Chimpanzees have long been known to kill their neighbors, and now researchers reporting in the June 22 issue of Current Biology, a Cell Press publication, think they have a motive. It appears that chimps, and especially small packs of males on patrol, kill one another to gain territory.
"The take-home is clear and simple," said John Mitani of the University of Michigan. "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 findings were made in a large group of chimpanzees living in Ngogo, Kibale National Park, Uganda. Those chimpanzees have been the subject of close observation by researchers over the course of a decade. During that time, the team directed by Mitani and David Watts of Yale University saw the Ngogo chimps kill 21 individuals from other groups. (Eighteen of those killings were observed directly, while the remaining three were deduced from circumstantial evidence.) The researchers believe that as many as 13 of the victims belonged to a single neighboring group, representing an "extremely high" rate of mortality due to intergroup violence.
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.
Mitani and his colleagues think the new territory most likely benefits the chimps by affording them with greater access to food. It's also possible that the larger territory will ultimately mean greater access to females, but it is still too early to tell.
It's clear that the attacks are triggered when bands of chimpanzees go out "on patrol" into the territory of a neighboring chimpanzee community. "Patrollers are quiet and move with stealth," Mitani said. "They pause frequently to scan the environment as they search for other chimpanzees. Attacks are typically made only when patrolling chimpanzees have overwhelming numerical superiority over their adversaries."
The Ngogo chimpanzees may be at an unusual advantage over their neighbors due to the impressive size of their community, which may explain the surprisingly high level of violence observed, the researchers say. There are more than 150 Ngogo chimps—about three times the number found in chimp communities studied elsewhere.
Despite humans' close evolutionary ties with chimpanzees, Mitani and his colleagues tend not to think that studies of chimps' aggressive tendencies will reveal much at all about the varied and complex reasons that humans go to war. In fact, they say, the findings may have more to tell us about why humans so often work together.
"Using our results to address an enduring question about why humans are an unusually cooperative species may prove to be a more productive line of inquiry," the researchers write. "Our observations indicate that territorial conflict leads chimpanzees in some groups to cede land to members of other groups as a consequence of lethal coalitionary aggression. In the process, chimpanzees in communities that gain territory obtain increased access to resources that are then available to others in the group. Whether this selective factor can override the fitness costs suffered by individuals who cooperate within groups remains a theoretically and empirically challenging problem."
The authors include John C. Mitani, University of Michigan, Ann Arbor, MI; David P. Watts, Yale University, New Haven, CT; and Sylvia J. Amsler, at University of Arkansas at Little Rock, Little Rock, AK.