Chimps' homicide motive determined

ANN ARBOR, Mich., June 23 (UPI) -- U.S. scientists say they've determined the reason chimpanzees often kill each other is to expand their territory.

Researchers led by Professors John Mitani of the University of Michigan and David Watts of Yale University said their findings were made by observing a large group of chimpanzees living in Ngogo in the Kibale National Park in Uganda.

"The take-home is clear and simple," Mitani said. "Chimpanzees kill each other. They kill their neighbors. Up until now, we have not known why. Our observations indicate that they do so to expand their territories at the expense of their victims."

The team said it saw the Ngogo chimps kill 21 individuals from other groups -- 18 killings were observed directly, while three were deduced from circumstantial evidence. With some of their competitors out of the way, the Ngogo chimpanzees began to use a large portion of new territory to the northeast of their previous range.

"Because the newly acquired territory corresponds to the area once occupied by many of the victims, we suggest that a causal link exists between the prior acts of lethal intergroup aggression and the subsequent territorial expansion," Mitani said, noting the new territory most likely benefits the chimps by affording them greater access to food.

The study that included postdoctoral researcher Sylvia Amsler of the University of Arkansas at Little Rock appears in the journal Current Biology.