Evolutionary Perspectives on Human Male Socioendocrinology: Insights from Cebu, Philippines

Humans have evolved a life history characterized by the production of highly immature offspring, the early weaning of those offspring, who then remain dependent for a decade or more, shortened inter-birth intervals, and high juvenile survivorship. This suite of characteristics is thought to have resulted in or have co-evolved with allocaregiving. Specifically, given the ecologies in which our bipedal hominin ancestors resided, it would have been virtually impossible for females to maintain this life history strategy if they were not cooperating with other individuals to raise young. There is a longstanding debate within anthropology as to whether fathers, grandmothers, or older siblings were the best candidates to cooperate with mothers. Outside of ethnographic observation, there have been few lines of evidence to support or refute whether fathers raised offspring. Male socioendocrinology is an under-explored area that may shed light on the evolution of human fatherhood. I draw on data from The Cebu Longitudinal Health and Nutrition Survey (CLHNS), which is an ongoing population-based birth cohort study of mothers and their infants (index children) born in 1983-84 in the metropolitan area of Cebu City, the Philippines, to test hypotheses on the ways in which men's hormones respond to parenthood and direct caregiving, over multiple time scales ranging from 4-5 year to minute-to-minute changes. The results of two separate studies show that men transitioning to partnered, first-time fatherhood experience a significantly greater decline in testosterone compared to men remaining single non-fathers. Those fathers who are most involved in childcare also generally had lower testosterone compared to other fathers. In a cross-sectional study, fathers were shown to have elevated prolactin, a hormone potentially linked to parenting effort. However, fathers showed short-term declines in prolactin and no change in testosterone when interacting with their children, suggesting long- and short-term patterns of hormonal change in fathers may be distinct. Consistent with the notion that males cooperated with mothers to raise young in some capacity during human evolution, multiple findings from the studies presented here suggest that male reproductive physiology has been shaped by natural selection to respond to committed parenting and involvement in offspring care.