Chimpanzees are "natural born killers," and their tendency toward lethal aggression is not a result of human influence, a new study finds. The study, published in Nature, looked at chimp-on-chimp killings in 18 chimp communities over a span of five decades and assessed how much those communities had been affected by human activities. Researchers found killings to be most common in the east African communities that had been least touched by humans, according to a press release. At the site most affected by human interference, in Guinea, no killings took place. "Patterns of lethal aggression ... show little correlation with human impacts," the authors say, "but are instead better explained by the adaptive hypothesis that killing is a means to eliminate rivals when the costs of killing are low."
Researchers concluded that, in addition to eliminating rivals, chimps kill to get better mates, food, resources, or access to territory. The study bolsters evidence that such killings are an evolved, adaptive tactic as chimps seek to pass on their own genes, rather than a consequence of deforestation or humans studying and feeding chimps. The findings are particularly interesting because, as the Washington Post reports, chimpanzees are the only animals other than humans who "go to war" with each other. But the study—which refutes another high-profile study blaming human interference for chimps' warlike tendencies, the Chicago Tribune reports—has inspired some debate. Two anthropologists tell the New York Times the researchers didn't actually establish that any of the communities studied were truly free from human interference. (Another fascinating recent study finds that primates are capable of abstract thought.)