Female hierarchy instability, male immigration and infanticide increase glucocorticoid levels in female chacma baboons

Abstract

Female baboons (Papio hamadryas spp.) must contend with myriad potential stressors on a daily basis. In a previous study on female chacma baboons, Papio hamadryas ursinus, living in the Okavango Delta of Botswana, increases in glucocorticoid (GC) concentrations were associated with female reproductive stage, male immigration and the threat of infanticide. Here, we extend this previous analysis to a larger data set with several additional potential stressors, including actual infanticide and instability in the female dominance hierarchy. A general linear mixed model showed that reproductive state, male immigration, infanticide, female rank instability and predation all had significant effects on GC levels. Lactating females’ GC levels increased in response to the arrival of immigrant males and increased even further when infanticide occurred. In contrast, cycling and pregnant females’ GC levels did not change. Females also showed elevated GCs in response to instability within their own dominance hierarchy, especially if their own ranks were at risk. Females’ stress responses were frequent, but specific to events that threatened their own lives, the lives of their offspring, or their dominance ranks.