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Chimpanzee gangs kill for land, new study shows

From the University of Michigan…

Bands of chimpanzees violently kill individuals from neighboring groups in order to expand their own territory, according to a 10-year study of a chimp community in Uganda that provides the first definitive evidence for this long-suspected function of this behavior.

U-M primate behavioral ecologist John Mitani’s findings are published in the June 22 issue of Current Biology. Mitani is the James N. Spuhler Collegiate Professor in the Department of Anthropology. His co-authors are David Watts, an anthropology professor at Yale University, and Sylvia Amsler, a lecturer in anthropology at the University of Arkansas at Little Rock. Amsler worked on this project as a graduate student at U-M.

During their decade of study, the researchers witnessed 18 fatal attacks and found signs of three others perpetrated by members of a large community of about 150 chimps at Ngogo, Kibale National Park.

Then, in the summer of 2009, the Ngogo chimpanzees began to use the area where two-thirds of these events occurred, expanding their territory by 22 percent. They traveled, socialized and fed on their favorite fruits in the new region.

“When they started to move into this area, it didn’t take much time to realize that they had killed a lot of other chimpanzees there,” Mitani says. “Our observations help to resolve long-standing questions about the function of lethal intergroup aggression in chimpanzees.”

Chimpanzees (along with bonobos) are humans’ closest living relatives. Anthropologists have long known that they kill their neighbors, and they suspected that they did so to seize their land.

“Although some previous observations appear to support that hypothesis, until now, we have lacked clear-cut evidence,” Mitani says.