For years, there have been two main theories about why chimpanzees, our closest evolutionary cousins, sometimes kill each other. One theory blames human encroachment on the chimpanzees' native habit in Africa. Another says that (male) chimps kill in the normal course of competition with rival groups.
A new study published in Nature appears to support the second theory. In short, it found that the numerical makeup of chimpanzee communities is roughly proportional to the "chimp murder rate."
"Variation in killing rates was unrelated to measures of human impacts," the authors, Michael L. Wilson, Christophe Boesch, et al., write in the abstract. "Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported."
To be sure, the knowledge that chimps will occasionally carry out organized killings on groups of rivals is nothing new. As early as the mid-1970s, researchers in Tanzania's Gombe National Park observed gangs of a half dozen or more male chimpanzees conducting lethal raids in neighboring territories.
As The New York Times wrote in 1988: "For some time after the pioneering studies of Jane Goodall and others, it was thought that chimps were generally peaceful, playful, sophisticated and easygoing. ... Then, from Ms. Goodall's own work, and in particular from her associate Richard Wrangham, it became evident that chimpanzee males engaged in active killing of other chimps and other primates."
Still, the question of how common the behavior was and why exactly it occurred remained open to debate.
In an article in 2011 published in Psychology Today, University of Notre Dame professor Darcia Narvaez summed up the argument for human impact. She noted that in the first 14 years that Goodall and Wrangham observed chimps at Gombe, "aggression patterns were no different from other primates (peaceful and unaggressive)."
Then, the behavior suddenly changed: "With hindsight, it turned out that human feeding of the chimpanzees, with its restrictions and control, deeply affected the behavior and culture of the chimpanzees, such as keeping large groups of animals near the feeding site, which promoted increased fighting among the males," Narvaez wrote in Psychology Today, citing The Egalitarians: Human and Chimpanzee, a 1991 book by Margaret Power.
In a rebuttal to Narvaez published soon after in Psychology Today, Kevin D. Hunt, an anthropology professor at Indiana University who had Goodall colleague Wrangham as his doctoral co-supervisor, concludes:
"[There] is irrefutable evidence that the threat of lethal violence has exerted a strong evolutionary force on chimpanzee nature, and its effects are visible on a minute-to-minute basis in chimpanzee society. It is the origin of the very unusual social bonding among male chimpanzees — they must hang together to protect against extra-group murderers."
As bleak as this sounds, Wrangham — although he adheres to the chimps-as-natural-born-killers theory in the book Demonic Males — finds cause for optimism when it comes to the ability of humans to change their own violent tendencies.
In observing bonobos (the closely related but less-violent cousins of chimpanzees), Wrangham observed peaceful communities based on a power-sharing arrangement between males and females. Chimps, by contrast, live in patriarchal groups where dominant males run roughshod over compliant females.
The reason for the difference, he concludes, is sex selection. Female chimps select aggressive males as mates; female bonobos don't.
"The example of the bonobos reminds us that females and males can be equally important players in a society," Wrangham is quoted in Harvard Magazine as saying. "And by giving us a model in which female action works in suppressing the excesses of male aggression, the bonobos show us that in democracies like our own, women's voices should be heard more than they are."
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