Chimpanzees use aggression in ways that repulse us when we see it in our own species. My University of Southern California colleague Christopher Boehm estimated that the rate of nonlethal violence among wild chimpanzees is greater than that of most human societies. A separate study by Harvard University primatologist Richard Wrangham and his coauthors found a similar “murder” rate between chimpanzees and traditional human hunter-gatherer societies, and a much higher rate of nonlethal aggression by chimpanzees.
Chimpanzees are the only primates other than us who routinely kill one another in the name of territory and resources. As in human societies, the killers are virtually always males, as Wrangham pointed out in his book Demonic Males. Chimpanzees lack the weapons we associate with efficient killers; they have hands and fingernails, not paws and claws. Their canine teeth, while impressive, are no match for those of a carnivore. And yet they carry out grisly attacks on members of their own and especially neighboring communities. Males sexually coerce females. And both males and females are known to commit infanticide.
Chimpanzees are the only primates other than us who routinely kill one another in the name of territory and resources.
Chimpanzees are not killing machines; 99 percent of their lives are spent in peace. Of course the same could be said about us. The potential for violent behavior is within each of us, but it surfaces only rarely, or never at all. And just as humans have myriad ways to defuse disputes before they reach a stage at which violence seems a feasible option, chimpanzees have many fail-safes that prevent lethal aggression from taking place. After minor squabbles they reconcile, and the ways in which they restore social harmony are as interesting and important as the violence that gets all the attention from scientists and the media.
Natural Born Killers?
Like every other mammal on the planet, chimpanzees have the capacity to inflict physical harm on one another. It’s harder to take a utilitarian approach to understanding violence in chimpanzees than it is in lower mammals. Chimpanzees who injure or kill one another are not immoral. They are amoral; their violence is a means to reach an end. We don’t get angry at lions for attacking each other or for killing zebras; that’s just what lions do. We tend to view great apes in a different light because of their close evolutionary connection to us. An entire wing of animal behavior research is founded on the idea that the roots of human morality may be found in the premoral behavior of nonhuman primates, with chimpanzees serving as a prime animal model. Most researchers have concluded that “might makes right” when it comes to chimpanzees’ treatment of one another, but that hasn’t stopped anthropologists from citing chimpanzee aggression as a potential example of how punitive violence may have its cultural origins in our own species.
There is a school of thought—a poorly informed one—that holds that chimpanzee aggression is somehow the product of human interference in their behavior. This charge was first raised shortly after Jane Goodall first observed the males of one Gombe community seeking out and attacking the males of the neighboring community. Since these two groups had recently split from one another, males were ganging up to kill their former comrades. Some anthropologists argued that the violence was precipitated by the presence of human researchers, or by humans provisioning chimps with bananas, or perhaps by humans altering the habitat, or perhaps even by the habituation process itself. The argument gained traction especially among scholars who view human societies as egalitarian and peaceful by nature. They argue that intergroup violence is the product of outside forces such as Western contact. We then learned that such killings between males of adjacent chimp communities happen at nearly every research site where the apes have been observed. There was even a report of wild, unhabituated male chimpanzees attacking a group of captive chimpanzees in a facility in Senegal that happened to be inside the wild males’ territory.
The stakes are high. If male chimpanzee violence is adaptive rather than pathological, we might infer that the same is true for humans. Male chimpanzees use violence to achieve resource-related goals—food and sex—by eliminating rivals for both. The argument that extreme violence is an aberration dissolved with more and more field observations of chimpanzee violence. For most of the past three decades there has been a consensus that violence is a normal, adaptive behavior among chimpanzees.
A 1991 book by anthropologist Margaret Power attempted to resurrect the idea of chimpanzees as peaceful by nature and violent only when their social behavior is disturbed by human influence. Anthropologists Robert Sussman and Joshua Marshack of Washington University, Saint Louis, made a similar claim. These authors claim that human-caused habitat disturbances, combined with small forest size and provisioning, can produce increased, even lethal, aggression. The skeptics’ case ignores the voluminous data on chimpanzee violence. The most frequently violent chimpanzees that we know of, at Ngogo in Uganda, live in one of the most pristine habitats in which chimpanzees have been studied.
In the soul of their biology, chimpanzees possess the potential for behaviors that we consider immoral when we see them in ourselves. When they occur in chimpanzees, they are simply strategic ways to achieve life goals.
Michael Wilson of the University of Minnesota and his colleagues recently analyzed the pattern of chimpanzee violence that has been compiled over a half century of field research. They considered potential predictors of violence that are human caused: habitat disturbance, provisioning, and size of the forest. Wilson and colleagues found that none of these factors predicted which chimpanzee populations displayed the most violence. Instead, the best predictors of violence were adaptive factors. Violent attacks are more likely when there is an imbalance in the number of males in the parties of two adjacent communities. Attacks are also predictable from male demography; attackers are normally sexually mature males, so the more males in a community, the more violence we see.
It might have been acceptable in the 1970s or 1980s to be skeptical about the adaptive nature of chimpanzee violence, but with the accumulated observations of wild chimpanzees since, it cannot be written off as “unnatural.” I suppose the modes of chimpanzee violence are “natural” in the sense that they occur routinely in the wild under a wide range of environmental conditions. In the soul of their biology, chimpanzees possess the potential for behaviors that we consider immoral when we see them in ourselves. When they occur in chimpanzees, they are simply strategic ways to achieve life goals. Deciding whether there is an evolutionary link between chimpanzee aggression and human violence is an important topic, but not one that should influence our interpretation of chimpanzee behavior.
Excerpted from The New Chimpanzee by Craig Stanford, Harvard University Press. Copyright 2018. All rights reserved.