Chimpanzees can be cute and cuddly, but they're also capable of murderous violence. They wage war against their own kind, savagely maiming and even killing their neighbors and their neighbors’ offspring.
Such behavior is rare among mammals. In fact, humans -- who are more closely related to chimps and bonobos than any other animal -- are the only other mammals known to inflict this level of lethal violence against others of their species.
What causes this violence in chimps, and can they teach us anything about our own propensity to wage war?
Some research has suggested that chimps are inherently peaceful, and that they turn violent only because of human interference. One landmark study conducted in the 1960s showed that chimps in Tanzania's Gombe National Park began attacking each other only after Jane Goodall and other primatologists began handing out bananas. When the banana feeding stopped, so did the violence -- a fact that led some researchers to conclude that we are to blame for chimps' violent behavior.
But other experts disagree. This includes the 30 researchers involved in a newly published study of chimp violence data from more than 420 combined years of observation in 18 chimpanzee communities. The study suggests that chimpanzees in the wild are inherently violent, and that human interference has nothing to do with the mayhem.
The Ngogo group of chimpanzees in Uganda, for instance, “turned out to be the most violent group of chimpanzees there is,” Michael Wilson, a University of Minnesota anthropologist and the co-author of the new study, told The New York Times. And that’s despite the fact that those chimps live in a pristine habitat seemingly untouched by humans.
According to Wilson and his collaborators, aggression evolved in chimps because it gives the animals a reproductive advantage over more chilled-out chimps.
Among the 150 or so chimp "murders" analyzed in the new study, males were the aggressors more than 90 percent of the time -- and most of the killings were "intercommunity attacks" in which a group of chimps ganged up on an individual chimp from another community.
The researchers said these findings support the hypothesis that chimps fight other groups to gain territory, mates, and food.
“Chimpanzees often fragment into temporary parties that travel and forage independently within their community’s home range,” Arizona State University anthropologist Joan Silk, who was not involved in the study, wrote in an editorial published in conjunction with the new study. “When parties of males encounter single individuals from other communities, they sometimes launch brutal assaults that leave victims gravely wounded or dead.”
The thoroughness and scope of the new study, which the authors say is the most comprehensive of its kind, has impressed many ape experts. As University of Manchester evolutionary biologist Susanne Shultz told the BBC, "There's a real effort to look across a really wide range of populations, and the results are very compelling."
But the study has its share of critics, including two anthropologists who told The New York Times that the researchers failed to adequately measure the human impact on chimp communities. The study looked at whether chimps were fed by people, as well as the size of the animals' territory and how much of their habitat had been affected by humans, but Rutgers University anthropologist Brian Ferguson said that human impact “can’t be assessed by simple factors.”
No matter what, experts caution against using the study to make judgments about our own violent tendencies. As Silk wrote in her editorial, "Humans are not destined to be warlike because chimpanzees sometimes kill their neighbors."
Study co-author David Morgan agreed.
“These findings remind me how special and different from chimpanzees we really are in terms of cooperation and empathy,” he told the Chicago Tribune. “Chimpanzees will cooperate but not at the scale or degree of humans.”
Maybe there's hope for us yet.