Chimps kill other chimps to expand territory

Chimpanzees kill other chimpanzees, according to a ten-year study conducted in a Uganda national park and released in the journal Current Biology in June, 2010. Primate behavioral ecologist Sylvia Amsler worked on a team that observed groups of male chimps patrolling the edges of their territory and targeting rivals for brutal killings.

Sylvia Amsler: We’ve known for quite some time, since the 1970’s, that chimpanzees regularly go on territorial boundary patrols looking for members of other chimpanzee communities. And when they find them and are able to isolate individuals, chimpanzees kill these other chimpanzees in these very dramatic episodes of lethal, coalitionary aggression. The question that’s been enduring has been why, from an evolutionary perspective, they’re engaging in this behavior.

The answer, according to the study, is to increase the chimpanzee community’s territory, and therefore their access to food.

Sylvia Amsler: This is not an unnatural, or aberrant behavior in any way, but actually has positive reproductive consequences for the males who do it. So although it seems very brutal, from an evolutionary perspective it really does make sense because these males are increasing the feeding territory. They and their mates and their offspring are all going to be more successful as a result of this behavior.

Amsler said that although community rivalry is universal among chimps, the group she studied is more frequently aggressive than normal, possibly because it’s a larger than average population with a high number of adult males. Amsler explained that chimpanzees’ warlike intentions are clear when they go on patrol.

Sylvia Amsler: On these territorial boundary patrols, chimpanzees change their behavior dramatically from what they normally do. Chimpanzees when they’re in the middle of the range are very noisy. They run around a lot. When we see these boundary patrols, they get extremely quiet and very cohesive. They move in a single file line out to the boundaries and beyond their boundaries, acting very cautious, startling very easily, and investigating any signs of chimpanzees they encounter such as feces or fruits that have been partially eaten. When they hear sounds of the neighboring chimpanzees they get very excited and very quickly and stealthily move toward those sounds. If they meet a large group of chimpanzees they usually make a lot of noise and then run back to their home territory. But if they’re able to isolate a single individual or just one or two individuals what we see is this lethal aggression.

While chimpanzees are one of our closest living relatives, Amsler cautioned against drawing parallels between chimpanzee and human aggression.

Sylvia Amsler: We are equally closely related to the other species of chimpanzee, the bonobo, which does not engage in this sort of lethal aggression and is more peaceful. Certainly there is some relationship between some aspects of human warfare – which is a very complex phenomenon – and what we see in chimpanzees. But what we’re also very interested in is the fact that in order to successfully compete with these other groups, the chimpanzees in one community have to cooperate very closely with one another. Humans also are a very cooperative species, so we are opening a discussion, potentially, about the origins of human cooperation as well.