Chimps Need No Help for Violence

chimpanzee-glockWhoever wrote the script for Dawn of the Planet of the Apes should have done some more homework. The primary antagonist of the film is Koba, a scarred bonobo who holds a grudge against humans for his mistreatment.

The only problem is that bonobos don’t seem to hold a grudge. Nor do they seem to have any violent tendencies whatsoever, for that matter.

The two closest species to humans are chimpanzees and bonobos, and one could argue that violence is one of the largest differences between the three . While humans kill each other for smudging their Pumas and chimpanzees have hundreds of documented instances of in-species killings , bonobos just don’t. Of four different bonobos communities constantly observed, there has been only one killing, and it can only be described as a suspected killing, at best.

This leads to some interesting questions. Did the last common ancestor of all three species have violent tendencies, and bonobos have evolved out of them? Was the last primate common to humans and chimps peaceful, and the two species since evolved same-species violence independently? Or was it some combination of the two?

There is a fourth option that many conservationists have taken to recently—that it is humans that are causing chimps to be aggressive, not their own nature. Perhaps by constricting their habitats, injecting ourselves into their lives, feeding some groups while not others, and otherwise interfering with their natural lives, we are the cause of their killings.

Michael Wilson says, “nope.”

Wilson, lead author of a recent paper published in Nature and a researcher at the University of Minnesota, looked at the question by gathering data from 15 chimp and 4 bonobo communities. Over the past five decades, there have been 152 instances of observed, inferred and suspected killings in 15 chimp communities.

After crunching the numbers, he determined that humans are not the cause of the observed aggression. Besides the best statistical models pointing to natural variables such as population size and density, there is plenty of anecdotal evidence.

The highest killing rate occurred at a relatively undisturbed and never-provisioned site, the least disturbed site had at least two suspected killings, and the site that was rated as the most disturbed by humans had zero.

What’s more, one would think that if humans were the cause, the communities would be getting more violent over time. Despite some claims that this is indeed happening, Wilson found no statistical increase in reports of killings during the past five decades. Sure there are more reported instances, but that’s because there are more communities being watched.

“The most important predictors of violence were thus variables related to adaptive strategies: species; age–sex class of attackers and victims; community membership; numerical asymmetries; and demography,” wrote Wilson in the paper. “We conclude that patterns of lethal aggression in [chimps] show little correlation with human impacts, but are instead better explained by the adaptive hypothesis that killing is a means to eliminate rivals when the costs of killing are low.”

The paper, “Lethal aggression in Pan is better explained by adaptive strategies than human impacts,” was published in Nature by Wilson, along with 29 co-authors at sites across Africa.

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About bigkingken

A science writer dedicated to proving that the Big Ten - or the Committee on Institutional Cooperation, if you will - is more than athletics.
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