Why Do Chimps Kill Each Other?

War—what is it good for? “Absolutely nothing” according to the refrain of a 1970 hit song. Many humans would agree with this sentiment. But a major new study of warfare in chimpanzees finds that lethal aggression can be evolutionarily beneficial in that species, rewarding the winners with food, mates, and the opportunity to pass along their genes. The findings run contrary to recent claims that chimps fight only if they are stressed by the impact of nearby human activity—and could help explain the origins of human conflict as well.

Ever since primatologist Jane Goodall’s pioneering work at Gombe Stream National Park in Tanzania in the 1970s, researchers have been aware that male chimps often organize themselves into warring gangs that raid each other’s territory, sometimes leaving mutilated dead bodies on the battlefield. Primatologists have concluded that their territorial battles are evolutionarily adaptive. But some anthropologists have resisted this interpretation, insisting instead that today’s chimps are aggressive only because they are endangered by human impact on their natural environment. For example, when humans cut down forests for farming or other uses, the loss of habitat forces chimps to live in close proximity to one another and to other groups. Feeding chimps can also increase their population density by causing them to cluster around human camps, thus causing more competition between them.

The researchers created a series of computer models to test whether the observed killings could be better explained by adaptive strategies or human impacts. The models incorporated variables such as whether the animals had been fed by humans, the size of their territory (smaller territories presumably corresponding to greater human encroachment), and other indicators of human disturbance, all of which were assumed to be related to human impacts; and variables such as the geographic location of the animals, the number of adult males, and the population density of the animals, which the team considered more likely to be related to adaptive strategies.

“The contrast could not be more stark” between how the two hypotheses fared, says William McGrew, a primatologist at the University of Cambridge in the United Kingdom, who praises the study as a “monumental collaborative effort.” Joan Silk, an anthropologist at Arizona State University, Tempe, agrees. The study “weighs competing hypotheses systematically,” she says. “Advocates of the human impact hypothesis … must challenge [the study’s] empirical findings, or modify their position.”

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