Chimps kill neighbours to expand territory

It has long been known that chimpanzees kill their neighbouring groups, but a motive has largely escaped researchers, until now.

A new study has, however, found that chimps, and especially small packs of males on patrol, kill one another to gain territory.

"The take-home is clear and simple. Chimpanzees kill each other. They kill their neighbours. 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," said John Mitani of the University of Michigan.

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 neighbouring group, representing an "extremely high" rate of mortality due to inter-group 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 inter-group 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 neighbouring chimpanzee community.

"Patrollers are quiet and move with stealth. 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," Mitani said.

The Ngogo chimpanzees may be at an unusual advantage over their neighbours due to the impressive size of their community, which may explain the surprisingly high level of violence observed, the researchers say.

The study has been reported in the June 22 issue of Current Biology, a Cell Press publication.