In the mid-1970s, after more than a decade of observing chimpanzees at Tanzania’s Gombe National Park, Jane Goodall witnessed an unexpected rupture: a previously unified community split into two hostile factions that began killing one another. That episode forced her to revise assumptions about chimpanzee behavior and revealed a capacity for brutal, sustained violence.
Until recently, that Gombe breakdown was the only well-documented case. A new paper in Science reports a second prolonged factional split — effectively a chimpanzee civil war — that has permanently divided the Ngogo community, the largest known wild chimpanzee group, in Uganda’s Kibale National Park. Ngogo once included nearly 200 individuals that moved in intermingling subgroups or clusters. Members of different clusters socialized, mated, hunted and cooperated to defend territory and repel outsiders.
Beginning in 2015, field researchers recorded a sudden shift. Aaron Sandel, the study’s lead author, describes a day when Western-cluster chimpanzees heard Central-cluster individuals nearby, grew unusually anxious, touched and reassured one another, then fled as the Central animals chased them. The two clusters then avoided each other for about six weeks — behavior never previously seen at Ngogo. Over the next few years that avoidance hardened into polarization. By 2018 the clusters were effectively separate communities, and lethal aggression followed.
The first documented killing was Errol, an adolescent male from the Central cluster whom researchers had watched grow up. In the seven years since, chimpanzees from the Western cluster have killed at least six other adults and 17 infants from the Central cluster. The violence has continued intermittently.
Why the split happened is not clear. The authors suggest several interacting factors: the community’s very large size, increased competition for food, intensifying male-to-male rivalry, and demographic shocks — notably the deaths of several adult males and one adult female in 2014 — that may have weakened key social ties. Importantly, the researchers note that chimpanzees lack human cultural constructs like religion or political ideology; their conflicts arise from social and ecological dynamics rather than abstract belief systems.
That distinction has implications for how we think about human violence. Michael Wilson, a primatologist not involved in the study, points out that collective violence can emerge without the ideological or cultural drivers often invoked in explanations of human civil wars. Sandel offers a different lesson: if factional violence among chimpanzees can erupt without human-style motives, perhaps the quality of everyday social relationships and the resilience of social bonds are crucial deterrents to large-scale conflict among people.
Anne Pusey, who worked with Goodall, finds the similarities between Ngogo and Gombe striking and discomfiting: ordinary social relationships can erode and harden into antagonism. Sandel’s personal takeaway is straightforward — if you behave toward others as if they were strangers, you risk becoming strangers — implying that repairing daily ties and letting old grievances fade might reduce the risk of escalation.
The Ngogo case adds to a small but growing record showing that chimpanzee communities can undergo long-lasting, lethal factionalism. It raises broader questions about how social networks, group size and demographic disruptions combine to destabilize communities, and what lessons about reconciliation and social cohesion humans might learn from our closest animal relatives.