Seasonal changes in plasma testosterone and glucocorticosteroids in free-living male yellow-pine chipmunks and the response to capture and handling

We measured plasma levels of testosterone, corticosterone, and cortisol in free-living male yellow-pine chipmunks to demonstrate the patterns of seaso nal variation and to assess the effects of capture and handling on hormone levels. We achieved the latter by modifying our standard trapping technique (blood samples collected within 1-3 h of capture) to obtain blood samples that allowed measurement of hormone levels within 3 min of capture (basal) and again 30 min later. By alternating the modified and standard trapping t echniques over 7 months of the active season we demonstrated that seasonal patterns of variation in steroid hormone levels can be accurately described with the simpler, standard trapping technique. Basal and 30-min post-captu re testosterone levels were high during mating and dropped to a persistentl y low level thereafter. Conversely, both cortisol and corticosterone were a t their seasonal low during mating and climbed to peak levels in June follo wing reproduction. Plasma glucocorticosteroid levels increased during the 3 0 min after capture and handling at all times of the active season, and the se elevated levels were similar to the levels obtained by Standard trapping . Testosterone levels during the mating period also increased in response t o capture and handling. The contrasting patterns of seasonal variation in g lucocorticosteroid and testosterone levels and the changes induced by captu re and handling suggest that when testosterone concentration is high, adren ocortical activity is suppressed.