every 500 years.

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The research relied on three decades of data dating to 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 chimpanzees shared territory, changed which cluster they most often affiliated with and mated with individuals from the other cluster—until 2015, says the study’s lead author Aaron Sandel, a primatologist at the University of Texas at Austin.

On June 24 of that year, Sandel was observing Ngogo chimpanzees when a Western party approached a Central one. Normally, 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 chimpanzees for two decades, didn’t know what to make of the scuffle. And Mitani is “rarely surprised by what they do,” Sandel adds.

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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.

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 may have been victims of Western chimpanzee aggression. Sandel’s team has documented at least two more attacks on Central males since the data analysis for this study ended in 2024.

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