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Peace Among Primates

By Robert M. Sapolsky | September 1, 2007 | 0 comments

Anyone who says peace is not part of human nature knows too little about primates, including ourselves.

It used to be thought that humans were the only savagely violent primate. “We are the only species that kills its own,” narrators intoned portentously in nature films several decades ago. That view fell by the wayside in the 1960s as it became clear that some other primates kill their fellows aplenty. Males kill; females kill. Some use their toolmaking skills to fashion bigger and better cudgels. Other primates even engage in what can only be called warfare—organized, proactive group violence directed at other populations.

Yet as field studies of primates expanded, what became most striking was the variation in social practices across species. Yes, some primate species have lives filled with violence, frequent and varied. But life among others is filled with communitarianism, egalitarianism, and cooperative child rearing.

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Patterns emerged. In less aggressive species, such as gibbons or marmosets, groups tend to live in lush rain forests where food is plentiful and life is easy. Females and males tend to be the same size, and the males lack secondary sexual markers such as long, sharp canines or garish coloring. Couples mate for life, and males help substantially with child care. In violent species, such as baboons and rhesus monkeys, the opposite conditions prevail.

The most disquieting fact about the violent species was the apparent inevitability of their behavior. Certain species seemed simply to be the way they were, fixed products of the interplay of evolution and ecology, and that was that.

And although human males might not be inflexibly polygamous or outfitted with bright red butts and six-inch canines designed for tooth-to-tooth combat, it was clear that our species had at least as much in common with the violent primates as with the gentle ones. “In their nature” thus became “in our nature.” This was the humans-as-killer-apes theory popularized by the writer Robert Ardrey, according to which humans have as much chance of becoming intrinsically peaceful as they have of growing prehensile tails.

That view always had little more scientific rigor than a Planet of the Apes movie, but it took a great deal of field research to figure out just what should supplant it. After decades’ more work, the picture has become quite interesting.

Some primate species, it turns out, are indeed simply violent or peaceful, with their behavior driven by their social structures and ecological settings. More importantly, however, some primate species can make peace despite violent traits that seem built into their natures. The challenge now is to figure out under what conditions that can happen, and whether humans can manage the trick ourselves.

Old primates and new tricks

To an overwhelming 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.

Two classic studies have shown that primates are somewhat independent from their “natures.” In the early 1970s, a highly respected primatologist named Hans Kummer was working in a region of Ethiopia 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 and quite different multilevel society. When confronted with a threatening male, the females of the two species react differently: A hamadryas baboon placates the male by approaching him, whereas a savanna baboon can only run away if she wants to avoid injury.

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. 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 absorbed 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 Frans de Waal of Emory University 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. In contrast, male stump tail macaques, which share almost all of their genes with their rhesus macaque cousins, display much less aggression, looser hierarchies, more egalitarianism, and more behaviors that promote group cohesion.

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. Finally, when the newly warm-and-fuzzy rhesus macaques were returned to a larger, all-rhesus group, their new behavioral style persisted.

This is nothing short of extraordinary. But it brings up one further 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—at least not within the relatively short time they were studied. For that, we need to move on to a 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, so did the amount of food tossed into its garbage dump. Baboons are omnivorous, and this “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 particular Forest Troop males who did this shared two traits: They were especially 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).

Soon afterward, tuberculosis, a disease that moves with devastating speed and severity in nonhuman primates, broke out in Garbage Dump Troop. Over the next year, most of its members died, as did all of the males from Forest Troop who had foraged at the dump. (Considerable sleuthing ultimately revealed that the disease had come from tainted meat in the garbage dump. There was little animal-to-animal transmission of the tuberculosis, and so the disease did not spread in Forest Troop beyond the garbage eaters.) The results were that Forest Troop was left with males who were less aggressive and more social than average, and the troop now had double its previous female-to-male ratio.

The social consequences of these changes were dramatic. There remained a hierarchy among the Forest Troop males, but it was far looser than before. Compared with other, more typical savanna baboon groups, high-ranking males rarely harassed subordinates and occasionally even relinquished contested resources to them. Aggression was less frequent, particularly against third parties. And rates of affiliative behaviors, such as males and females grooming each other or sitting together, soared. There were even instances, now and then, of adult males grooming each other—a behavior nearly as unprecedented as baboons sprouting wings.

This unique social milieu did not arise merely as a function of the skewed sex ratio (with half the males having died); other primatologists have occasionally reported on troops with similar ratios but without a comparable social atmosphere. What was key was not just the predominance of females but the type of male who remained. The demographic disaster—what evolutionary biologists term a “selective bottleneck”—had produced a savanna baboon troop quite different from what most experts would have anticipated.

But the largest surprise did not come until some years later. Female savanna baboons spend their lives in the troop into which they are born, whereas males leave their birth troop around puberty; a troop’s adult males have thus all grown up elsewhere and immigrated as adolescents. By the early 1990s, none of the original low aggression/high affiliation males of Forest Troop’s tuberculosis period was still alive; all of the group’s adult males had joined after the epidemic. Despite this, the troop’s unique social milieu persisted—as it does to this day, some 20 years after the selective bottleneck. In other words, adolescent males that enter Forest Troop after having grown up elsewhere wind up adopting the unique behavioral style of the resident males. As defined by both anthropologists and animal behaviorists, “culture” consists of local behavioral variations, occurring for nongenetic and nonecological reasons, that last beyond the time of their originators. Forest Troop’s low aggression/high affiliation society constitutes nothing less than a multigenerational benign culture.

Continuous study of the troop has yielded some insights into how its culture is transmitted to newcomers. Genetics obviously plays no role, nor apparently does self-selection: Adolescent males that transfer into the troop are no different from those that transfer into other troops, displaying on arrival similarly high rates of aggression and low rates of affiliation. Nor is there evidence that new males are taught to act in benign ways by the residents. One cannot rule out the possibility that some observational learning is occurring, but it is difficult to detect, given that the distinctive feature of this culture is not the performance of a unique behavior but the performance of typical behaviors at atypically extreme rates.

To date, the most interesting hint about the mechanism of transmission is the way recently transferred males are treated by Forest Troop’s resident females. In a typical savanna baboon troop, newly transferred adolescent males spend years slowly working their way into the social fabric; they are extremely low ranking—ignored by females and noted by adult males only as convenient targets for aggression. In Forest Troop, by contrast, new male transfers are inundated with female attention soon after their arrival. Resident females first present themselves sexually to new males an average of 18 days after the males arrive, and they first groom the new males an average of 20 days after they arrive, whereas normal savanna baboons introduce such behaviors after 63 and 78 days, respectively. Furthermore, these welcoming gestures occur more frequently in Forest Troop during the early post-transfer period, and there is four times as much grooming of males by females in Forest Troop as elsewhere. From almost the moment they arrive, in other words, new males find out that in Forest Troop, things are done differently.

At present, I think the most plausible explanation is that this troop’s special culture is not passed on actively but simply emerges, facilitated by the actions of the resident members. Living in a group with half the typical number of males, and with the males being nice guys to boot, Forest Troop’s females become more relaxed and less wary. (This is so, in part, because in a typical baboon troop, a male who loses a dominance interaction with another male will often attack a female in frustration.) As a result, they are more willing to take a chance and reach out socially to new arrivals, even if the new guys are typical jerky adolescents at first. The new males, in turn, finding themselves treated so well, eventually relax and adopt the behaviors of the troop’s distinctive social milieu.

Natural born killers?

Are there any lessons to be learned here that can be applied to human-on-human violence—apart, that is, from the possible desirability of giving fatal cases of tuberculosis to aggressive people? Can human behavior be as malleable—and as peaceful—as Forest Troop’s?

Any biological anthropologist opining about human behavior is required by long-established tradition to note that for 99 percent of human history, humans lived in small, stable bands of related hunter-gatherers. Game theorists have shown that a small, cohesive group is the perfect setting for the emergence of cooperation: The identities of the other participants are known, there are opportunities to play games together repeatedly (and thus the ability to punish cheaters), and there is open-book play (players can acquire reputations). And so, those hunter-gatherer bands were highly egalitarian. Empirical and experimental data have also shown the cooperative advantages of small groups at the opposite human extreme, namely in the corporate world.

But the lack of violence within small groups can come at a heavy price. Small homogenous groups with shared values can be a nightmare of conformity. They can also be dangerous for outsiders. Unconsciously emulating the murderous border patrols of closely related male chimps, militaries throughout history have sought to form small, stable units; inculcate them with rituals of pseudokinship; and thereby produce efficient, cooperative killing machines.

Is it possible to achieve the cooperative advantages of a small group without having the group reflexively view outsiders as the Other? One often encounters pessimism in response to this question, based on the notion that humans, as primates, are hard-wired for xenophobia. Some brain-imaging studies have appeared to support this view in a particularly discouraging way. There is a structure deep inside the brain called the amygdala, which plays a key role in fear and aggression, and experiments have shown that when subjects are presented with a face of someone from a different race, the amygdala gets metabolically active—aroused, alert, ready for action. This happens even when the face is presented subliminally, which is to say, so rapidly that the subject does not consciously see it.

More recent studies, however, should mitigate this pessimism. Test a person who has a lot of experience with people of different races, and the amygdala does not activate. Or, as in a wonderful experiment by Susan Fiske, of Princeton University, subtly bias the subject beforehand to think of people as individuals rather than as members of a group, and the amygdala does not budge. Humans may be hard-wired to get edgy around the Other, but our views on who falls into that category are decidedly malleable.

In the early 1960s, a rising star of primatology, Irven DeVore of Harvard University, published the first general overview of the subject. Discussing his own specialty, savanna baboons, he wrote that they “have acquired an aggressive temperament as a defense against predators, and aggressiveness cannot be turned on and off like a faucet. It is an integral part of the monkeys’ personalities, so deeply rooted that it makes them potential aggressors in every situation.” Thus the savanna baboon became, literally, a textbook example of life in an aggressive, highly stratified, male-dominated society. Yet in my observation of Forest Troop, I saw members of that same species demonstrate enough behavioral plasticity to transform their society into a baboon utopia.

The first half of the twentieth century was drenched in the blood spilled by German and Japanese aggression, yet only a few decades later it is hard to think of two countries more pacific. Sweden spent the 17th century rampaging through Europe, yet it is now an icon of nurturing tranquility. Humans have invented the small nomadic band and the continental megastate, and have demonstrated a flexibility whereby uprooted descendants of the former can function effectively in the latter. We lack the type of physiology or anatomy that in other mammals determine their mating system, and have come up with societies based on monogamy, polygyny, and polyandry. And we have fashioned some religions in which violent acts are the entrée to paradise and other religions in which the same acts consign one to hell. Is a world of peacefully coexisting human Forest Troops possible? Anyone who says, “No, it is beyond our nature,” knows too little about primates, including ourselves.

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About The Author

Robert M. Sapolsky, Ph.D., is the John A. and Cynthia Fry Gunn Professor of Biological Sciences and a professor of neurology and neurological sciences at Stanford University. His most recent book is Monkeyluv: And Other Essays on Our Lives as Animals. A longer version of this essay appeared in Foreign Affairs.


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