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.

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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.