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

Abstract

We measured plasma levels of testosterone, corticosterone, and cortisol in free-living male yellow-pine chipmunks to demonstrate the patterns of seasonal 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 techniques 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-capture testosterone levels were high during mating and dropped to a persistently low level thereafter. Conversely, both cortisol and corticosterone were at their seasonal low during mating and climbed to peak levels in June following reproduction. Plasma glucocorticosteroid levels increased during the 30 min after capture and handling at all times of the active season, and these elevated levels were similar to the levels obtained by standard trapping. Testosterone levels during the mating period also increased in response to capture and handling. The contrasting patterns of seasonal variation in glucocorticosteroid and testosterone levels and the changes induced by capture and handling suggest that when testosterone concentration is high, adrenocortical activity is suppressed.