GILL, S.A.*; ALFSON, E.D.; HAU, M.; Princeton University; Princeton University; Princeton University: The myth of the passive sex: hormonal control of female aggression in a year-round territorial bird

The hormonal control of territorial aggression in female vertebrates is still unclear. We therefore studied buff-breasted wrens (Thryothorus leucotis), a resident of the neotropics, in which females aggressively defend the territory year-round. We asked whether (1) females distinguish between the sex of the intruders in their aggressive responses, (2) female aggression differs between pre-breeding and breeding periods, and (3) steroid hormones were correlated in either period with female aggression. We conducted simulated territorial intrusions (STIs) with live female, male or paired decoys combined with playback of song to free-living pairs of wrens in central Panama. Females displayed robust aggressive responses to all stimuli, but responded more intensely to female decoys than either male or paired decoys. Response intensity did not differ between pre-breeding and breeding periods. Following STIs, most females had detectable plasma concentrations of testosterone (T), 5 &alpha dihydrotestosterone (DHT), and dehydroepiandrosterone (DHEA), whereas estradiol was undetectable in most females. T concentrations following female STIs were elevated in females tested during the pre-breeding season relative to when they were caring for young and relative to control females. T and DHT concentrations were correlated, but neither DHT nor P varied predictably between seasons or stimuli. DHEA was present in the highest concentration of any steroid hormone, and may be an important precursor of sex steroids for regulation of year-round territoriality in females. Our results suggest that both T and DHEA may play a role in facilitating aggression in female buff-breasted wrens.