In the mid-1970s, after more than a decade studying chimpanzees in Tanzania’s Gombe National Park, Jane Goodall witnessed a shocking rupture: a once-cohesive chimp group split into two hostile factions that began killing one another. That episode changed her view of chimpanzees’ nature, revealing a brutal side she hadn’t expected.
Until recently, that Gombe conflict was the only documented case. Now, a team publishing in Science describes a second prolonged “civil war” that has permanently divided the Ngogo community, the largest known wild chimpanzee group, in Uganda’s Kibale National Park. The Ngogo group once numbered nearly 200 individuals organized in intermingling subgroups or “clusters.” Males and females from different clusters socialized, mated, hunted together and cooperated to repel outsiders.
Beginning in 2015, researchers observed a sudden change. Aaron Sandel, the study’s lead author, recalls a June day when members of the Western cluster heard chimpanzees from the Central cluster nearby. The Western chimps grew unusually tense, touched and reassured one another, then fled while the Central chimps chased them. The two clusters avoided one another for six weeks — behavior never before seen at Ngogo. Over the following years, polarization deepened. By 2018 the clusters were effectively separate groups, and lethal violence followed.
The first observed deadly attack claimed Errol, an adolescent male from the Central cluster whom researchers had watched grow up. In the seven years since, Western chimpanzees have killed at least six other adults and 17 infants from the Central cluster. The violence continues.
Why the split occurred remains uncertain. The authors suggest several contributing factors: very large group size, competition for food, heightened male-to-male competition, and the deaths of several adult males and one adult female in 2014 that may have weakened social networks. The study emphasizes that chimpanzees, like other animals such as lions or wolves, do not have human constructs like religion or ideology to explain their conflicts.
That absence matters for understanding human violence, some researchers argue. Michael Wilson, a primatologist not involved in the study, notes that violent group conflicts can emerge without the cultural or ideological drivers often invoked in human civil wars. Sandel takes a more hopeful angle: if chimpanzee violence can erupt without human-style motives, perhaps the quality of interpersonal relationships and the strength of social bonds are more important in preventing large-scale violence among people than we often assume.
Anne Pusey, who worked with Goodall and was not involved in the Ngogo study, found the parallels with Gombe striking and uncomfortable: social relationships can erode and lead to antagonisms that previously didn’t exist. Sandel reflects on a personal takeaway: when you act like a stranger, you become one — suggesting that repairing or maintaining everyday social ties and letting old grudges fade might help avert escalations.
The Ngogo case adds to a small but growing record showing that chimpanzees can undergo enduring, lethal factionalism. It also raises questions about how social networks, group size, and demographic shocks interact to destabilize communities — chimpanzee and human alike — and what lessons about reconciliation and social cohesion we might draw from our closest animal relatives.