Technology & Science

Chimps kill for land: 10-year study

Chimpanzees violently kill other chimps to expand their own territory, according to a study in Uganda that provides the first evidence that such attacks are motivated by a desire for others' land.

Chimpanzees violently kill other chimps to expand their own territory, according to a study in Uganda that provides the first evidence that such attacks are motivated by a desire for others' land.

Over the course of their 10-year study, U.S. researchers witnessed 18 killings and found signs of three others perpetrated by members of a 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 per cent. They traveled, socialized and fed on their favourite fruits in the new region.

"Our observations indicate that chimpanzees at Ngogo have expanded their territory at the expense of a neighboring community. Territorial expansion followed a series of lethal … attacks that formed an especially large source of mortality," primate behavioural ecologist John Mitani, from the University of Michigan, and colleagues write in the journal Current Biology.

Chimpanzees (along with bonobos) are our closest living relatives. Anthropologists have long known that they kill their neighbours, and they suspected that they did so to seize their land, but the researchers say their study is the first to provide clear-cut evidence of this.

The researchers witnessed attacks when the chimps were on routine, stealth "boundary patrols" into neighbouring territory, moving silently and in single file while looking for signs of other chimps.

Sylvia Amsler, a lecturer in anthropology at the University of Arkansas at Little Rock, describes one attack while she and a colleague were following a group of 27 adult and adolescent males and one adult female:

"They had been on patrol outside of their territory for more than two hours when they surprised a small group of females from the community to the northwest," Amsler said in a release. "Almost immediately upon making contact, the adult males in the patrol party began attacking the unknown females, two of whom were carrying dependent infants."

1 infant killed, another wounded

The Ngogo chimps killed one of the infants almost immediately. They fought for half an hour in an unsuccessful attempt to take the other from its mother. The Ngogo chimpanzees then rested for an hour, holding the female and her infant captive before resuming their attack.

"Though they were never successful in grabbing the infant from its mother, the infant was obviously very badly injured, and we don't believe it could have survived," Amsler said. In most of the attacks, chimpanzee infants were killed, probably because they are easy targets.

Scientists are still not sure if the chimpanzees' ultimate motive in gaining territory is resources or mates, since the attacks could attract new females to the Ngogo community.

Renowned primatologist Jane Goodall first described such killings, but her findings were criticized because she and her colleagues used food to gain the chimps' trust. Some researchers believe this affected the animals' behaviour. The Michigan researchers didn't use food.

Mitani cautions against drawing any connections to human warfare and suggests instead that the findings could speak to the origins of teamwork.

"Warfare in the human sense occurs for lots of different reasons," Mitani said. "I'm just not convinced we're talking about the same thing."

Instead, he suggests the killings may in fact be better seen as a form of co-operative behaviour. 

"In the process, our chimpanzees have acquired more land and resources that are then redistributed to others in the group," said Mitani.