Seasonal changes in glucocorticoid and testosterone concentrations in free-living arctic ground squirrels from the boreal forest of the Yukon

We examined how glucocorticoid and testosterone concentrations changed from spring to summer by livetrapping free-living populations of arctic ground squirrels (Spermophilus parryii). The primary glucocorticoid was found to b e cortisol, with corticosterone below measurable concentrations in most ind ividuals. Livetrapping elicited a strong stress response in both sexes: bre eding males and females trapped in spring had free cortisol concentrations 4 and 34 times, respectively, those of base-line animals. The maximum corti costeroid-binding capacity (MCBC) was unaffected by trapping and was about 3 times higher in breeding females than in breeding males. Over the active season, MCBC values were lowest in all male classes (juveniles, nonreproduc tive adults, and reproductive adults), being less than half those in all fe male classes; pregnant females had values approximately twice those of juve nile females. However, free cortisol concentrations were similar in all fem ale classes and in juvenile males and about half those in adult males. Live trapping increased testosterone concentrations in males over those found in samples from base-line males, and testosterone concentrations did not affe ct MCBC values. Testosterone concentrations in livetrapped animals differed significantly among male classes, with nonreproductive males maintaining c oncentrations 64% of those in breeding males and 10 times those in juvenile s.