Hallmarks of the human life cycle include early weaning and a fertility rate unusually high for a large bodied, high-investment primate, which requires a flexible social network of offspring provisioning and care to sustain. Past research in birds in which males provide care shows that testosterone is often reduced as males become fathers and their priorities shift. Consistent with this model, our work at Cebu was the first to use longitudinal data to show that men experience a dramatic decline in testosterone as they transition into new fatherhood (see NY Times coverage here). Because no similar effects are seen in other closely-related primates (or in most mammals), our work suggests that paternal care of offspring has likely been sufficiently important to human reproductive success to favor the evolution of male capacities to adjust hormones and priorities as men enter culturally-defined fathering roles. Under the leadership of former PhD advisee Lee Gettler (now Associate Professor and Department Chair at Notre Dame), along with recent postdoc Stacy Rosenbaum (now Assistant Professor of Anthropology at Michigan), our group has explored multiple dimensions of human life history and reproductive strategy at Cebu, including the plasticity of life history strategies, endocrine correlates of caregiving, the social and familial predictors of cooperative care, and testing models of maturational acceleration in response to cues of mortality and environmental harshness.
Figure: Decline in salivary testosterone during a 4.5 year period of follow-up among 26 year old men who were all single non-fathers at baseline (Gettler et al PNAS 2011). Men who transitioned into new fatherhood experienced the largest declines in testosterone.