Correlates of stress in free-ranging male chacma baboons, Papio hamadryas ursinus

Abstract

In male baboons, social instability is known to increase concentrations of glucocorticoids (GCs), a class of steroid hormones associated with the vertebrate stress response. The stress response may be particularly marked among male chacma baboons, whose social groups often demonstrate a strong mating skew and high rates of infanticide. Here we test this prediction and explore additional correlates of variation in concentrations of GCs measured in faeces. We collected 482 faecal samples from 13 males over 14 months. Changes in the male hierarchy were the major source of GC variation. Such instability occurred as the result of a new immigrant male's take-over of the alpha position, a resident male's rise to the alpha position, and the rise in rank of a young natal male. All three events were correlated with elevated GCs among all males and higher rates of male–male aggression than were found during periods of stability. The interaction between rank and instability had a significant effect on GC concentrations; low-ranking males had higher GCs during stable periods, whereas high-ranking males had higher GCs during unstable periods. Males that dispersed had elevated GCs in the month following immigration but not at other times, including the month preceding emigration. Finally, entering into a sexual consortship led to a temporary increase in a male's GC measures. By contrast, seasonal changes and a male's age were unrelated to GCs. GC variation in chacma males was primarily explained by temporary elevations following events that relate to reproductive fitness.