Ugandan Chimpanzees Split Into Rival Factions and Engage in Collective Violence

Researchers have documented the first clearly verified permanent split of the world’s largest known community of wild chimpanzees, an event that triggered years of lethal violence between former social partners. The study, published in the journal Science on April 9, 2026, analyzed the Ngogo chimpanzee population in Uganda’s Kibale National Park to understand how social networks can collapse into warring factions.

The findings are based on a massive longitudinal dataset, including 30 years of demographic data, 24 years of social network analysis and 10 years of GPS tracking. This data allowed scientists to track the transition of a single cohesive community into two rival groups, identified as the Western and Central clusters.

The Three Phases of Social Collapse

The researchers identified three distinct phases that led to the permanent fissure of the Ngogo community. The process began in 2015 with an abrupt shift in social dynamics, where relationships became polarized into the two distinct clusters.

Following this initial polarization, the chimpanzees spent two years increasingly avoiding members of the rival cluster. During this period, interactions across the two groups became rare, and male chimpanzees from the Western cluster began patrolling their territory and exhibiting increased aggression toward those in the Central cluster.

By 2018, the split became permanent. The community fractured into two separate groups with their own territories: a Western group consisting of 83 chimpanzees and a Central group consisting of 107 chimpanzees.

Escalation to Collective Violence

Once the fission was complete, the smaller Western group began conducting targeted raids into the territory of the larger Central group. Between 2018 and 2024, researchers documented 17 infant deaths and seven lethal attacks on adult males.

The study suggests these figures may be an undercount of the total violence. Between 2021 and 2024, an additional 14 adult and adolescent males from the Central group died or disappeared unexpectedly, despite showing no previous signs of illness.

As a result of this prolonged conflict, the Western group has since surpassed its rival to become the dominant force within the jungle.

What’s especially striking is that the chimpanzees are killing former group members. The new group identities are overriding cooperative relationships that had existed for years.

Aaron Sandel, associate professor of anthropology at UT Austin

Social Drivers and Historical Context

The split in 2015 coincided with a shift in the male dominance hierarchy. This change occurred one year after the deaths of several adult males who researchers believe may have functioned as social bridges that held the larger community together.

Prior to the split, the Ngogo community operated under a fission-fusion dynamic, where individuals moved between flexible subgroups and maintained social ties across the entire community. The loss of bridging individuals and the subsequent polarization transformed these former companions and groomers into enemies.

This event mirrors a rare occurrence observed by Jane Goodall in the 1970s in Gombe, Tanzania, where a community split into two factions and the males of one group killed all the males of the rival group over four years, along with one female.

While the Gombe case was long considered an anomaly, genetic evidence suggests that such permanent splits are rare events that occur approximately once every 500 years.

Implications for Understanding Conflict

Lead author Aaron Sandel cautioned against labeling the event a civil war, but noted that the observation of polarization and collective violence provides a rare window into the chimpanzee mind.

Because the conflict involved former social partners rather than strangers from different communities, the study offers scientists insight into the relational dynamics that may also shape conflict and collective violence in human societies.