Acute and chronic effects of an aromatase inhibitor on territorial aggression in breeding and nonbreeding male song sparrows

Many studies have demonstrated that male aggression is regulated by testost erone. The conversion of testosterone to estradiol by brain aromatase is al so known to regulate male aggression in the breeding season. Male song spar rows (Melospiza melodia morphna) are territorial not only in the breeding s eason, but also in the nonbreeding season, when plasma testosterone and est radiol levels are basal. Castration has no effect on nonbreeding aggression . In contrast, chronic (10 day) aromatase inhibitor (fadrozole) treatment d ecreases nonbreeding aggression, indicating a role for estrogens. Here, we show that acute (1 day) fadrozole treatment decreases nonbreeding territori ality, suggesting relatively rapid estrogen effects. In spring, fadrozole d ecreases brain aromatase activity, but acute and chronic fadrozole treatmen ts do not significantly decrease aggression, although trends for some behav iors approach significance. In gonadally intact birds, fadrozole may be les s effective at reducing aggression in the spring. This might occur because fadrozole causes a large increase in plasma testosterone in intact breeding males. Alternatively, estradiol may be more important for territoriality i n winter than spring. We hypothesize that sex steroids regulate male aggres sion in spring and winter, but the endocrine mechanisms vary seasonally.