SCHROCK, SE*; YOUNG, RC; KETTERSON, ED; Indiana Univ., Bloomington; Indiana Univ., Bloomington; Indiana Univ., Bloomington: Is the aggressive response of captive female dark-eyed juncos to repeated intrusions mediated by testosterone?

Hormonal mediation of intrasexual aggression has received less attention in females than in males. In some bird species, testosterone (T) has been suggested as a mediator of intrasexual aggression, while in other species T does not appear to be crucial for aggression. Previous research with female dark-eyed juncos (Junco hyemalis) found that prolonged exposure to experimentally increased T enhances aggression in females; however, endogenous levels of T did not co-vary with aggressive behavior or dominance status after a single intrusion. In an attempt to resolve this inconsistency, we speculated that prolonged or repeated exposure to challengers might induce a rise in circulating T. Thirty females were allowed 48 hours to become residents in indoor breeding aviaries (2.6m x 2.1m x 2.4m) and to form pair bonds with males. We then exposed them to three repeated, hour-long intrusions by either a conspecific female (experimental trials) or a heterospecific (control trials). Balancing for order of presentation, each female acted as her own control by being exposed to both a conspecific and a heterospecific intruder on mornings separated by a minimum of 14 days. Plasma samples were collected before first and after last intruder exposure in each treatment. Behavior of the resident female, including aggression toward both the intruder female and the resident male, was noted. Analysis of behavioral data by matched pairs revealed significantly more aggressive behavior by females when confronted with a conspecific than when confronted with a heterospecific. T, E2, P, and CORT will be measured using a competitive binding radioimmunoassay following column separation and using a specific antibody. Findings from this study will serve to further our understanding of the hormonal mediation of female aggression.