The largest group of wild chimpanzees known to scientists has permanently split in two. In a study published in Science , researchers from The University of Texas at Austin and other institutions report the first clearly documented permanent fission in wild chimpanzees and the sustained intergroup violence that followed. The findings draw on three decades of field observations of the Ngogo chimpanzees of Kibale National Park, Uganda, a population featured in the Netflix documentary series Chimp Empire .
The community was cohesive for the first two decades of research. Individual chimpanzees moved between flexible subgroups, or "clusters," and maintained social ties across the community — a fission-fusion dynamic typical of the species, in which individuals temporarily separate and reunite. In 2015, however, the team witnessed signs of polarization, with the Western and Central clusters increasingly avoiding each other. This shift coincided with a change in the male dominance hierarchy and came one year after the deaths of several adult males who may have functioned as bridges holding the larger community together.
By 2018, the fission was complete. The chimpanzees now belonged to two distinct groups — Western and Central — with separate territories. What followed was a series of lethal attacks by the Western group on members of the Central group. Between 2018 and 2024, researchers observed or inferred with high confidence seven attacks on adult males and 17 on infants.
"What's especially striking is that the chimpanzees are killing former group members," says Aaron Sandel, associate professor of anthropology at UT Austin and the study's lead author. "The new group identities are overriding cooperative relationships that had existed for years."
In many primate species, large groups regularly split into smaller ones, often reducing competition for resources. But in chimpanzees, permanent fissions are extraordinarily rare. Genetic evidence suggests they occur roughly once every 500 years. The only previously reported case took place in the 1970s at Gombe, Tanzania, during Jane Goodall's long-term study. But that case has remained a subject of debate in part because the chimpanzees there were provisioned with food by researchers. At Ngogo, the chimpanzees were never provisioned, and the picture is more complete, thanks to nearly three decades of study by John Mitani, emeritus professor of anthropology at the University of Michigan, and a large team of researchers and Ugandan field staff.
"I would caution against anyone calling this a civil war," says Sandel. "But the polarization and collective violence that we have observed with these chimpanzees may give us insight into our own species."
The authors describe their findings as a challenge to the hypothesis that human warfare, including civil war, is driven primarily by cultural markers of group identity such as ethnic or religious differences.
"If relational dynamics alone can drive polarization and lethal conflict in chimps without language, ethnicity, or ideology, then in humans, those cultural markers might be secondary to something more basic," says Sandel. "If that's true, then we may have the potential to reduce societal conflicts in our personal lives, and that gives me hope. As our paper concludes, it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace."
Science
Lethal conflict following group fission in wild chimpanzees
9-Apr-2026
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