Despite the fact that variation in age of first reproduction is one of the most important life history characteristics determining lifetime reproductive success, little work has been done describing the interindividual variation in this trait. While some work has demonstrated the important of heritable factors affecting age at first reproduction, such as dominance rank and genetic heritability, the majority of the variation comes from as yet unexplained environmental influences. In this dissertation, I examine several environmental factors that may affect variation in the age of first reproduction in adolescent female rhesus macaques from a single birth cohort on Cayo Santiago, Puerto Rico. Chapter 2 demonstrates that dominance rank is important for determining age at first reproduction, and that this may in fact be related to differences in body condition. These body condition differences are likely the cause for variation in latency to begin cycling in the mating season, which was an important factor in determining whether or not a female became pregnant. Chapter 3 examines the changing physiology and social behavior between the juvenile and adolescent period by examining the changes in stress physiology and behavior, as well as grooming behavior. Results demonstrate, as predicted, that stress levels do increase during puberty as do grooming rates with other adult females. Chapter 4 explores the function of a sexual signal present only in adolescent females, the hindquarter swelling. However, the swelling does not seem to fit into either of the two existing models of explanation for this trait, and may in fact be a non-functional side effect of reacting to newly circulating female sex hormones. Finally, Chapter 5 explores the development of a perceptual bias for preferentially looking at darker red male faces . Together, these studies provide a comprehensive look at the changing physiology and behavior of adolescents as they enter adulthood, and begin to explain variation in the important life history trait of age at first reproduction.