Hormones, rank, and aggression during periods of social stability: examining the Challenge Hypothesis and the Dual Hormone Hypotheses in male white-faced capuchins (Cebus imitator)

Social dominance hierarchies shape access to resources in many group-living species, yet the endocrine mechanisms associated with dominance rank remain unclear, especially in multi-male groups. To examine how testosterone, glucocorticoids, male dominance rank, and male-male aggression interact under conditions of social stability, we tested three frameworks in wild white-faced capuchins (Cebus imitator): the Challenge Hypothesis (i.e., testosterone increases with reproductive competition), rank-related stress responses (i.e., rank and glucocorticoids are positively related), and the Dual Hormone Hypothesis (i.e., testosterone's effects on status-seeking behaviours are strongest when glucocorticoids are low). We combined behavioural observations with endocrine data from 982 fecal testosterone (fT), 984 fecal glucocorticoids (fGC), and 978 paired fT and fGC samples from 14 males across three groups during a 17-month field study. Hypotheses were examined using Bayesian multilevel models. Support for our hypotheses was mixed. Male-male aggression was not associated with hormonal measures. However, consistent with the Challenge Hypothesis, high-ranking males exhibited higher fT concentrations than lower-ranking males, and there was also some evidence of elevated fGC with rank, consistent with the rank-related stress hypothesis. Higher rank was also associated with larger fT-fGC values and a negative association with fT × fGC, consistent with the Dual Hormone Hypothesis. These associations disappeared when alpha-males were excluded from analyses, indicating that endocrine correlates of status differ substantially between alpha and subordinate males. Together, our results suggest that the endocrine mechanisms underlying status are context dependent and heavily influenced by alpha male physiology.