Growth Trajectory Evident at Birth Affects Age of First Delivery in Female Monkeys

Growth trajectories established early in life have proven to be important determinants of metabolism, health in adulthood, and ultimate mortality. The age of sexual maturation may also be set early in development, perhaps etched in utero. The following study used growth curve modeling to investigate the degree to which birth weight and weight gain before sexual maturation constrained the timing of reproduction in 147 female rhesus monkeys living under standardized social and nutritional conditions. Although size at birth by itself did not determine age of reproductive maturation, it was strongly associated with the subsequent developmental growth trajectory, which in turn predicted age at first offspring. In contrast to human studies indicating that small birth size is followed by a postnatal "catch-up" growth phase that accelerates menarche, growth trajectories remained distinctive in small and large infant monkeys. Thus, it was the sustained and stable disparity in size already evident at birth and amplified through development that accounted for variation in the age of adult sexual maturity.