Lethal aggression among chimps has been viewed as either primitive warfare or the result of human interference. Now, researchers show that chimp-on-chimp violence is normal -- nay, adaptive -- behavior that gives the perpetrators an edge in the evolutionary sense. The study was published in Nature this week.

Scientists trying to understand the factors contributing to aggressive behavior in our closest relatives have proposed two theories on why apes of the same species will kill each other. First, lethal violence may be the result of adaptive strategies to enhance access to valuable resources (like food and mates) by eliminating competition when the costs of killing are low. Alternatively, it may have emerged as a result of habitat change or altered food supplies because of human impacts.

To see if there’s more support for one hypothesis than the other, a huge international team led by Michael Wilson from the University of Minnesota analyzed 18 chimpanzee and four bonobo communities over five decades in well-studied sites across Africa. The researchers measured human impact based on whether the community had been fed, the size of the area they inhabited, and any disturbance or deforestation.

In that time, the team observed one suspected killing in bonobos and 152 reported killings in 15 of the chimp communities. Over a third of these killings were seen directly, while others were inferred from mutilated bodies or disappearances. This variation in killing rates, the team says, was not related to human impacts.

“Chimpanzees sometimes kill other chimpanzees, regardless of whether human impacts are high or low, whereas bonobos were not observed to kill, whatever the level of human impacts,” Wilson says in a university statement. “Based on our results, it’s clear that lethal aggression is something that chimpanzees naturally do.” To the right is Ferdinand, the alpha male of the Kasekela community, standing bipedally in the midst of a charge display.

Killings increased in larger populations and groups with a high number of males, and most killings were carried out by males against other males from neighboring groups.

Sometimes nursing infants were killed, and in those cases, the attackers removed them from the mother without killing her. The killings were most common in east African communities that were least affected by human interference of any kind. One dense community had large numbers of males who banded together in coalitions to conduct raids on neighboring troops. Pictured below are Ngogo males listening for neighbors during territorial boundary patrols, when killings often take place. These observations support the view that killing is an evolved tactic, according to study coauthor John Mitani from University of Michigan in a news release.

The team also used a series of computer models to test out the different explanations. The models that assumed that killings are related to adaptive strategies were nearly seven times as strongly supported as models that assumed human impacts are mostly responsible, Science reports.

Images: John Mitani (top, bottom), Ian Gilby (middle)

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