Of all of the world's species, humans and chimpanzees are some of the only to engage in coordinated attacks on other members of their same species. Jane Goodall was among the first to introduce the occurrence of lethal inter-community killings and since then primatologists and anthropologists have long debated the concept of warfare in this genus. Research theories have pointed to increased gains and benefits of killing off competitors and opening up increased access to key resources such as food or mates. In contrast, others have argued that warfare is a result of human impact on chimpanzees, such as habitat destruction or food provisioning, rather than adaptive strategies.

New research from an international coalition of ape researchers, published September 18 in the journal Nature, has shed new light on the subject, suggesting that human encroachment and interference is not, as previous researchers have claimed, an influential predictor of chimp-on-chimp aggression.

The study began as a response to a growing number of commentators claiming that chimpanzee violence was caused by human impacts. "This is an important question to get right. If we are using chimpanzees as a model for understanding human violence, we need to know what really causes chimpanzees to be violent," said University of Minnesota researcher Michael L. Wilson, lead author on the study.

"Humans have long impacted African tropical forests and chimpanzees, and one of the long-standing questions is if human disturbance is an underlying factor causing the lethal aggression observed," explained co-author David Morgan, PhD, research fellow with the Lester E Fisher Center for the Study and Conservation of Apes at Lincoln Park Zoo in Chicago. Morgan has studied chimpanzees deep in the forests of Republic of Congo for 14 years. "A key take-away from this research is that human influence does not spur increased aggression within or between chimpanzee communities."

A team of 30 ape researchers assembled extensive data sets spanning five decades of research gathered from 18 chimpanzee communities experiencing varying degrees of human influence. In all, data included pattern analysis of 152 killings by chimpanzees. The key findings indicate that a majority of violent attackers and victims of attack are male chimpanzees, and the information is consistent with the theory that these acts of violence are driven by adaptive fitness benefits rather than human impacts.

"Wild chimpanzee communities are often divided into two broad categories depending on whether they exist in pristine or human disturbed environments," explained Morgan. "In reality, however, human disturbance can occur along a continuum and study sites included in this investigation spanned the spectrum. We found human impact did not predict the rate of killing among communities.

"The more we learn about chimpanzee aggression and factors that trigger lethal attacks among chimpanzees, the more prepared park managers and government officials will be in addressing and mitigating risks to populations particularly with changing land use by humans in chimpanzee habitat," explained Morgan.