Other than humans, chimps are the most warlike primates -- but why? (Lincoln Park Zoo)

When rival communities of chimps interact, death is often the result. That may not sound out of the ordinary in the human world, but it's quite unusual for other animals -- other than chimps, we seem to be the only ones who go to war. But while some researchers have argued that chimpanzees evolved to fight their rivals, others have maintained that the chimp wars we've witnessed are caused by human intervention.

According to a new study in Nature, chimps come by those warrior instincts honestly. In the project, researchers compiled and analyzed data from over five decades of research on 18 different chimp communities, as well as four groups of bonobos -- the chimpanzees' very close but very gentle cousin.

Together, the data showed that chimp-on-chimp killings weren't more likely to occur when human interference like feedings or habitat destruction occurred. And bonobos simply didn't kill each other, even given the same human disturbances.

The researchers say this indicates a fundamental intra-species violence in chimps, but leading supporters of the opposite view are unimpressed. "I am surprised that [the study] was accepted for publication,"  Robert Sussman, an anthropologist at Washington University in St. Louis, told Science Magazine. Even the groups that the research team claimed were free from human disturbance were probably impacted by our interference, Sussman said.

In an accompanying article in Nature, behavioral ecologist Joan Silk (who wasn't involved in the study), points out that our desire to disprove an evolutionary basis for warfare may be wishful thinking. Our perception of primate behavior, she writes, is often skewed so that "morally desirable features, such as empathy and altruism, have deep evolutionary roots, whereas undesirable features, such as group-level violence and sexual coercion, do not."

In other words, we might just be looking for proof that we ourselves aren't violent at the core -- so results like these can be tough to swallow.

But whatever the truth is about chimp violence, we shouldn't apply it too much to human nature. "Humans are not destined to be warlike because chimpanzees sometimes kill their neighbors," Silk writes.