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Peace Among Primates (Part 2)

(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

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.

 

(To be con­tin­ued, in a third and final install­ment, on Sat­ur­day April 19th).

Robert SapolskyRobert 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.

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