Scientists find out why chimpanzees kill their neighbors

By Stephen Beech

Chimps in the wild engage in deadly battles with rival troops to increase their reproductive success, suggests new research.

The Ngogo chimps of Uganda’s Kibale National Park have long been known for violent clashes with their primate neighbors, often leading to fatalities - a phenomenon described as “chimpanzee warfare.”

Now, a team of anthropologists has discovered "clear links" between lethal aggression, territorial expansion, and increased reproductive success among wild chimps.

Their study, published in the journal Proceedings of the National Academy of Sciences (PNAS), offers rare evidence linking lethal conflict to reproductive benefits, providing insight into the evolution of violence.

They say their findings provide the clearest evidence yet that territorial expansion after lethal conflict can directly boost reproductive success.

Following a series of coordinated attacks that claimed the lives of at least 21 chimps, 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.

Professor Brian Wood, who led the study in collaboration with University of Michigan Professor John Mitani, says it provides valuable evidence about the evolutionary roots of intergroup aggression and its fitness consequences for chimpanzees.

Anthropologist Wood, from the University of California, Los Angeles (UCLA), said: “Our findings provide the first direct evidence linking coalitionary killing between groups to territorial gain and enhanced reproductive success in chimpanzees.”

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 three to just 8% afterward.

Mitani said, "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."

But he says the extent to which births and survival rates ballooned was a surprise, adding: “What we saw were very high numbers."

Mitani has been part of a research team that's observed this group of chimps for more than three decades.

About 15 years ago, the researchers witnessed the chimps overtake the territory of neighboring chimps that they had killed.

But the question remained as to what evolutionary advantage the behavior might provide, which the team has now shown to be reproductive benefits.

After ruling out other explanations, the research team concluded that territorial expansion improved female nutrition and overall health, leading to higher fertility and survival rates among their young.

The researchers also tested alternative theories.

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 the researchers' fruit abundance in Ngogo’s core territory remained stable or even declined slightly after the expansion.

Wood said: “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."

He added: "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."