Washington: Chimpanzees have long been known to kill one another, but a motive has largely escaped researchers, until now. Chimps kill their neighbors, a study released Monday says, in order to expand their territory.
The decade-long look into the violent behavior of our closest living relatives could provide clues into the evolution of human cooperation, the researchers said.
"The take-home is clear and simple," John Mitani of the University of Michigan said in the study published in the journal Current Biology.
"Chimpanzees kill each other. They kill their neighbors," he said. "Our observations indicate that they do so to expand their territories at the expense of their victims."
Mitani, an anthropology professor, is principal author of the study in which researchers observed and documented 18 fatal attacks, and found conclusive evidence of three other chimpanzee killings, by members of a large group of some 150 of the primates in Ngogo, in Uganda`s Kibale National Park.
The attacks primarily occurred when smaller parties of chimpanzees went "on patrol" into the territory of neighboring chimpanzee communities.
As a result of these campaigns, tracked for 10 years by the researchers, the Ngogo chimpanzees suddenly extended their territory by 22 percent during the summer of 2009. The animals quickly perused their newly acquired lands and enjoyed increased access to resources such as food and, possibly, more females.
"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 said.
Sylvia Amsler, a professor of anthropology at the University of Arkansas who conducted field research for this project, was among the researchers who observed a violent raid in the northwest of the chimpanzees` Ngogo territory.
She said the patrol group of 27 adult and teenage males and one adult female surprised a smaller band of female chimps who belonged to the neighboring community. Two of the females carried young chimps.
The male attackers killed and cannibalized one of the small chimps and seriously injured the other as they tried unsuccessfully to wrest it from its mother`s arms.
Since chimpanzees, both the common chimpanzee and the bonobo, are the closest relative to humans, researchers often study the animals` behavior for clues about our own evolutionary origins and behavior.
But the study stressed that its basic findings were not expected to shed much light on the complex reasons for humans to go to war, although they could reveal clues about human cooperation.
"Using our results to address an enduring question about why humans are an unusually cooperative species may prove to be a more productive line of inquiry," the researchers wrote, citing the successful results of collaboration and teamwork, including the redistribution of extra resources to group members.