Research into the lives of animals in their natural environments has revealed a rich tapestry of complex social relationships and previously unsuspected social and mating systems. The evolution of this behavior is increasingly well understood. At the same time, laboratory scientists have made significant discoveries about how steroid and peptide hormones act on the nervous system to shape behavior. An exciting and rapidly progressing hybrid zone has developed in which these two fields are integrated, providing a fuller understanding of social behavior and the adaptive functions of...