Chimpanzees expand their territory by attacking and killing neighbours

Gangs of chimpanzees carry out violent attacks on individuals from rival groups in order to secure more resources or mates, a 10-year study in Uganda has found.

During that time scientists recorded 18 attacks and found signs of three others carried out by a large, male-dominated community of chimpanzees at Ngogo in Kibale National Park.

In summer last year, the aggressor chimpanzees finally began to occupy the area where two-thirds of their attacks had occurred, expanding their territory by more than a fifth.

According to the scientists, led by John Mitani, a primate behavioural ecologist at the University of Michigan, the chimps then travelled, socialised and ate in the new territory.

"When they started to move into this area, it didn't take much time to realise that they had killed a lot of other chimpanzees there," said Mitani. "Our observations help to resolve long-standing questions about the function of lethal intergroup aggression in chimpanzees."

The findings are published today in the journal Current Biology.

Anthropologists have long suspected that chimpanzees, humans' closest living relatives, kill neighbours for land, but they have lacked any hard evidence until now.

Sylvia Amsler, an anthropologist at the University of Arkansas at Little Rock and a member of the research team, said that the attacks usually occured when the chimpanzees were on routine boundary patrols in neighbouring teritory. In one attack that she witnessed, 27 adult and adolescent males and one adult female had been on patrol outside their territory for more than two hours when they surprised a small group of females from a nearby community.

"Almost immediately upon making contact, the adult males in the patrol party began attacking the unknown females, two of whom were carrying dependent infants," she said.

The Ngogo party quickly killed one of the infants and fought for 30 minutes to wrest the other from its mother, but were unsuccessful. After an hour-long break, during which time they held the female and her infant captive, they carried on with 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," said Amsler.

Despite their decade of observations, the researchers said they were still not sure if the objective of the attacks had been more resources or more mates.

Mitani warns against using the research to draw conclusions about warfare among humans, instead arguing that his study provides insights into primate teamwork. "Warfare in the human sense occurs for lots of different reasons. I'm just not convinced we're talking about the same thing."

He added: "What we've done at the end of our paper is to turn the issue on its head by suggesting our results might provide some insight into why we as a species are so unusually cooperative. The lethal intergroup aggression that we have witnessed is cooperative in nature, insofar as it involves coalitions of males attacking others. In the process, our chimpanzees have acquired more land and resources that are then redistributed to others in the group."