The largest group of wild chimpanzees known to scientists has permanently split in two — something that is extremely rare.
Scientists with Arizona State University and The University of Texas at Austin are the first to clearly document the permanent divide and the intergroup violence that followed. The study draws 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, where 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 chimpanzees belonged to two distinct groups 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 killed former group members,” said Aaron Sandel, associate professor at The University of Texas Austin and lead author. “The new group identities overrode cooperative relationships that existed for years.”
The scientists explained that 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.
“Interestingly, in the Ngogo study, most of the active aggression has been instigated by the smaller, but more cohesive, of the two daughter groups formed by the fission,” said Kevin Lee, an ASU anthropology alumnus and research affiliate with the Ngogo Chimpanzee Project.
“Normally, larger groups are able to out-compete smaller groups, sometimes even causing the extinction of the latter. However, through a series of targeted attacks, the West group has reversed their initial numeric disadvantage, and have been able to push the Central group out of much of their formerly shared territory.”
To quantify when the split in this group happened, the researchers turned to network scientist Yixuan He, assistant professor at the School of Mathematical and Natural Sciences at ASU. He was one of the AI-mathematics leads for this study.
“I led the construction of weighted networks from the chimpanzee time series data, innovatively creating a principled way of fusing multiple networks into one based on hierarchies of distances — similarities,” he said.
“After constructing the networks, I led the analysis of their community structures, important individuals and network statistics, to quantify and visualize when the split happened, and tried to see whether polarization was developed before the actual split.”
The authors describe their findings as a challenge to the hypothesis that human warfare is driven primarily by cultural markers of group identity such as ethnic or religious difference. This study indicates that cultural differences don’t need to be present to cause group conflict; changing relationships alone can split a group and lead to violence.
“Chimpanzees have been the subject of more long-term studies than perhaps any other animal,” said Kevin Langergraber, an ASU primatologist and co-director of the Ngogo Chimpanzee Project. “This study shows that despite this, they still have interesting and important things to teach us. I hope that they will be around in the future for these sorts of studies to continue.”
Langergraber is a professor at the School of Human Evolution and Social Change and a research scientist at the Institute of Human Origins at ASU. Throughout his extensive career and research, he advocates for the conservation of chimpanzees.
The article, “Lethal conflict following group fission in wild chimpanzees,” was published in the journal Science. Other authors include Sebastían Ramírez Amaya, an alumnus of the School of Human Evolution and Social Change who died in April 2022 while doing fieldwork.
Article adapted from The University of Texas Austin.