Ugandan Chimps Waged History's Bloodiest Recorded Conflict, Expanding Territory 22%

For a decade, the males of Ngogo moved through Kibale National Park's forest in single file, silently, before fanning out to kill. By the time the conflict ended in 2009, at least 21 chimpanzees from neighboring groups had died, the Ngogo community had seized 2.5 square miles of new territory to the northeast, and researchers were left with a question that still lacks a clean answer: why did it start?

The Ngogo group, with more than 140 individuals, is unusually large for a chimpanzee community. John Mitani, professor emeritus of anthropology at the University of Michigan, has studied them since 1995. The first intergroup killing his team observed came in 1999, and over the following decade the Ngogo males conducted coordinated, stealthy raids targeting adults and juveniles, males and females alike, in what researchers now describe as the bloodiest recorded conflict in chimpanzee history, surpassing even Jane Goodall's documentation of the Kasakela-Kahama war at Gombe Stream National Park in Tanzania from 1974 to 1978. That four-year war ended with the deaths of all males in the Kahama community and temporarily expanded Kasakela territory from 12 to over 15 square kilometers before a third community, the Kalande, reversed the gains. Ngogo dwarfs it: ten years, 21 confirmed kills, and a 22% territorial expansion that held.

What that expansion produced in biological terms is now documented with striking precision. A study published in the Proceedings of the National Academy of Sciences, led by Brian Wood, associate professor of anthropology at UCLA, in collaboration with Mitani, found that Ngogo females gave birth to 15 offspring in the three years before the territorial expansion. In the three years after, that figure rose to 37. Infant mortality before age three dropped from 41% to 8%. "With additional land and the resources it contained, females could feed better and use that added nutrition to produce more infants," Mitani said. "And mothers, now in better energetic condition, were more successful raising their young."

The research team tested alternative explanations and eliminated them. High infant mortality before the expansion might have triggered compensatory births, but the data showed the opposite pattern. A shift in fruit availability might explain the reproductive surge, but fruit abundance in Ngogo's pre-expansion territory remained stable or declined slightly during that period. "Our findings provide the first direct evidence linking coalitionary killing between groups to territorial gain and enhanced reproductive success in chimpanzees," Wood said.

What the study cannot resolve is the conflict's origin. The "imbalance of power hypothesis," examined at length by anthropologist R. Brian Ferguson in his 2023 Oxford University Press book "Chimpanzees, War, and History: Are Men Born to Kill?", proposes that groups launch coordinated lethal aggression when they perceive a numerical or strategic advantage. Ngogo, with its exceptional size, fits the profile. But the precise trigger for this specific conflict remains unconfirmed.

That uncertainty carries weight well beyond primatology. Chimpanzees and bonobos are humankind's closest living relatives, and understanding what drives coordinated lethal violence in chimps bears directly on questions about the evolutionary origins of human warfare. As Kibale and other primate habitats continue to shrink under development pressure, the Ngogo findings also sharpen an urgent conservation question: what happens to chimpanzee group dynamics and reproductive outcomes when habitats contract and neighbor pressure intensifies. The victors at Ngogo reproduced at more than double their previous rate. The fate of the displaced groups remains the harder data to collect.