Combined Aromatase Inhibitor and Antiandrogen Treatment Decreases Territorial Aggression in a Wild Songbird during the Nonbreeding Season

Abstract

Male song sparrows (Melospiza melodia morphna) defend territories throughout the year in western Washington State. In the nonbreeding season (autumn and winter), aggression and song are robustly expressed but plasma testosterone (T) levels are basal. Also, castration does not decrease nonbreeding territoriality. In this field experiment, we asked whether nonbreeding aggression is independent of T. T can act via androgen receptors or T can be aromatized to 17β-estradiol (E2) and act via estrogen receptors. We treated free-living nonbreeding birds with an aromatase inhibitor (ATD) and an androgen receptor antagonist (flutamide) in combination. We then challenged subjects with a live decoy and playback of tape-recorded songs. ATD+flutamide treatment decreased several aggressive behaviors. However, ATD+flutamide treatment did not affect body condition, suggesting that subjects were healthy and that foraging behavior was not reduced. As expected, ATD+flutamide treatment increased plasma T, probably by blocking negative feedback on luteinizing hormone. Surprisingly, ATD+flutamide treatment increased plasma E2. Most other studies using aromatase inhibitors have not measured plasma E2. However, it is possible that ATD+flutamide treatment decreased local E2 levels in the brain but not in plasma. Finally, ATD+flutamide treatment increased plasma corticosterone, perhaps in response to increased plasma T or E2. To our knowledge, these are the first data to suggest that nonbreeding territoriality is regulated by endogenous steroid hormones. Nongonadal production of sex steroids may support aggression in the nonbreeding season.