Scientists Document First Permanent Split and Lethal Civil War in Wild Chimpanzee Group

A study published April 9, 2026, in the journal Science documents the first clearly recorded permanent split of a wild chimpanzee community accompanied by lethal intergroup violence, following nearly three decades of field observations at Kibale National Park in Uganda. The Ngogo chimpanzee community, the largest known group of wild chimpanzees anywhere, peaked at around 200 members — roughly four times the typical group size of approximately 50. Researchers led by primatologist Aaron Sandel of the University of Texas at Austin, with senior author John Mitani, a professor emeritus at the University of Michigan, documented how the community fractured into two factions, the Western and Central groups, and how the Western group then launched coordinated lethal attacks against the Central group between 2018 and 2024. The violence claimed 28 lives in total, including seven adult males and 17 infants, with additional deaths recorded in 2025 and early 2026. The study draws on observations spanning from 1995 onward, making it one of the longest continuous records of wild chimpanzee behavior ever compiled.

A single incident in 2015 set off years of fracture Researchers pinpointed the first visible sign of the rift to June 24, 2015, when two subgroups met in the center of the shared territory and the Western chimpanzees fled, triggering what the team described as a six-week avoidance phase of a length not previously observed. The community had until that point operated under a fission-fusion dynamic, with flexible subgroups coexisting peacefully within a single cohesive community. The fracture coincided with a change in the alpha male hierarchy around 2015, when a chimpanzee named Jackson deposed another male, and followed the deaths of six adult chimpanzees in 2014 who had reportedly served as social bridges between subgroups. Starting in 2016, males of the Western group began conducting patrol walks along the emerging boundary; by 2017, the Central group responded with patrols of its own and the first direct fights occurred. An epidemic in early 2017 killed 25 chimpanzees, including 14 adults, and one of the last Western males who still maintained contacts with the Central group died during that outbreak, potentially accelerating the breakdown. By the end of 2017, two distinct groups had formed, and the split was sealed by 2018, with the Western group comprising 10 males and 22 females aged 12 and older, and the Central group comprising 30 males and 39 females.

Ngogo community fracture: key events: — ; — ; — ; — ; — ; —

Smaller Western group drove all documented attacks Despite being the numerically weaker faction, the Western group was responsible for all observed attacks against the Central group, a pattern researchers described as striking. The violence took the form of biting, pounding with hands, dragging, and kicking, targeting primarily adult males and infants.

„Biting, pounding the victim with their hands, dragging them, kicking them - mostly adult males, but sometimes adult females participate in the attacks” — Aaron Sandel via Reuters

What made the conflict scientifically significant was not the violence itself — chimpanzees are known to attack and kill members of rival neighboring groups — but the fact that the aggressors and victims had grown up together, cooperated, and maintained social bonds for years before the split.

„It is hard for me to wrap my head around the fact that yesterday's friend turned into today's foe. Males in the two groups grew up with each other, knew each other their entire lives and cooperated and collaborated with each other, benefiting in the process” — John Mitani via Reuters

By 2021, the violence escalated further to include young chimpanzees as targets, and 14 additional members of the Central group disappeared without a trace by 2024. The study notes the attacks continued beyond the formal observation window, with one adult male, one adolescent male, and two infants killed in 2025 and early 2026.

500 (years) — estimated genetic interval between chimpanzee group splits

Gombe precedent revisited — and why Ngogo matters more The only previously known comparable event was the Gombe Chimpanzee War documented by the late primatologist Jane Goodall, who died on October 1, 2025, in Tanzania during the 1970s. In that case, a community in Gombe National Park split and the northern group systematically killed all members of the southern group over four years. That case was long considered a possible anomaly, partly because the chimpanzees involved had been partially fed by researchers. A separate study published in the journal Proceedings of the Royal Society B, led by Joseph Feldblum of Duke University, reconstructed the social dynamics of the Gombe community using data from 1967 to 1975, finding that polarization and avoidance preceded the violence by several years, beginning as early as 1970.

The Ngogo case carries greater scientific weight than the Gombe precedent because the animals were never artificially fed and the observation record spans nearly 30 years, removing a key source of doubt that had surrounded the earlier findings. Researchers and outside commentators pointed to the group's exceptional size as the most likely root cause: with nearly 200 individuals including more than 30 adult males, the Ngogo community had grown beyond what Pan troglodytes social structures can sustain. Roman Wittig of the research organization CNRS in Lyon, who was not involved in the study, noted that the Ngogo group had grown so large partly because between 1999 and 2010 it had nearly wiped out a neighboring group and integrated surviving females. Genetic evidence cited in the study suggests that permanent splits of this kind occur roughly once every 500 years in chimpanzee populations, making the Ngogo case an exceptionally rare natural event. Aaron Sandel cautioned against drawing direct equivalences to human conflict, while acknowledging the broader implications.

„These changing identities and group dynamics observed in human civil wars rarely have a parallel in other animals, but one exists in chimpanzees” — Aaron Sandel via Le Soir

The study also warns that any human activity disrupting chimpanzee social cohesion — including deforestation, climate disruption, or epidemics — could make such conflicts more frequent than the genetic baseline suggests.