Chimps kill their neighbors—now we know why

Since Jane Goodall first described it in 1979, scientists have known that chimpanzee bands make organized raids into neighboring territories where they attack and kill chimpanzees from other groups. What they didn’t know was why.

Some researchers speculated that the goal of these raids was to seize territory; others thought that it might be a way of recruiting females from a victimized group into the raiders’ band. Some anthropologists argued that such attacks only took place in chimp communities that had been disturbed by human interventions such as providing food.

Researchers Sylvia Amsler, a biological anthropologist at the University of Arkansas at Little Rock, David Watts, at Yale University in New Haven, Connecticut, and John Mitani at the University of Michigan in Ann Arbor, say that they’ve proved that the goal of this form of chimpanzee aggression is to take over territory.

They describe their findings in the June 21, 2010 edition of Current Biology.

Amsler and her colleagues studied a large band of chimpanzees—the Ngogo group—in Uganda’s Kibale National Park for more than ten years. They observed dozens of organized raids, usually by large numbers of adult and adolescent males, although occasionally with females as well. The Ngogo band’s raids were focused on a region to the northeast of their home range.  In ten years of what the researchers refer to as “patrols,” the Ngogo chimps killed 21 individuals from that neighboring band.

That represents a very high mortality rate—some 5 to 17 times higher than has been measured among human hunter-gatherer societies, and 23 to 75 times higher than what’s been seen in other chimpanzee studies.

The researchers identify two factors that may account for this unusual rate of violence and killing. The Ngogo habitat is unusually rich, so the band has grown to some 150 individuals, about three times larger than the average chimpanzee band. That means that there are a lot more adult and adolescent males available to form large and successful raiding parties.

Amsler and her colleagues believe that their findings rule out the theory that chimps only kill each other as a result of human interference. They point out that neither the Ngogo band nor its neighbors have ever been provisioned by humans, and that the raiding they’ve observed is clearly adaptive. The Ngogo band has increased its territory by some 22 percent at the expense of its neighbors. As a result, more of its members are likely to survive and reproduce.

Although the chimpanzee raids are eerily similar to human intergroup aggression and violence, the researchers are reluctant to discuss any linkage.

They do speculate, however, that the high level of within-group cooperation that lets chimpanzees form these patrols and carry out lethal raids on their neighbors may tell us something about the evolution of high levels of cooperative behavior within human groups.

It would be ironic if the researchers are right, and that it was our primate ancestors cooperating among themselves in order to compete aggressively with outsiders that made us as cooperative as we are today.

Journal reference: John C. Mitani, David P. Watts and Sylvia J. Amsler, “Lethal intergroup aggression leads to territorial expansion in wild chimpanzees,” Current Biology Vol. 20 No. 12.

For a more in-depth read, click here to see my Suite101 story about this research.

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Robert Adler

For the institute