Masculinization and Defeminization in Altricial and Precocial Mammals: Comparative Aspects of Steroid Hormone Action

Altricial and precocial species follow different developmental trajectories possibly reflecting different reproductive strategies. This chapter describes evidence that this distinction may have heuristic value in understanding the nature of steroidal influences on masculinization and defeminization. Across both types of mammals, defeminization was found to utilize estrogenic metabolites of androgens. However, the evidence of this requirement was stronger in altricial than precocial species. Unambiguous evidence of defeminization by nonaromatizable androgens was found only in the precocial rhesus monkey. The role of aromatization in masculinization was less clear, with little evidence in any species that mounting potential differentiated under either androgenic or estrogenic influence. When all aspects of male sexual and social behavior were considered, altricial species relied more on aromatization for masculinization than did precocial species. No evidence was found in human males that the actions of estrogenic compounds were necessary for normal male sexual differentiation. These apparent differences between altricial and precocial species in the hormonal actions producing masculinization and defeminization might be an artifact of which species have become favored laboratory subjects. Alternatively, they may reflect a deeper organizing principle resulting from the different life strategies of altricial and precocial species. Resolving this issue awaits a broader comparative investigation of sexual differentiation than is currently available today. Phoenix et al. (1959) closed their landmark paper proposing that steroid hormones organized the sexual characteristics of the developing nervous system with the following: “We are assuming that testosterone or some metabolite acts on these central nervous tissues in which patterns of sexual behavior are organized.” This caution concerning whether testosterone (T) or its metabolites were the active agents in organizing the nervous system has assumed a central position in the 40 years of research following the paper’s publication. While the Kansas group might have been cautious about which steroid was responsible for CNS