Killing by chimpanzees is not caused by human disturbance

KCP researchers contributed long-term data to a collaborative study, just published in the journal Nature, examining patterns of intraspecific killing in chimpanzees and bonobos. Considerable evidence supports the idea that chimpanzees employ lethal aggression as part of an adaptive strategy to reduce the coalitionary strength of their neighbors and expand their territories. Some anthropologists, however, have maintained that escalated aggression in chimpanzees is the result of human influence, such as provisioning or habitat encroachment, and that in the absence of these factors, chimpanzees would be peaceful and egalitarian. The question is an important one, because chimpanzees are often used as a model for understanding the evolution of human violence.

The study, led by Michael Wilson at University of Minnesota, focused on 152 killings (58 observed, 41 inferred, and 53 suspected) recorded in 18 chimpanzee and 4 bonobo communities. Males were the most frequent attackers in the sample (92% of participants in killings), and they mostly targeted other males as victims (73%). Most killings occurred between communities, and attackers greatly outnumbered their victims (median 8:1 ratio). All of these findings support the idea that lethal killing is an adaptive strategy in this species, with coalitions of males taking advantage of low-cost opportunities to kill competitors. By contrast, rates of killing across chimpanzee communities were not related to human impacts, and bonobos were not observed to kill, whatever the level of human impacts.

“People have long been interested in chimpanzees as a way to better understand the evolution of human behavior,” says Wilson. While the new study answers one question, many more remain. “It’s still an open question whether this sort of violence is something that has happened continuously in human evolutionary history, or whether it arose independently in humans and chimpanzees. Perhaps our common ancestor, which we believe lived five to seven million years ago, also had high rates of violence, too.”

Image (by Andrew Bernard): Kanyawara male Bud, bloodied from an aggressive encounter with males from a neighboring community.

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