Testosterone and cortisol are negatively associated with ritualized bonding behavior in male macaques

Neuroendocrine research on the formation of social bonds has primarily focused on the role of nonapeptides. However, steroid hormones often act simultaneously to either inhibit or facilitate bonding. Testosterone is proposed to mediate a trade-off between male mating effort and nurturing behavior; therefore, low levels are predicted during periods of nurturing infant care and social bonding. In species where social bonding and support regulates hypothalamic-pituitary-adrenal axis activity, we also expect glucocorticoid levels to be low during bonding periods. We investigated how immunoreactive urinary testosterone (iuT) and cortisol (iuC) were related to triadic male-infant-male interactions – a ritualized male bonding behavior – as well as infant care in male Barbary macaques ( Macaca sylvanus ). We collected >3000 hours of behavioral observation data during full-day focal animal follows from 14 adult males and quantified iuT and iuC from 650 urine samples. As predicted, both iuT and iuC were negatively correlated with rates of triadic interactions within-subjects in the hours preceding urination. We found no relationship between iuT and iuC with triadic interactions between-subjects. Infant care was weakly positively correlated to iuT and iuC within-subjects, but not between-subjects. The observed negative relationship between iuT and triadic interactions may be beneficial to lower competitive tendencies between adult males and to not inhibit bond formation. Lowered iuC could reflect increased bonding and perceived social support as triadic interactions predict future coalition formation in this species. Additionally, lowered iuC may be reflective of buffered tensions between males. The positive relationship of iuT and iuC with infant care suggests that the handling of infants in may be less nurturing but rather protective or competitive in this species. Measuring steroid hormones in relation to bonding and nurturing can help us interpret behaviors within the ecological contexts that they occur.