Key Takeaways:
The Ngogo group of wild chimpanzees in Uganda expanded its territory after its members killed at least 21 chimpanzees in neighboring groups.
In the three years after the territorial expansion, the fertility of Ngogo females doubled and the survival rates of their offspring dramatically increased.
The study offers rare evidence linking intergroup lethal conflict to reproductive benefits, providing insight into the evolution of coalitionary violence.
The Ngogo chimpanzees of Uganda's Kibale National Park have long been known for violent clashes with neighboring groups, often resulting in deaths — a phenomenon sometimes described as "chimpanzee warfare."
Now, a new study led by UCLA anthropologist Brian Wood, in collaboration with John Mitani of the University of Michigan, provides the clearest evidence yet that territorial expansion after lethal conflict can directly boost reproductive success. Following a series of coordinated attacks that claimed at least 21 lives, the Ngogo group's territory grew by 22%. In the years that followed, females gave birth more often, and their infants were far more likely to survive.
Published in Proceedings of the National Academy of Sciences (PNAS), the study provides valuable evidence about the evolutionary roots of intergroup aggression and its fitness consequences for chimpanzees. The research was funded in part by the National Science Foundation.
"Our findings provide the first direct evidence linking coalitionary killing between groups to territorial gain and enhanced reproductive success in chimpanzees." said Wood.
The numbers tell a striking story. In the three years preceding the territorial expansion, Ngogo females gave birth to 15 offspring. In the three years after, they gave birth to 37 — more than doubling their fertility rate. Infant survival also improved dramatically: from a 41% chance of death before age 3 to just 8% afterward.
"In retrospect, we knew what happened. We were observing all these births and there are good theoretical and empirical reasons for thinking something like this might happen," said Mitani, who is professor emeritus of anthropology at U-M. Still the extent to which births and survival rates ballooned was a surprise. "What we saw were very high numbers," said Mitani.
Mitani has been part of a team that's observed this group of chimpanzees for more than three decades. About 15 years ago, the researchers witnessed the chimps overtake the territory of neighboring chimps that they had killed. The question remained as to what evolutionary advantage this behavior might provide, which the team has now shown to be these reproductive benefits.
After ruling out other explanations, the research team, which also includes David P. Watts of Yale University and Kevin E. Langergraber of Arizona State University, concluded that territorial expansion improved female nutrition and overall health, leading to higher fertility and survival rates among their young.
The team also tested alternative hypotheses. One possibility was that females reproduced more frequently because infant mortality was high — a pattern sometimes seen in primates — but the data showed the opposite: both fertility and infant survival improved. Another possibility was that changes in food availability might explain the results, but fruit abundance in Ngogo's core (pre-expansion) territory remained stable or even declined slightly after the expansion.
"These findings help us understand why chimpanzees, and perhaps our own early ancestors, evolved a capacity for coordinated violence. When food is scarce, territorial gains can translate into real reproductive advantages. 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," said Wood.
The research was performed with permission from the Uganda Wildlife Authority, the Uganda National Council for Science and Technology and the Makerere University Biological Field Station.
