58 - Sexual Differentiation of Behavior in Nonhuman Primates

Abstract

Sexually dimorphic behavior in nonhuman primates reflects behavioral predispositions organized by prenatal androgens. The rhesus monkey has been the primary primate model for understanding the hormonal organization of sexually dimorphic behavior. Historically, high prenatal androgen doses were administered to females to investigate the masculinizing and defeminizing effects of androgens. Such treatments masculinized juvenile and adult copulatory behavior and defeminized female-typical sexual initiation to adult estrogen treatment. Testosterone, and the nonaromatizable androgen 5α-dihydrotestosterone, produced similar effects suggesting that estrogenic metabolites of androgens are not critical for masculinization and defeminization in rhesus monkeys. Long duration androgen treatments masculinized both behavior and genitalia suggesting that socializing responses to the females' male-like appearance may have produced the behavioral changes. Treatments limited to 35 days, early or late in gestation, differentially affected behavioral and genital masculinization demonstrating direct organizing actions of prenatal androgens. Recent studies exposed fetal females to smaller doses of androgens or blocked endogenous androgen action using the anti-androgen flutamide. Low-dose androgen treatment significantly masculinized infant vocalizations, but produced no behavioral defeminization. Females receiving flutamide late in gestation showed masculinized infant vocalizations and defeminized interest in infants. Both late androgen and flutamide treatment hypermasculinized some male juvenile behaviors. Early flutamide treatment blocked full male genital masculinization, but did not alter their juvenile or adult behavior. The role of neuroendocrine feedback mechanisms in the effects of flutamide is discussed. Sexually differentiated behavior ultimately reflects both hormonally organized behavioral predispositions and the social experience that converts these predispositions into behavior.