A Natural History of Peace

Robert M. Sapolsky

From Foreign Affairs, January/February 2006

Summary:  Humans like to think that they are unique, but the study of other primates has called into question the exceptionalism of our species. So what does primatology have to say about war and peace? Contrary to what was believed just a few decades ago, humans are not "killer apes" destined for violent conflict, but can make their own history.

Robert M. Sapolsky is John A. and Cynthia Fry Gunn Professor of Biological Sciences and Professor of Neurology and Neurological Sciences at Stanford University. His most recent book is "Monkeyluv: And Other Essays on Our Lives as Animals."

To some extent, the age-old "nature versus nurture" debate is silly. The action of genes is completely intertwined with the environment in which they function; in a sense, it is pointless to even discuss what gene X does, and we should consider instead only what gene X does in environment Y. Nonetheless, if one had to predict the behavior of some organism on the basis of only one fact, one might still want to know whether the most useful fact would be about genetics or about the environment.

The first two studies to show that primates were somewhat independent from their "natures" involved a classic technique in behavioral genetics called cross-fostering. Suppose some animal has engaged in a particular behavior for generations -- call it behavior A. We want to know if that behavior is due to shared genes or to a multigenerationally shared environment. Researchers try to answer the question by cross-fostering the animal, that is, switching the animal's mother at birth so that she is raised by one with behavior B, and then watching to see which behavior the animal displays when she grows up. One problem with this approach is that an animal's environment does not begin at birth -- a fetus shares a very intimate environment with its mother, namely the body's circulation, chock-full of hormones and nutrients that can cause lifelong changes in brain function and behavior. Therefore, the approach can be applied only asymmetrically: if a behavior persists in a new environment, one cannot conclude that genes are the cause, but if a behavior changes in a new environment, then one can conclude that genes are not the cause. This is where the two studies come in.

In the early 1970s, a highly respected primatologist named Hans Kummer was working in Ethiopia, in a region containing two species of baboons with markedly different social systems. Savanna baboons live in large troops, with plenty of adult females and males. Hamadryas baboons, in contrast, have a more complex, multilevel society. Because they live in a much harsher, drier region, hamadryas have a distinctive ecological problem. Some resources are singular and scarce -- like a rare watering hole or a good cliff face to sleep on at night in order to evade predators -- and large numbers of animals are likely to want to share them. Other resources, such as the vegetation they eat, are sparse and widely dispersed, requiring animals to function in small, separate groups. As a result, hamadryas have evolved a "harem" structure -- a single adult male surrounded by a handful of adult females and their children -- with large numbers of discrete harems converging, peacefully, for short periods at the occasional desirable watering hole or cliff face.

Kummer conducted a simple experiment, trapping an adult female savanna baboon and releasing her into a hamadryas troop and trapping an adult female hamadryas and releasing her into a savanna troop. Among hamadryas, if a male threatens a female, it is almost certainly this brute who dominates the harem, and the only way for the female to avoid injury is to approach him -- i.e., return to the fold. But among savanna baboons, if a male threatens a female, the way for her to avoid injury is to run away. In Kummer's experiment, the females who were dropped in among a different species initially carried out their species-typical behavior, a major faux pas in the new neighborhood. But gradually, they assimilated the new rules. How long did this learning take? About an hour. In other words, millennia of genetic differences separating the two species, a lifetime of experience with a crucial social rule for each female, and a miniscule amount of time to reverse course completely.

The second experiment was set up by de Waal and his student Denise Johanowicz in the early 1990s, working with two macaque monkey species. By any human standards, male rhesus macaques are unappealing animals. Their hierarchies are rigid, those at the top seize a disproportionate share of the spoils, they enforce this inequity with ferocious aggression, and they rarely reconcile after fights. Male stump tail macaques, in contrast, which share almost all of their genes with their rhesus macaque cousins, display much less aggression, more affiliative behaviors, looser hierarchies, and more egalitarianism.

Working with captive primates, de Waal and Johanowicz created a mixed-sex social group of juvenile macaques, combining rhesus and stump tails together. Remarkably, instead of the rhesus macaques bullying the stump tails, over the course of a few months, the rhesus males adopted the stump tails' social style, eventually even matching the stump tails' high rates of reconciliatory behavior. It so happens, moreover, that stump tails and rhesus macaques use different gestures when reconciling. The rhesus macaques in the study did not start using the stump tails' reconciliatory gestures, but rather increased the incidence of their own species-typical gestures. In other words, they were not merely imitating the stump tails' behavior; they were incorporating the concept of frequent reconciliation into their own social practices. When the newly warm-and-fuzzy rhesus macaques were returned to a larger, all-rhesus group, finally, their new behavioral style persisted.

This is nothing short of extraordinary. But it brings up one last question: When those rhesus macaques were transferred back into the all-rhesus world, did they spread their insights and behaviors to the others? Alas, they did not. For that, we need to move on to our final case.

LEFT BEHIND

In the early 1980s, "Forest Troop," a group of savanna baboons I had been studying -- virtually living with -- for years, was going about its business in a national park in Kenya when a neighboring baboon group had a stroke of luck: its territory encompassed a tourist lodge that expanded its operations and consequently the amount of food tossed into its garbage dump. Baboons are omnivorous, and "Garbage Dump Troop" was delighted to feast on leftover drumsticks, half-eaten hamburgers, remnants of chocolate cake, and anything else that wound up there. Soon they had shifted to sleeping in the trees immediately above the pit, descending each morning just in time for the day's dumping of garbage. (They soon got quite obese from the rich diet and lack of exercise, but that is another story.)

The development produced nearly as dramatic a shift in the social behavior of Forest Troop. Each morning, approximately half of its adult males would infiltrate Garbage Dump Troop's territory, descending on the pit in time for the day's dumping and battling the resident males for access to the garbage. The Forest Troop males that did this shared two traits: they were particularly combative (which was necessary to get the food away from the other baboons), and they were not very interested in socializing (the raids took place early in the morning, during the hours when the bulk of a savanna baboon's daily communal grooming occurs).