A decade of vicious, internecine battles between chimps has been documented in a new paper in Current Biology.
Twenty-one chimps were killed or fatally wounded in Kibale National Park, Uganda, over this time period by a group of male chimps from a large community in a region of the park called Ngogo. As many as 13 of the victims may have belonged to a single neighbouring group, representing an extremely high rate of mortality due to intergroup violence, exceeding median rates of mortality due to intergroup violence reported for humans in agricultural and hunter-gatherer populations and compared to the median rate suffered by individuals in 9 well-studied chimp communities. The motive appears to be territory.
John Mitani from the University of Michigan in Ann Arbor and his colleagues spent 10 years observing the 150 Ngogo chimps. They even watched 18 of the killings (the other three were inferred from carcasses and faeces containing chimp remains).
Most of the time, male chimps behave like rowdy, loud frat boys, but once every 10 to 14 days, they act like cooperative adults and wage war. The New York Times reports:
A band of males, up to 20 or so, will assemble in single file and move to the edge of their territory. They fall into unusual silence as they penetrate deep into the area controlled by the neighbouring group. They tensely scan the treetops and startle at every noise… If a single chimp has wandered into their path, they will attack. Enemy males will be held down, then bitten and battered to death. Females are usually let go, but their babies will be eaten.
After the bloodshed, the Ngogo chimps usurp the area once occupied by their victims, literally enjoying the fruits (genus Morus) of their labour. “The take-home is clear and simple,” Mitani says (news release). “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.”
The Ngogo chimp patrols have expanded their 29-square-kilometre territory over the years, but during the summer of 2009, they nabbed a sizable 6.4-square-kilometre chunk to the northeast – increasing their land by 22%. With bountiful land and resources, the males grow stronger and their females – who likely increase in number – have more babies.
While the study has implications for the evolution of cooperation, the authors caution against extrapolating their study into the realm of human warfare. “Invariably, some will take this as evidence that the roots of aggression run very deep,” Mitani says (Time). But even if that were true, “we operate by a moral code chimps don’t have” he adds.
Image: NIH