In the film Planet of the Apes, one of the primary laws of ape society is “Ape shall never kill ape.” It would appear though that Caesar’s law, like the rest of the film, is fiction.

Very few animals engage in warfare. In this case warfare can be taken to mean coordinated, organized attacks by a group on another group of the same species for personal gain. Humans, of course, engage in this behavior and so do chimpanzees. For many years primatologists and anthropologists, beginning with the noted primatologist Jane Goodall, have noted lethal inter-community killings among groups of chimpanzees.

There has been debate for many years on whether this is natural chimpanzee behavior or whether it is influenced by human interaction, such as loss of habitat or food provisioning. A recent article, published in the September 18 edition of the journal Nature, closely examines this question and shows that chimp on chimp violence is naturally occurring and not due to human interaction.

“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, in a statement.

A team of 30 ape researchers examined five decades worth of data sets on 18 chimpanzees communities who experienced varying degrees of interaction with humans. That data included information on 152 chimpanzee-on-chimpanzee killings.

The researchers found that a majority of the victims of the killings were male and that the violence is driven by “adaptive fitness benefits” and not human impacts.

“Wild chimpanzee communities are often divided into two broad categories depending on whether they exist in pristine or human disturbed environments. 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,” said David Morgan, PhD, research fellow with the Lester E Fisher Center for the Study and Conservation of Apes at Lincoln Park Zoo in Chicago.

These findings agree with a 2010 study by another of the paper’s authors John Mitani, primate behavioral ecologist at the University of Michigan. In that study, Mitani found that groups of chimpanzees will kill over land. Over the course of a decade, Mitani and his colleagues witnessed 18 fatal attacks and found signs of three others.

The killers were members of a community of about 150 chimps at Ngogo, Kibale National Park. According to the researchers that community then began to use the land where two-thirds of the killings had occurred.

“When they started to move into this area, it didn’t take much time to realize that they had killed a lot of other chimpanzees there. Our observations help to resolve long-standing questions about the function of lethal intergroup aggression in chimpanzees,” said Mitani in a statement at the time.

The researchers behind the current study hope that their work will help with chimpanzee conservation efforts.

“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,” said Morgan.

It might be tempting to assume that because violent behavior is natural to chimpanzees, that it is also natural to humans. That analogy is, however, much too simplistic.

It is well known that humans and chimpanzees are closely related, however, humans are just as closely related to bonobos and bonobos do not demonstrate violent inter-species behavior. The study does, however, provide additional information on violent behavior and may, one day, help researchers understand how this behavior evolved and why it continues.