Stress, Evolution, and Sociality in Wild Female Baboons

Abstract

Organisms that successfully respond to threats have higher fitness than those who fail to mount appropriate responses. The vertebrate stress response system has evolved as a suite of physiological and behavioral strategies for coping with such threats, but chronic activation of this system can carry long-term physiological costs. I studied a population of wild olive baboons (Papio cynocephalus anubis) and used demographic and hormonal data to investigate how social parameters influence exposure to stressors and also to consider the adaptive value of the chronic stress response. Social circumstances such as rank and access to social support have been associated with differential exposure to the physiological costs of stress including exposure to the stress hormone, cortisol. I found that neither rank nor social support correlated significantly with cortisol exposure in adult female baboons. In contrast, a juvenile female’s social circumstances strongly influence her exposure to cortisol; maternal rank, a living mother, and the presence of multiple female kin were all significant predictors of cortisol exposure. I also considered the evolutionary history of the vertebrate stress response system. The system’s major weakness has always been believed to be its enigmatic predisposition for chronic and pathological activation. I propose that this tendency for continued activation in the face of lasting challenges can carry its own adaptive benefits and is not, as has been long assumed, merely maladaptive. In reaction to a severe and prolonged stressor, the baboons implemented a number of cortisol-mediated coping strategies, suggesting that sustained activation of the stress response system may promote lasting changes in behavior and physiology that can promote survival. Thus, the long-term risk of pathology may be outweighed by more immediate survival benefits.