Female social dominance over males is unusual in mammals, yet characterizes most Malagasy lemurs, which represent almost 30% of all primates. Despite its prevalence in this suborder, both the evolutionary trajectory and proximate mechanism of female dominance remain unclear. Potentially associated with female dominance is a suite of behavioral, physiological and morphological traits in females that implicates ‘masculinization’ via androgen exposure; however, relative to conspecific males, female lemurs curiously show little evidence of raised androgen concentrations. In order to illuminate the proximate mechanisms underlying female dominance in lemurs, I observed mixed‐sex pairs of related Eulemur species, and identified two key study groups ‐‐ one comprised of species expressing female dominance and, the other comprised of species (from a recently evolved clade) showing equal status between the sexes (hereafter ‘egalitarian’). Comparing females from these two groups, to test the hypothesis that female dominance is an expression of an overall masculinization of the female, I 1) characterize the expression of female dominance, aggression, affiliation, and olfactory communication in Eulemur; 2) provide novel information about the hormonal and neuroendocrine correlates associated with the expression of female dominance; 3) investigate the activational role of the sex-steroid hormones in adult female Eulemur using seasonal correlates of hormonal and behavioral change; and 4) examine the specific role of estrogen in the regulation and expression of sex-reversed female behavior in these species. In doing so I highlight significant behavioral and physiological differences between female-dominant and egalitarian Eulemur and show that female dominance is associated with a more masculine behavioral and hormonal profile. I also suggest that these behavioral and hormonal differences may be the result of fundamental differences in the biosynthetic pathway associated with estrogen production. Moreover...