Exogenous testosterone and the adrenocortical response in dark-eyed juncos

Abstract

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 ongoing research into the effects of the steroid hormone testosterone (T) in Dark-eyed Juncos (Junco hyemalis), we measured the corticosterone (B, asteroid hormone secreted by the adrenal 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 individual 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 difference 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 statistically 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.