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Chimps Kill Neighbors to Gain New Territory

A decade of observation of one large community finds they take over land after attack

MONDAY, June 21, 2010 (HealthDay News) -- It has long been known that chimpanzees sometimes kill their neighboring primates, and now new research suggests that they are apparently motivated to do so by a desire to gain territory.

The finding, published in the June 22 issue of Current Biology, stems from work with a large group of more than 150 chimpanzees living in Ngogo, Kibale National Park, in Uganda. This community of chimpanzees has killed 21 chimps belonging to neighboring groups over a decade of observation, while moving into territory that had previously been outside their home base.

"The take-home is clear and simple. 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," John Mitani, of the University of Michigan, said in a news release from the journal's publisher.

"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 added.

The study authors indicated that access to more food would likely have been the driving force behind the chimps desire to acquire more territory. Access to more female chimps might also explain the killer instinct, they theorized.

The killings took place during "patrols," when packs of chimps often creep into a neighbor's jurisdiction.

"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 chimps, the team pointed out, are particularly violent, perhaps because of the larger-than-normal size of their community, which is about three times as big as others that have been studied.

Regardless, the authors do not believe that the chimp behavior can explain the underlying motivations behind human warfare. Instead, they suggest that their findings might in fact explain why human beings tend to cooperate with one another.

More information

For more on chimpanzees and aggression, visit Discover Chimpanzees.

SOURCE: Current Biology, news release, June 21, 2010
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