Masculinization of female mammals: lessons from nature.

Although varying degrees of genital masculinization are a reasonably common phenomenon in the world of female mammals, the majority of such variation has not been investigated. In this chapter we have described research on the "masculinized" genitalia of moles and hyenas. Such research raises intriguing possibilities regarding the coordinated role that androgens, estrogens and peptide hormones (e.g., relaxin) might play, at different stages of sexual differentiation and development, in preparing genital tissues for their functional roles in reproduction. Such studies also suggest that non-androgenic mechanisms need to be considered. Arnold (1996) and Carruth et al. (2002) have recently presented the argument for broadening our view of sexual differentiation of brain and behavior, emphasizing direct genetic effects. A similar view has been presented for the Tammar wallaby, where formation of a scrotum, or a pouch, is a direct consequence of the presence/absence of two X chromosomes (Pask and Renfree, 2001). Although our research on moles and hyenas has not yet yielded such definitive results, the research reviewed in this chapter calls attention to processes that could well operate in other mammals, including humans.