Fractured Society

Two hundred chimps are embroiled in a rare "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 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.