Fifteen years ago, researchers working deep in Uganda’s Kibale National Park watched something unsettling unfold. The Ngogo chimpanzees, one of the largest known chimp communities, began killing members of neighboring groups and pushing into their territory. The violence was clear. The motive was not.
Why would chimps take such a risky step? What evolutionary payoff could possibly justify it?
A new long-term study, led by John Mitani of the University of Michigan and published in Proceedings of the National Academy of Sciences, has finally answered the question. And the answer is surprisingly straightforward. The Ngogo chimpanzees gained a major reproductive advantage.
“Chimpanzees ultimately kill their neighbors to gain a reproductive advantage,” Mitani said. “The extent of the boost we saw was remarkable.”
Mitani and his colleagues have followed this community for more than 30 years, so they had an unusually detailed record of life before and after the territorial takeover. When they compared these two periods side by side, the pattern was impossible to miss.
Before the Ngogo chimps expanded their territory, females gave birth to 15 infants across three years. After the expansion, that number shot up to 37. It wasn’t a small bump. It was a doubling.
Even more striking was what happened after the infants were born. In the years before expansion, young chimps had a 41 percent chance of dying before their third birthday. Afterward, that rate dropped to just 8 percent.
“What we saw were very high numbers,” Mitani said. “They were so dramatic that they simply could not be sustained long term.”
And indeed, the birth numbers eventually tapered off. But for a brief window of time, the Ngogo chimps experienced one of the most substantial reproductive booms ever documented in great apes.
At its core, the explanation comes back to two things: food and safety.
Once the Ngogo community controlled more land, mothers had access to more feeding opportunities. That meant less competition, better nutrition, and more energy to carry a pregnancy to term and care for a newborn.
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If you are a baby ape, your entire survival is tied to your mother’s condition. A well-fed mother can nurse more effectively, defend her infant more aggressively, and recover faster when stressed. More food means healthier babies.
There is another factor too. A darker one.
Infanticide is a leading cause of death for young chimpanzees. These killings often come from rival groups. When the Ngogo chimps expanded their territory, they eliminated many of the very individuals who posed this threat.
More space, more food, fewer enemies. The evolutionary payoff becomes clearer.
Although researchers have long debated whether intergroup killings bring measurable benefits to chimpanzees, this is the first study to offer direct evidence.
“Our findings provide the first direct evidence linking coalitionary killing between groups to territorial gain and enhanced reproductive success,” said Brian Wood of the University of California, Los Angeles, lead author of the new report.
The team included scientists from Yale University and Arizona State University, all of whom have decades of combined experience studying wild chimpanzees. Their conclusion is simple. In this case, violence brought a clear fitness advantage.
Because chimpanzees and bonobos are our closest living relatives, it is tempting to draw parallels between this behavior and human warfare. But Mitani urges caution.
“We last shared a common ancestor with chimpanzees 6 to 8 million years ago,” he said. “During that time, we have changed in many ways.”
Humans have evolved into an unusually cooperative and prosocial species. We routinely help strangers. We build large, peaceful societies. And even with more than 8 billion people on the planet, we manage to coexist with only occasional outbreaks of conflict.
Chimpanzees, in contrast, show intense hostility toward neighboring groups. Their territorial aggression is a stable, predictable part of their biology. That difference highlights just how far human social behavior has diverged.
This research gives scientists a clearer picture of the evolutionary pressures that shaped chimpanzee behavior. It also helps anthropologists explore deeper questions about the origins of human cooperation and aggression.
Most importantly, it demonstrates how long-term field studies can reveal patterns that would otherwise remain invisible. Some answers only emerge when researchers are willing to follow the same animals for decades.
Journal Reference
- Wood, B. M., Watts, D. P., Langergraber, K. E., & Mitani, J. C. (2025). Female fertility and infant survivorship increase following lethal intergroup aggression and territorial expansion in wild chimpanzees. Proceedings of the National Academy of Sciences, 122(47), e2524502122. DOI: 10.1073/pnas.2524502122
