For nearly 30 years, researchers at Ngogo in Uganda’s Kibale National Park watched the world’s largest known wild chimpanzee community share food, groom one another, and patrol their territory as a single group. Then, around 2015, something shifted. Old alliances began to fracture. By 2018, the community of roughly 190 chimpanzees had split permanently into two factions. What followed was not a quiet separation. It was organized, lethal violence between animals that once lived side by side.
A peer-reviewed study published in the journal Science in May 2026 documents at least seven coordinated attacks on adult males between 2018 and 2024, carried out by former groupmates against former groupmates. Additional violence targeted adolescents and infants. The researchers behind the paper, led by a team based at the University of Texas at Austin, describe it as the first clearly recorded case of a permanent community fission in wild chimpanzees followed by sustained, organized killing.
John Mitani, a primatologist at the University of Michigan and a co-author of the study, told reporters that the scale of the post-fission violence was unlike anything the team had anticipated. “We watched these chimpanzees live together for years,” Mitani said in the AAAS press release accompanying the paper. “To see them turn on each other with this level of coordination and lethality was deeply unsettling.” The press release called the event a rare “civil war” among wild chimpanzees.
A community too large to hold together
Most wild chimpanzee communities number between 20 and 80 individuals. Ngogo’s group, at roughly 190 members before the split, was an outlier by any measure. That size gave researchers an extraordinary dataset but may also have planted the seeds of the fracture. Large communities generate more competition for food, mates, and social rank, and they strain the alliance networks that hold chimpanzee societies together.
Field teams first detected social polarization within the group around 2015. Individuals who had once ranged freely across the territory began clustering into two distinct subgroups, one centered in the western part of the range and the other in the central zone. By 2018, the division was permanent: 83 chimpanzees in the Western faction, 107 in the Central faction, according to figures reported by the research team and consistent with institutional summaries accompanying the Science paper.
The split did not bring peace. Instead, it triggered a pattern of lethal raiding that the researchers tracked for six years. The seven confirmed attacks on adult males represent a floor, not a ceiling. The AAAS press release explicitly noted that the true death toll is likely higher because many individuals simply disappeared during the study period, and field teams could not always determine whether a missing chimpanzee had been killed, died of disease, or moved beyond the observation area.
Violence that escalated over time
One of the study’s most striking findings is that the killing did not taper off as the two groups settled into separate territories. Instead, the violence appears to have intensified. The Science paper reports that post-fission infanticide increased in frequency after 2021, suggesting the conflict deepened rather than burned out.
Lethal aggression is not unprecedented at Ngogo. A separate, earlier study published in the journal Primates documented inter-community infanticide by male chimpanzees at the site in the late 1990s, establishing that this population has a history of fatal violence. But the post-fission pattern represents something different in scale and character: sustained, repeated raids between groups whose members share a recent common social history.
The attacks were overwhelmingly male-driven. Coalitions of males from one faction targeted males from the other in coordinated assaults, a pattern consistent with what primatologists call “lethal coalitionary aggression.” What remains poorly documented is the role of females. Whether females actively chose sides during the split, altered their ranging patterns to avoid conflict zones, or bore disproportionate costs from the upheaval is not detailed in the study’s published materials.
What researchers still cannot explain
The study provides a detailed chronology and a confirmed body count, but it does not identify a clear trigger for the original polarization. No primary quantitative data linking environmental pressures, such as fruit scarcity or seasonal drought, to the timing of the 2015 fracture have been presented in the paper’s abstract or accompanying press materials. That leaves open a fundamental question: Did ecological stress push the community apart, or was the split driven primarily by social dynamics among competing male alliances?
The answer matters beyond Ngogo. Across equatorial Africa, chimpanzee habitat is fragmenting as forests shrink and human activity encroaches. If large communities are inherently unstable and prone to violent fission, then conservation strategies that focus on protecting big populations in intact forests may need to account for the possibility that those populations could tear themselves apart. But the current data from Ngogo cannot answer that question directly, and no official statements from Ugandan park authorities about how the fission has affected chimpanzee numbers or habitat management have appeared in available reporting.
Why the Ngogo dataset carries unusual weight
Community fissions have been suspected at other chimpanzee study sites, most notably at Gombe in Tanzania, where Jane Goodall’s team observed a group split in the 1970s that was followed by lethal attacks. But direct, long-term documentation of the process from start to finish has been elusive. What sets Ngogo apart is the depth of the baseline data. After roughly three decades of continuous observation, field researchers could identify nearly every individual by sight. They knew who associated with whom, who groomed whom, and who patrolled with whom before the split occurred.
That familiarity allowed the team to confirm, with a level of certainty rarely possible in wildlife research, that the aggressors and victims in the post-fission raids were not strangers from a neighboring territory. They were former allies. The longitudinal, individual-level dataset gives the findings a weight that shorter or less detailed studies could not achieve.
Still, the researchers were careful not to overstate their numbers. By acknowledging that disappearances likely mask additional deaths, they set a methodological standard that strengthens rather than undermines the paper’s credibility. The seven confirmed lethal raids are what the evidence supports. The full toll of the Ngogo split remains unknown.
A conflict with no clear end in Kibale’s forest
As of the study’s publication in May 2026, the violence between the Western and Central factions at Ngogo had not stopped. The two groups now occupy adjacent territories in one of Africa’s most important remaining tracts of tropical forest, and the raids documented through 2024 show no sign of a stable truce. For the researchers who spent decades watching these chimpanzees live as a single community, the question is no longer whether the split happened. It is whether the killing will eventually subside, or whether Ngogo’s “civil war” is simply what happens when the largest chimpanzee society ever recorded breaks apart.
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*This article was researched with the help of AI, with human editors creating the final content.
