1Department of Anthropology, University of Texas at Austin, 2Institute for Mind and Biology, University of Chicago, 3Department of Anthropology, Northwestern University, 4Department of Anthropology, New York University, 5Department of Anthropology, University of New Mexico
April 14, 2016 2:45, A 602
Social status and mating effort can impact hormone concentrations in male primates. In particular, dominance rank has been associated with variation in glucocorticoid concentrations, and seasonal changes in glucocorticoid concentrations have been documented in seasonal breeding primates. However, there is considerable variation in the relationship between these three factors across nonhuman primate studies. Here, we use a multi-group comparison to examine the relationship between glucocorticoid concentrations, dominance rank, and mating effort in high-ranking male rhesus macaques. More specifically, we tested the predictions that 1) alpha males have significantly higher glucocorticoid concentrations than other high-ranking males, and 2) males have higher glucocorticoid concentrations when mating effort is highest. We collected behavioral observations and fecal samples (N=224) for high-ranking males in 9 social groups (N=21) of free-ranging rhesus macaques for 5 months on Cayo Santiago, Puerto Rico. Dominance rank and month were significant predictors of glucocorticoid variation. Alpha males had consistently higher glucocorticoid concentrations than other high-ranking males (b=0.164, P=0.03, CI: 0.013; 0.314) and glucocorticoid concentrations were higher at the beginning of the mating season (when mating effort was at its peak), declining linearly as the mating season progressed (b=-0.153, P<0.001, CI: -0.200; -0.106). Male age, group size, the ratio of adult males to females, and the overall stability of the group’s male dominance hierarchy throughout the mating season did not affect glucocorticoid concentrations. In sum, we show that even among the highest-ranking males in a group, alpha males experienced particularly high physiological stress, likely associated with mating effort.