Combined aromatase inhibitor and antiandrogen treatment decreases territorial aggression in a wild songbird during the nonbreeding season

Male song sparrows (Melosyiza melodia morphna) defend territories throughou t the year in western Washington State. In the nonbreeding season (autumn a nd winter), aggression and song are robustly expressed but plasma testoster one (T) levels are basal. Also, castration does not decrease nonbreeding te rritoriality. In this field experiment, we asked whether nonbreeding aggres sion is independent of T. T can act via androgen receptors or T can be arom atized to 17 beta-estradiol (E-2) and act via estrogen receptors. We treate d free-living nonbreeding birds with an aromatase inhibitor (ATD) and an an drogen receptor antagonist (flutamide) in combination. We then challenged s ubjects with a live decoy and playback of tape-recorded songs. ATD + flutam ide treatment decreased several aggressive behaviors. However, ATD + flutam ide treatment did not affect body condition, suggesting that subjects were healthy and that foraging behavior was not reduced. As expected, ATD + flut amide treatment increased plasma T, probably by blocking negative feedback on luteinizing hormone. Surprisingly, ATD + flutamide treatment increased p lasma E-2. Most other studies using aromatase inhibitors have not measured plasma E-2. However, it is possible that ATD + flutamide treatment decrease d local E-2 levels in the brain but not in plasma. Finally, ATD + flutamide treatment increased plasma corticosterone, perhaps in response to increase d plasma T or E-2. To our knowledge, these are the first data to suggest th at nonbreeding territoriality is regulated by endogenous steroid hormones. Nongonadal production of sex steroids may support aggression in the nonbree ding season. (C) 1999 Academic Press.