In her recent post, Darcia Narvaez asks, "Is violence in our genes? Do chimpanzees in the wild want to kill others?" She concludes that evidence that lethal violence plays a critical role in chimpanzee society is flimsy. Psychologist Narvaez relies on a 2009 book by Donna Hart and Bob Sussman, and a 20-year-old book by Margaret Power. These authors maintain that deadly chimp-on-chimp violence is so rare we can conclude little from the few instances where it has been seen, and that in many cases the evidence is over-interpreted and exaggerated.
Full disclosure: My doctoral co-supervisor was Richard Wrangham, coauthor of Demonic Males, a book many view as the quintessence of the chimps-are-murderers view. In responding to the rarity of our observations of chimpanzee murders, I return to that old saw, the proverbial Martian anthropologist sent to Earth to study our society. There are a number of Martians studying humans, and most of them find humans to be a relatively peaceful lot, the occasional murder notwithstanding.
One Martian anthropologist returns to Mars, however, with the astonishing theory that human society is divided into units called nations. She learned that humans invest enormous resources in building weapons, and maintain thousands of their members in readiness to defend against potentially hostile groups. Violence occurs most often when members of one coalition crosses a border. Even when only a few individuals are killed, border crossings can escalate into a mobilization of people and machinery, leading to mass killings and the devastation of whole cities.
This hypothesis is greeted with skepticism, even scorn. Others have long studied humans, and have seen no evidence of mass killings or border crossings. And where are these weapons? No, Martian scholars conclude, humans form nations only to pool resources to build highways, regulate trade and control the minuscule number of humans who truly are violent. This war hypothesis is a just-so story, cobbled together from over-extrapolating paltry data.
Lesson: A behavior may be extremely important even if it is rare and difficult to observe. Not surprisingly, to me at least, chimpanzee lethal violence is rare and similarly difficult to observe. Despite this difficulty, 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.
It takes 15 years to grow a chimp, if every community lost a chimp a month, there would soon be no chimps; evidence of this is far from paltry, including one chimpanzee killing documented on film, from first wraa-bark to the dead body lying on the ground. Those who disagree with my perspective attempt to throw doubt on individual data points by citing a few episodes where the cause of death truly is doubtful, as if these few cases were all the scientific community has to rely on in our attempt to understand lethal violence.
Let's consider the evidence from the four chimpanzee populations that are perhaps the best-studied: Gombe, Mahale (both in Tanzania), Tai (Ivory Coast) and Kibale (Uganda). Some maintain that Jane Goodall failed to find violence among Gombe chimpanzees on her own, and it was only when other more bloody-minded scholars arrived on the scene that killings became a focus of attention. A quick perusal of Goodall's 1986 chapter on aggression in The Chimpanzees of Gombe will show that this is not true. Moreover, Goodall was a coauthor on a 2008 publication by Williams and colleagues that documented the cause of death in a staggering 130 cases at Gombe. Williams, Goodall and colleagues found that after illness (58 percent of deaths), murder by other chimpanzees was the most common cause of death (20 percent of deaths).
At Mahale, Nishida (1996) reported on circumstances that he felt were clear evidence that chimpanzee Ntologi had been lethally attacked by members of his own community. I mention this one incident because Nishida's description of it is so compelling. In the '70s Nishida and his colleagues watched as every male member of Nishida's original study group, the so-called K-group, disappeared; he and his colleagues believe Ntologi's community, the M-group, killed most of them.
At Tai, Boesch and colleagues published an article in 2008 documenting inter-group killings in which they wrote: "In the past three years, two cases of fatal inter-community attacks have been observed." They recount one attack that lasted 39 minutes. When the victim was near death, an attacking male "bit his arm and the noise of the breaking bones could be heard" meters away, after which the victim appeared to have died.
At Kibale-Ngogo, Mitani, Watts and colleagues report that they "observed the Ngogo chimpanzees kill or fatally wound 18 individuals from other groups." Mitani and Watt's team taped, from beginning to end, one horrifically brutal attack in which the victim died.
Richard Wrangham and Dale Peterson's book Demonic Males is commonly cited as the epitome of the exaggerated chimpanzee violence genre, so I have avoided discussing Wrangham and colleagues' comprehensive review of lethal violence among chimpanzees, "Comparative Rates of Violence in Chimpanzees and Humans." I find their argument, based on data from 75 chimp-on-chimp killings, to be utterly convincing.
I agree with Dr. Narvaez when she writes: "We have ourselves to blame, not selfish genes, not evolution... We can change the practices and beliefs" to become nonviolent. Many researchers who have documented violence among primates would also agree with her. Certainly Wrangham agrees with her on this point. In Demonic Males, Wrangham and Peterson wrote: "We are blessed with an intelligence that can...draw us away" from violence, from a demonic evolutionary history.
Christophe Boesch, Catherine Crockford, Ilka Herbinger, Roman Wittig, Yasmin Moebius, and Emmanuelle Normand (2008). Intergroup conflicts among chimpanzees in Taï National Park: lethal violence and the female perspective. American Journal of Primatology. 70: 519-532.
Jane Goodall, (1986). The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, MA: Harvard University Press.
Image from ZakuSeven, Flickr.com.
Donna Hart and Robert W. Sussman (2009). Man the Hunted: Primates, Predators, and Human Evolution. Boulder, CO: Westview Press.
John C. Mitani, David P. Watts, DP; Sylvia J. Amsler (2010) Lethal intergroup aggression leads to territorial expansion in wild chimpanzees. Current Biology 12: R507-R508.
Darcia Narvaez (2011). Male chimps and humans are genetically violent---NOT!
Toshisada Nishida (1996). The death of Ntologi, the unparalleled leader of M Group. Pan Africa News 2(2): 9-11.
Margaret Power (1991). The Egalitarians, Humans and Chimpanzees: An Anthropological View of Social Organization. Cambridge, England: Cambridge University Press.
J.M. Williams, E.V. Lonsdorf, M.L. Wilson, J. Schumacher-Stankey, J. Goodall, and A.E. Pusey (2008). Causes of death in the Kasekela chimpanzees of Gombe National Park, Tanzania. American Journal of Primatology 70: 766-777.
Richard Wrangham & D. Peterson (1996). Demonic Males: Apes and Origins of Human Violence. Boston: Houghton Mifflin.
Richard W. Wrangham, Michael L. Wilson and Martin N. Muller (2006) Comparative rates of violence in chimpanzees and humans. Primates 47: 14-26.
Image: ZakuSeven, Flickr.com.