Sex steroid hormones play key roles in the development of gender differences in brain and behavior, a process known as sexual differentiation. Males are usually more aggressive than females, but individual variation in aggression is extensive. There are numerous studies on inbred laboratory lines of vertebrate animals, but some types of inferences are limited because of the extreme genetic homogeneity of such animals. The extent and nature of naturally occurring variation in the hormonal mechanisms mediating variation in aggression that occurs among closely related species, which may be more characteristic of animals in out-bred populations such as humans, are not well understood. The long-term goal of the PI's research program is to investigate how closely related species vary in the neuroendocrine mechanisms contributing to variation in the degree to which the sexes differ in aggression. In one of the study species, males exhibit high levels of territorial aggression but females do not. In the second, both sexes have high levels of territorial aggression. And in the third species, both sexes have low levels of territorial aggression. The proposed research will examine hypotheses related to androgen receptors and androgen-mediation of aggression. Distribution and abundance of androgen receptors in regions of the brain involved in mediating aggression in most vertebrates will be compared, using immunohistochemical techniques with brain tissues of adult males and females of each study species, collected from periods of naturally high and naturally low aggression. The effects of gonadal steroids on the distribution of brain androgen receptors and cellular localization of the receptor will also be determined. Finally, the androgen dependence of aggression in adults of the "feminized" species (with low male aggression) will be experimentally determined. This program will also contribute to a detailed neuroendocrine study of female aggression, an area that has received relatively little attention, except in the context of maternal aggression, compared to aggression in males.