Hormonal correlates of paternal care differences in the Hylobatidae

Only one of the 15 species of monogamous hylobatids, the siamang (Symphalangus syndactylus), demonstrates direct paternal care in the form of infant-carrying, providing a unique model for examining hormonal correlates of paternal care differences between siamangs and gibbons. We used behavioral data and fecal hormone analysis to investigate (1) differences in monthly percent father-infant proximity in relation to monthly fecal androgen metabolite concentrations from infant birth to the late postpartum period between siamangs and gibbons, (2) the pattern of change in fecal androgen and fecal estrogen metabolite concentrations during the 8-week peripartum period between siamangs and gibbons, and (3) the change in mean fecal glucocorticoid metabolite concentrations at 1-month postpartum from individual baseline between siamangs and gibbons. Father-infant proximity increased as androgen concentrations decreased over the postpartum period in siamangs but not in gibbons. Androgen concentrations increased around birth in siamangs during the 8-week peripartum period, but exhibited a decreasing trend around birth in gibbons. Estrogen concentrations increased from pre- to postpartum in siamangs during the 8-week peripartum period, but exhibited a decreasing trend from pre- to postpartum in gibbons. The difference in mean glucocorticoid metabolite concentrations from baseline was greater in siamangs than gibbons. Our data suggest a relationship between specific steroid hormone patterns and differences in paternal care among the hylobatids, warranting further investigation of such proximate mechanisms.