PROJECT SUMMARY/ABSTRACT Humans form social attachments and develop friendships throughout our lifetimes, with fundamental consequences for wellbeing. Decades of research have established the importance of the neuropeptide oxytocin (OT) for social bond formation with reproductive partners. In contrast, very little is known about the mechanisms supporting selective affiliation between non-mate peers. Prairie voles exhibit a human-like social structure, forming specific and selective relationships with mates and same-sex peers, unlike other laboratory rodents. The goal of this project is to understand the roles oxytocin plays in shaping the selectivity of peer relationships in prairie voles, particularly its dual functions in promoting acceptance of familiar partners and avoidance of unfamiliar individuals. We will use complementary neuropharmacological and genetic manipulations to test the necessity and sufficiency of oxytocin signaling for partner approach and stranger avoidance and aggression. We make use of natural contrasts between reproductive (mate) and non- reproductive (same-sex peer) relationships in prairie voles, as well as species comparisons between peer relationships in prairie voles and prior studies in meadow voles to understand the specificity and generality of these mechanisms. By manipulating oxytocin across multiple circuits, we have a unique opportunity to determine how OT influences selectivity across brain regions and relationship types. This work has the potential to elucidate the neural basis of peer relationships, and to provide the foundation for understanding how prosocial and antisocial factors of relationships are related and mediated. Our long-term goal is to better understand how friendship-like relationships are established and maintained, and how the mechanisms underlying social relationships in prairie voles can be translated to human relationships.