Why are chimps violent? Is it our fault?

A analysis of five decades of research on chimpanzees and their apparently more peaceful cousins, bonobos, finds that habitat destruction by humans is not to blame for chimp violence.  

Brandon Wade/The Humane Society of the United States and Chimp Haven/AP
A 4-year-old chimpanzee relaxes in the trees at Chimp Haven sanctuary in Keithville, La. A new study published Wednesday in Nature suggests that chimpanzees developed aggressive behaviors through natural selection rather than as a result of human disturbances, as previously thought.

New analysis of 50 years worth of chimpanzee and bonobo research is changing the way we think about the nature of violence.

It’s been 35 years since primatologist Jane Goodall shocked the world with her account of chimpanzees brutally ganging up on lone individuals from neighboring communities. Since then, scientists have proposed a variety of theories about how chimps came to exhibit such vicious behavior.

Many believed that humans' deforestation and hunting are to blame for placing new constraints on resources. Dr. Goodall began to wonder if she had somehow sparked such aggression by giving bananas to chimps that visited her camp at the Gombe National Park in Tanzania.

However, a new study published Wednesday in the scientific journal Nature dismisses the idea that humans have managed to turn chimps against each other.

If humans aren’t to blame, what drives chimpanzees to gang up on each other in ways that other non-human primates, including the nearly genetically identical bonobo, do not?

For this study, an international team of researchers compared 54 years worth of chimpanzee and bonobo research, including some new, yet to be published observations. The researchers identified 152 gang killings among chimpanzees in 15 out of 18 communities surveyed, and just one suspected killing among the four bonobo communities studied. (Bonobo research has been severely limited in recent decades due to political strife in the Democratic Republic of Congo.)

These statistics may fit into the popular caricatures of bonobos as free-loving hippies that spend all their time fornicating and violent murderous chimps that engage in human-like warfare but the discrepancy likely has more to do with accessibility to resources and balances of power, says study author Michael Wilson, an anthropologist at the University of Minnesota.

For one thing, bonobos’ legendary promiscuity isn’t actually that much more pronounced than chimpanzees', Prof. Wilson says. For another, violence isn’t as prominent a feature of chimp culture as many people believe.

“In reality, chimps are very much like people,” Wilson says. “They spend a lot of time just going about their daily business, looking for food, and socializing.”

The incidence of concerted violence seen among chimps tends to occur when a group of male chimpanzees patrol their territory together in search of food or a fertile female and they happen across a lone individual from another community. Only when this balance of power is in the gang’s favor do the chimps attack, Wilson says.

When one group of chimps encounters another group of chimps, as frequently happens if a desirable fruit grows on the border between the two territories, the neighboring groups will run around and try to chase the other off, but they will not initiate a battle. This suggests that they are not engaging in violent acts for the sake of violence or simply because their innate nature is to be aggressive.

At the same time, there is clearly some psychological difference between chimps and bonobos, says Harvard University biological anthropologist Richard Wrangham, lead author of the study.

“It’s clearly in their psychology. It’s not just that they live in different areas and are subject to different environmental contexts,” Prof. Wrangham says. “If you have bonobos and chimps side by side in captivity, they’re very different species.”

Chimps and bonobos share 99 percent of the same DNA, but they have followed very different evolutionary paths. We don’t know very much about the evolutionary regimes that shaped the innate characteristics seen today, but analysis of skull structure suggests that bonobos evolved from a chimpanzee-like ancestor rather than the other way around, Wrangham explains.

Wrangham suggests that that connection between a chimpanzee and a bonobo is similar to that found between a wolf and a domesticated dog. What’s becoming clear to scientists is that somewhere during the long-term history of the species, bonobos experienced selection for reduced aggression. What evolutionary pressures led to that selection process remains unclear.

However, Wrangham theorizes that the presence – or lack thereof – of gorillas may have contributed to the divergent tendencies towards this type of aggression between bonobos and chimpanzees.

Chimpanzees tend to share territories with gorillas while bonobos do not. Because bonobos do not have to compete with gorillas for food, they are able to form more stable community groups that are less likely to leave individuals alone and unprotected than chimps, Wrangham says.

This evidence that aggression in chimps arose organically through the process of natural selection suggests that while violent nature may be ingrained, it is not necessarily innate. This is an important distinction when attempting to draw parallels between human and chimpanzee tendencies toward violence, evolutionary anthropologist Jane Silk suggests in a companion article published in Nature.

“The data tell us that there are some ecological and demographic circumstances in which the benefits of lethal aggression exceed the costs for chimpanzees, nothing more,” she writes. “Humans are not destined to be warlike because chimpanzees sometimes kill their neighbours.”

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