Variation in early-life testosterone within a wild population of red deer

Individual differences in circulating hormone concentrations can affect life-history traits throughout an animal's life. Despite this, relatively little is known about the potential drivers or consequences of individual variation in hormone levels, particularly in early life. In animals showing maternal care, early development is often dependent on maternal characteristics and condition. It is therefore possible that individual hormone profiles early in life are dependent on condition-linked characteristics of the mother. Using data from a long-term study of a wild red deer (Cervus elaphus) population, we investigated the potential role of maternal effects on offspring early-life testosterone concentrations and the relationship between these testosterone levels and juvenile survival. Most of the variation among neonatal calves was accounted for by their age and sex. Both sexes showed a steep decline in testosterone levels within 24 h of birth, although concentrations were consistently higher in males, and females showed a steeper decline in testosterone after 24 h. Furthermore, male calves born in years after a brother had lower concentrations than those who were preceded by a sister or who were firstborns. We did not find any evidence of repeatable differences among mothers in the testosterone levels of their calves, but there was significant interannual variation across the 17-year study period. We also found early-life testosterone to be associated with calf survival, but only among individuals already at higher mortality risk: male calves born to first-time mothers were increasingly less likely to survive with higher neonatal testosterone concentrations. These results support the suggestion that a neonate's circulating testosterone concentrations can be linked to both individual and maternal characteristics and that interindividual variation in these levels can have implications for juvenile fitness within a wild mammal population.