Female Age of First Reproduction at Cayo Santiago: Heritability and Shared Environments

Abstract

Sexual maturation is a major transition in mammalian life histories including relatively long-lived, late-maturing primates. Age of first reproduction (AFR) in female primates is widely documented to vary among population members and to correlate with population density and social dominance. Research with Cayo Santiago macaques was among the first to identify these patterns. While explanations for variation in female AFR have centered around priority of access to limited resources and avoidance of stress, less attention has been drawn to potential genetic variation for AFR. Furthermore, the “dual inheritance” of genes and dominance rank in nepotistic female macaques implies these effects may be confounded. Heritability estimates for AFR at Cayo Santiago are small (≈0.15), but significantly greater than zero implicating genetic variation in this life history trait. However, predicted breeding values for AFR are randomly distributed among rank-levels, which points to common environmental effects, rather than inter-matriline genetic differences, as the primary causes of rank-related variation in AFR. In addition, interannual variation in population density, climate, and colony management also result in strong cohort effects on AFR. Maternal identity explained no variance in AFR, although some maternal characteristics do influence AFR, which requires greater clarity in describing the presence or absence of maternal effects on this important life history trait.