A Tuberculosis Outbreak Made One Kenyan Baboon Troop the Friendliest in the Country

Robert Sapolsky earned his BA at Harvard in 1978 and went straight from graduation to the Masai Mara in Kenya, where he set up camp near a wild troop of olive baboons and started measuring their stress hormones. The trick was that he could anesthetize the animals long enough for a blood draw and a clean cortisol reading — the body's chronic-stress hormone — without the act of being captured itself spiking the levels. He returned every summer for the next 25 years, eight to ten hours a day for four months at a stretch, using radio collars and individual face recognition to track the same animals over their lives.

The core finding accumulated steadily. In a stable baboon hierarchy, the alphas had low cortisol, low blood pressure, low LDL cholesterol; the lower-ranking males had elevated cortisol, atherosclerosis, and signs of immune suppression. The chronic stress of subordination, in other words, was producing measurable cardiovascular disease in animals that lacked any of the modern lifestyle risk factors people usually blame. Sapolsky generalized the results in his 1994 book Why Zebras Don't Get Ulcers.

Around 1983 he watched a piece of natural experimentation he could not have engineered. The Forest Troop, the group he was tracking, fed at a tourist-lodge garbage pit. The most aggressive, dominant males were the ones who got there first, ate spoiled meat contaminated with bovine tuberculosis, and died. Within a couple of seasons, more than half the adult males in the troop were dead, and almost all the casualties were from the high-ranking, hostile end of the hierarchy. The troop's remaining culture flipped. Aggression dropped. New males joining from outside adopted the calmer norms within months. The pattern persisted for two decades after the original cohort had died out.