(Editor’s Note: A few days ago we published the first installment of this Peace Among Primates series, by neuroscientist Robert Sapolsky. Today we publish the second installment. Next Saturday, April 19th, you can come back and read the third and final part in the series.)
Peace Among Primates (Part 2)
Anyone who says peace is not part of human nature knows too little about primates, including ourselves.
–By Robert M. Sapolsky
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.
(To be continued, in a third and final installment, on Saturday April 19th).
– 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. He wrote the classic Why Zebras Don’t Get Ulcers: An Updated Guide to Stress, Stress Related Diseases and Coping. His most recent book is Monkeyluv: And Other Essays on Our Lives as Animals. A longer version of this essay appeared in Foreign Affairs. We bring you this post thanks to our collaboration with Greater Good Magazine, a UC-Berkeley-based quarterly magazine that highlights ground breaking scientific research into the roots of compassion and altruism.