Simple SummaryIn most primates, the “challenge hypothesis” predicts increases in male–male aggressive behavior and testosterone during reproductive challenges, i.e., mating competition, or during periods of social instability, especially in seasonal breeders. However, few studies have investigated the role of stress hormones, such as glucocorticoids, and social rank in non-seasonal primates. In this study, we tested some predictors of the “challenge hypothesis” during a period of social stability in a non-seasonal primate. We collected data on aggressive behavior (male-to-male and male-to-female), copulation, and fecal samples (n = 700) to quantify testosterone and glucocorticoid metabolites (fTm and fGCm, respectively) in seven adults of stumptail macaques (Macaca arctoides) living in semi-captivity. We found that fTm and fGCm increased during times of copulation, although neither of the two hormones were associated with aggression between males. Besides, fGCm levels were positively associated with male-to-female aggression, indicating a possible social stress challenge. Both fTm and fGCm levels were higher in higher- and middle-ranking males. Our findings partially support the “challenge hypothesis” in stumptail males and highlight the importance of glucocorticoid secretion during challenging situations; our results also indicate that during periods of social stability, testosterone is not associated with aggression, at least in a non-seasonal primate.AbstractThe “challenge hypothesis” predicts higher male–male aggressive behavior along with increases in testosterone levels during times of reproductive challenges and social instability. In addition, in some primate species, higher glucocorticoid levels can be observed as well, but this is usually modulated by dominance rank. We studied rank-related aggressive behavior, mating activity, and fecal testosterone and glucocorticoid metabolites (fTm and fGCm) in male stumptail macaques (Macaca arctoides) in order to test some predictions of the “challenge hypothesis”. Over a 20-month period, we collected data on aggressive behavior and copulation, as well as fecal samples (n = 700) to quantify fTm and fGCm in seven adult stumptail males living in captivity. During periods of mating activity, male-to-male aggression increased in higher- and middle-ranking males. Neither fTm nor fGCm levels predicted male-to-male aggression. fGCm levels (but not fTm) were positively associated with male-to-female aggression; however, this association was pronounced during periods of mating activity. fGCm levels differed according to social rank, with middle-ranking males having the highest levels. Both hormones were higher during periods of mating activity, but only in higher- and middle-ranking males. Taken together, our findings partially support the challenge hypothesis in a non-seasonal primate and shed some light on the unique social and mating system of the stumptail macaque.