The Ngogo chimpanzee group in Uganda expanded its territory after killing 21 neighboring chimps about 15 years ago. A PNAS follow-up shows births more than doubled (15 to 37 in comparable three-year windows) and infant survival to age three rose from 59% to 92%. Researchers attribute these demographic gains to improved resources after territorial expansion, while noting any link to human evolution remains speculative.
Violent Land Grab Boosted Births and Survival in Uganda's Ngogo Chimps
The Ngogo chimpanzee group in Uganda expanded its territory after killing 21 neighboring chimps about 15 years ago. A PNAS follow-up shows births more than doubled (15 to 37 in comparable three-year windows) and infant survival to age three rose from 59% to 92%. Researchers attribute these demographic gains to improved resources after territorial expansion, while noting any link to human evolution remains speculative.
11:11 AM, 11/19/2025Science
About 15 years ago, the Ngogo chimpanzee community in Kibale National Park, Uganda, launched a series of lethal raids on neighboring groups. The attacks killed 21 chimpanzees, and the Ngogo group expanded its territory by more than 20 percent into areas formerly occupied by their victims. A follow-up demographic study published in PNAS finds that the takeover was followed by clear and measurable gains for the Ngogo community.
Study findings
University of Michigan ecologist John Mitani and colleagues compared births and infant survival before and after the violent expansion. In the three years before the raids, Ngogo chimpanzees produced 15 offspring; in the three years after, they produced 37. Infant survival to age three rose sharply as well, from 59% before the takeover to 92% afterward.
Why the gains occurred
Previous research indicates that improved maternal nutrition raises fertility in wild chimpanzees. The added territory appears to have increased access to food and other resources, improving female condition and reproductive output. The authors conclude that, for this group, "lethal intergroup aggression" translated into a substantive reproductive advantage.
"Chimps, ultimately, kill their neighbors to gain a reproductive advantage," said John Mitani in a statement accompanying the paper.
Co-author Brian Wood, an anthropologist at UCLA, emphasized that while these findings show how violent territorial gains can benefit nonhuman primates, modern humans have developed social mechanisms that reduce or avoid such cycles of intergroup violence. "Humans have, thankfully, evolved an extraordinary capacity to resolve and avoid such conflicts, offering a way to escape cycles of food scarcity, territorial violence, and zero-sum competition among neighboring groups," he said.
Broader implications
The authors note that whether similar dynamics influenced the evolution of the last common ancestor of chimpanzees and humans 6–8 million years ago remains speculative. The Ngogo case demonstrates that territorial conquest can create demographic advantages in at least some primate societies, but extrapolating from chimp populations to human evolution requires caution.
Study citation: Mitani et al., PNAS (follow-up study on Ngogo chimpanzees).