Energetic costs of social dominance in wild male baboons

In vertebrates, glucocorticoids can be upregulated in response to both psychosocial and energetic stressors, making it difficult to identify the cause of elevated glucocorticoid concentrations when both types of stressors are present. This problem has been particularly challenging in studies of social dominance rank in wild animals. In contrast to glucocorticoids, thyroid hormone concentrations are largely unaffected by psychosocial stressors and therefore offer a better estimate of energetic challenges. Here, we measured faecal metabolites of both triiodothyronine (mT3) and glucocorticoids (fGC) in wild baboons and assessed how these hormonal profiles vary with male dominance rank. We found that alpha males have lower mT3 and higher fGC than males of other ranks, indicating sustained energetic costs of alpha status. By contrast, low-ranking males have higher mT3 but similar fGC concentrations than non-alpha high-ranking males, reflecting their lower exposure to energetic stressors but greater vulnerability to psychosocial stressors than higher-ranking males. We also found that mate-guarding of fertile females, a behaviour expressed at higher rates by alpha males, partly explains the energetic costs of high social status. These findings offer evidence of the different types of costs experienced by low- and high-ranking animals.