Male chimpanzees reduce aggression when social relationships in their group are unstable

A high social status has substantial health and fitness benefits for males and females of many social animal species, including humans. However, attaining and maintaining dominance often comes with elevated energetic costs. "To investigate costs associated with dominance acquisition and maintenance, we examined potential sources of energetic and psychosocial stressors that male chimpanzees might be exposed to on a daily basis, and particularly during periods when males compete intensively over dominance status and mating opportunities," says Anna Preis, first author of the study.

The authors found that urinary cortisol levels were higher in all males during unstable compared to stable dominance periods, whereas aggression rates showed the reversed pattern, with higher rates in stable periods. In contrast to a previous study in another chimpanzee population, dominance rank and urinary cortisol levels were not associated in either period. These findings indicate that status maintenance is not associated with elevated physiological stress for dominant Tai male chimpanzees, and suggest that status competition and the uncertainty of dominance relationships within the group exposes all males to psychosocial stress, despite lower rates of aggressive interactions. This supports a number of other studies showing that unpredictability in social relationships is particularly stressful in primates, -- including in humans.

"Our study shows that open aggression was not the source of the high stress levels of males during periods of heightened male-male competition, but that males were affected by the social uncertainty induced by these competitive periods," says Preis. "We investigated two contexts that induce social instability, competition over status and competition over mating opportunities. For both contexts we found the same pattern: All males, irrespective of dominance rank, had higher stress levels during periods with high male-male competition, while aggression rates and intensities were higher in periods with low male-male competition."