FIGURE 1 ∣.

Sexual differentiation into masculinized and feminized physical phenotypes. (A) This classical model characterizes the differential development of the primordial gonad into testes or ovaries, depending on the presence of the SRY gene found on the Y chromosome. As testes develop, high levels of androgens, including testosterone and dihydrotestosterone (DHT), act on highly sensitive androgen receptors to developmasculinized internal (Wolffian duct) and external genitalia. Anti-Müllerian hormone is also produced, causing the Müllerian ducts to degenerate. In the absence of the SRY gene (in an XX genotype), the ovaries produce very low amounts of androgens, causing the Wolffian ducts to degenerate and allowing for feminization of external genitalia. In typical females anti-Müllerian hormone is also not produced, allowing the Müllerian ducts to develop into feminized internal genitalia. (B) In a social monogamy behavioral phenotype, we hypothesize that changes in the response to androgens produced by the testes during prenatal development may lead to changes in the masculinization of the external genitalia in XY genotype individuals. While testosterone production remains high to allow for the masculinization of the internal genitalia, decreases either in the production of DHT or in the sensitivity of the androgen receptor to either testosterone or DHT decrease the extent of masculinization of the external genitalia, a common physical feature of males of socially monogamous species.