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EXOGENOUS TESTOSTERONE AND THE ADRENOCORTICAL RESPONSE IN DARK-EYED JUNCOS

Hormonal manipulations with implants allow examination of the costs and benefits of behaviors and physiologic states mediated by a given hormone. As a part of on- going research into the effects of the steroid hormone testosterone (T) in Dark-eyed Juncos (Junco hyemalis), we measured the corticosterone (B, a steroid hormone secreted by the ad- renal in response to stress ) response to the stress of capture and handling in males treated with T (T-males) and in control males (C-males). Although B may be essential for energy mobilization, chronic or repeated exposure to elevated levels of B can have many negative effects. Because T mediates many behaviors that may increase the likelihood that an indi- vidual will encounter stressors, we predicted that plasma B would rise more rapidly in T- males than in controls. In the first few minutes post-capture, the increase in B levels was significantly higher in T-males than in controls. B levels in samples collected 10, 30, and 60 min post-capture were consistently higher in T-males than in C-males; however, the differ- ence was not statistically significant. Because previous work has shown that T-males reduce their parental contribution, we compared females that were mated to T-males and C-males (hereafter T- and C-females). B levels of T-females increased sharply in the first few minutes post-capture, whereas in C-females they did not; however, the responses were not statisti- cally different. Males had higher initial levels and a greater B-response to stress than females when data were compared irrespective of treatment. Our results suggest that the behaviors or physiological changes induced by T are potentially costly and that such costs may in part be incurred through elevated B. Received 25 November 1997, accepted 5 May 1998. IN MOST NORTH-TEMPERATE BIRDS, circulating levels of testosterone (T) in males are elevated during the sexual phase of the breeding season