A research plan is outlined which combines two ordinarily separate lines of investigation, namely, the reciprocal interaction between hormones and behavior and the role of behavior in the evolution and maintenance of species, bringing both to bear on outstanding questions in behavioral and evolutionary biology. This integrative approach to animal behavior, which spans three levelsμphysiology, behavior, and evolutionμshould not only yield answers to specific questions within each level of analysis, but, by taking into account the adaptive value of each in nature, also demonstrate interrelationships between the leve.