For more than two decades, the Ngogo chimpanzee community in Uganda’s Kibale National Park was the largest ever recorded, a sprawling society of roughly 200 individuals who groomed each other, hunted together, and patrolled their borders as one group. Then, sometime before 2018, the community fractured. Former allies split into rival factions. And then they started killing each other.
A study published in May 2026 in the journal Science, led by primatologist Aaron Sandel and colleagues, presents what researchers are calling the first clearly documented chimpanzee “civil war”: a permanent group split followed by sustained, lethal raids between individuals who once lived side by side. The findings, drawn from roughly 30 years of continuous field observation at Ngogo, challenge a foundational assumption in primatology, that chimpanzees direct deadly violence outward at strangers, never inward at their own.
A community tears itself apart
The Sandel et al. study traces how the Ngogo community gradually polarized into two factions, labeled Western and Central. Social bonds frayed. Ranging patterns diverged. By 2018, the split had hardened into a permanent fission, with each faction occupying its own territory within the park.
What followed was not avoidance but organized aggression. Coordinated raiding parties crossed into the opposing faction’s range and killed chimpanzees they had once groomed and shared meat with. The study reports that 13 chimpanzees were killed in lethal raids between the two factions after the split, with the Western group responsible for the majority of the attacks. The researchers were able to identify specific individuals on both sides of the violence because Ngogo’s chimpanzees have been habituated to human observers for decades, allowing the kind of individual-level tracking that is rare in primate field research.
“This is the first case where we can clearly show that chimpanzees who lived together, groomed together, and cooperated for years turned to killing each other after their community split apart,” Sandel said in a statement accompanying the study’s publication.
A companion perspective piece in Science frames the events explicitly as a civil war, defining the term as lethal conflict among former community members following a permanent split. The authors note that while intergroup killings between rival chimpanzee communities have been documented before, most famously during the Gombe “Four-Year War” observed by Jane Goodall’s team in the 1970s, the Ngogo case is distinct. At Gombe, the Kahama splinter group was small and the documentation less systematic. At Ngogo, researchers had decades of baseline data on the social relationships that preceded the fracture, making the before-and-after comparison far more rigorous.
Why the violence may pay off, and why it may not
Separate quantitative research at Ngogo offers one explanation for why lethal raids persist among chimpanzees at all. A study by David Watts and colleagues, published in tandem, found that lethal intergroup aggression correlates with territorial expansion, and that the resulting gains in land translate into higher female fertility and better infant survival. In evolutionary terms, killing pays: males who raid successfully leave more descendants.
But applying that logic to the Ngogo civil war is not straightforward. The reproductive-benefit data come from raids against neighboring communities, groups the attackers had no prior social bonds with. When former allies fight each other, the calculus changes. Each killing potentially destroys a hunting partner, a coalition ally, or a male who once helped defend the group’s borders. The cooperative networks built over years of shared life carry real value, and the cost of dismantling them may offset whatever territory the victors gain. The existing data cannot yet quantify that tradeoff.
What triggered the split
The study documents the fracture and its aftermath in detail, but the precise cause remains an open question. Field researchers who have spent years with the Ngogo chimpanzees have floated several hypotheses: shifts in male leadership, the destabilizing effects of disease outbreaks, and a slow erosion of social cohesion that may have built up over time as the community grew unusually large. These ideas draw on deep familiarity with the group rather than formal statistical tests, and no single trigger has been isolated.
The question of what happens next is equally unresolved. The Western and Central factions now occupy adjacent territories in Kibale National Park. Whether they will stabilize into separate communities with recognized borders, as chimpanzee groups elsewhere have done, or continue raiding each other is something only continued observation will answer.
What this changes
Before Ngogo, the working model in primatology held that chimpanzee lethal violence was an outward-facing strategy: kill strangers, seize their territory, reap the reproductive rewards. The Kibale evidence shows that the same capacity can turn inward when a community breaks apart, producing organized, sustained killing between individuals who share history, genetic ties, and learned behaviors.
That pattern carries an uncomfortable resonance. Organized internal warfare, factions forming within a group and then waging lethal campaigns against each other, was long considered a uniquely human phenomenon. The Ngogo findings do not mean chimpanzees experience conflict the way humans do. They lack language, ideology, and the political structures that shape human wars. But the research does suggest that the evolutionary raw materials for sustained intragroup violence existed millions of years before our species emerged, embedded in a social primate lineage we share.
Ngogo is also the setting of the Netflix documentary series “Chimp Empire,” which introduced millions of viewers to the community’s complex social dynamics. The split and its violent aftermath unfolded during and after the period covered by the show, adding a grim new chapter to a story many audiences already feel connected to.
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*This article was researched with the help of AI, with human editors creating the final content.
