Bands of chimpanzees violently kill others from rival groups in a bid to expand their own territory, says a new research which provides the first definitive evidence for this behaviour.During a decade-long study of a chimp community in Uganda, by University of Michigan (UM) primate behavioural ecologist John Mitani, the researchers witnessed 18 fatal attacks and found signs of three others perpetrated by members of a large community of about 150 chimps at Ngogo, Kibale National Park.
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 percent. They travelled, socialized and fed on their favourite fruits in the new region.
“When they started to move into this area, it didn’t take us much time to realise that they had killed a lot of other chimpanzees there,” Mitani said.
“Our observations help to resolve long-standing questions about the function of lethal intergroup aggression in chimpanzees,” he added.
Chimpanzees (along with bonobos) are humans’ closest living relatives. Anthropologists have long known that they kill their neighbours and have suspected that they did so to seize land.“Although some previous observations appeared to support that hypothesis, until now we lacked clear-cut evidence,” Mitani said.
The bouts occurred when the primates were on routine, stealth “boundary patrols” into neighbouring territory.Study co-author Sylvia Amsler, anthropologist at the University of Arkansas, who conducted the field work, described one of the attacks she witnessed far to the northwest of the Ngogo territory, while following 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.“Almost immediately upon making contact, the adult males in the patrol party began attacking the females, two of whom were carrying dependent infants,” she added.
The Ngogo patrollers seized and killed one of the infants fairly quickly. They fought for 30 minutes to wrestle the other from its mother, but unsuccessfully.The Ngogo chimpanzees then rested for an hour, holding the female and her infant captive. Then they resumed 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 think it could have survived,” Amsler said.In most of the attacks in this study, chimpanzee infants were killed. Mitani believes this might be because infants are easier targets than adult chimpanzees, said an UM release.Mitani says these findings disprove suggestions that the aggression is due to human intervention.These findings were published in the Tuesday edition of Current Biology.
