Martin Muller at Kibale chimpanzee site
Martin Muller observing chimpanzee behavior at the Kibale site.

A study published in the journal “Nature,” examines how chimpanzees kill in the wild. The study, titled “Lethal aggression in Pan is better explained by adaptive strategies than human impacts,” features collaborative research that concludes lethal aggression in chimpanzees is an adaptive behavior, rather than a side effect of human disturbance.

The research, which examined five decades of observed behavior among 18 chimpanzee communities and four bonobo communities, concludes that male chimpanzees were the most frequent attackers, and that communities with a large number of males are likely to be more bellicose.  The study says the violence is almost always against chimpanzees in other communities.

The research also found that there is no correlation between the amount of human impact on a community and the rate of lethal attacks. UNM Associate Professor of Anthropology and Co-director of the Kibale Chimpanzee Project in the Kibale National Park in southwestern Uganda Martin Muller said, “There has been a debate for a long time between anthropologists who contend that violence in chimpanzee communities might be the result of human interference, and those argue that this is an evolved behavior, through which chimpanzees reduce the coalitionary strength of their neighbors and expand their territories.” 

Muller travels to Uganda regularly where he spends time supervising the long-term observational research done at the site and conducting observations of chimpanzee behavior. He has witnessed two fatal attacks. Muller says male chimpanzees patrol the perimeter of their territory frequently and will kill a male from another community if he is alone, and thus vulnerable.

The paper, which analyzes known cases of killing, concludes that chimpanzees prefer to attack when they greatly outnumber the victim. In the cases reviewed in the paper the median ratio of attackers to victim is 8 to 1.

Muller said on one occasion he was observing a group of chimpanzees feeding in a fruit tree when they heard calls from the neighboring community, just over the border. The males in the group he was observing climbed down from the tree, hugged and comforted each other, then moved silently toward the other group in single file. There were many female chimpanzees in the other group but the males from the group focused their attack on the only male present. They killed him.

The data included information from 152 killings of chimpanzees by other chimpanzees. In those killings, 58 were observed directly, 41 were inferred and 53 were suspected. The bonobo communities had less violence.The actual sites varied from a great deal of human impact to no human impact. One interesting finding from the study concludes that ecological factors in communities that experienced less violence apparently allowed relatively high social interaction, which reduced the risk of lethal attack by minimizing the changes of being caught alone.

The study was funded by the National Science Foundation with additional funding from numerous other sources that are supporting the long-term studies that contributed data to this research.