The evolution of monogamy and paternal care in mammals creates an enigma. Our long-term goal is to understand the significance of male parental behavior to the evolution of monogamy in mammals. The immediate and specific objective is to test the importance of male care through field investigations of the monogamous, biparental California mouse, Peromyscus californicus. The fitness effects of male care will be determined in the field by removing the father and measuring maternal time budget (via radiotelemetry) and energy expenditure (via doubly-labeled water), and juvenile growth and survivorship. Genealogical relations will be determined by transfer of fluorescent pigments and by DNA fingerprinting. The proposed research will provide new insights into the underlying causes of monogamy and biparental behavior in mammals, and may reveal mechanisms that operate in other mammals, such as humans, that might otherwise go undetected. Peromyscus californicus provides a model system for studies that bear directly on current concerns about human paternal behavior, monogamy and family systems. These data will be of considerable importance to theories and models of parental investment, sexual selection, and the evolution of mating systems.