The Challenge Hypothesis argued that testosterone (T) plays a major role in establishing reproductive strategies and, since its inception, numerous empirical studies have supported this hypothesis. Testosterone (T) regulates male reproductive behavior by acting through androgen receptors (AR), or via estrogen receptors (ER) when T is aromatized to estrogen. To examine the relationship between steroid receptors and mating strategy, we used prairie voles as a model system. Prairie vole exhibit distinct population differences, with voles from Illinois (IL) being highly social, forming pair bonds and bi-parental care, and classified as monogamous, while Kansas (KN) voles are less social and display polygynous characteristics. Cushing et al. (SFN abst 957.13, 2001) showed an inverse relationship between ERα and social behavior, with KN males displaying significantly more ERα in the bed nucleus of the stria terminalis (BST) and medial amygdala (mAMY), limbic areas associated with reproduction and social behavior...