Testosterone increases activity but not daily energy expenditure in captive male dark-eyed juncos, Junco hyemalis.

Plasma testosterone (T) levels in male dark-eyed juncos peak early in the breeding season, then decline. If T enhances opportunities for reproductive success, as suggested by previous experiments, why does elevated T not occur naturally? To address this question, we prolonged the early peak level throughout the breeding season and explored potential energetic costs of maintaining elevated T. We measured daily energy expenditure (DEE) of treated males (T-males) and controls (C-males) using doubly labelled water (DLW). We also conducted behaviour scans of T- and C-males housed in outdoor aviaries. DEE was not higher in T-males than in C-males. However, T-males did increase locomotion and foraging and decrease rest and self-maintenance. These results suggest that elevated T may increase the contribution of some components of DEE and lower the contribution of others. Furthermore, the T-induced decrease in allocation of time to rest and maintenance may represent a long-term cost that has led to selection against the maintenance of elevated T beyond the natural early spring peak. Copyright 2000 The Association for the Study of Animal Behaviour.