Buck, C.L.*; Barnes, B.M.: Influence of Staged Encounters on Testosterone and Corticosterone in Free-living Arctic Ground Squirrels

In a field study on the North Slope of Alaska we determined levels of plasma testosterone and corticosterone for male arctic ground squirrels after emergence from hibernation in spring, throughout the mating season, and prior to their immergence into hibernation in late summer. In mid April reproductively active males emerged with testes descended into the scrotum and with intermediate levels of plasma testosterone. Testosterone levels remained constant at 2.9 � 0.29 ng/ml for about 1 week before significantly increasing to the active season maximum of 4.6 � 0.42 ng/ml. A lesser peak in late summer of 2.7 � 0.17 ng/ml not associated with testicular maturation was observed before levels decreased to the annual minimum of 1.7 � 0.57 ng/ml prior to entrance to hibernation. Corticosterone levels did not vary significantly across season and averaged 4.3 � 0.10 ng/ml. We tested the responsiveness of plasma testosterone and corticosterone to challenges by conspecific males by staging aggressive encounters between males and comparing plasma levels of these animals to those of unmanipulated animals. The effect of staged encounters on testosterone level depended on time of year. Testosterone levels in spring were significantly up-modulated after a staged encounter and were positively correlated to the relative intensity of the encounter, whereas staged encounters in late summer had no significant effect on testosterone level. Corticosterone levels in both spring and late summer were significantly increased by staged encounters and were positively correlated to the relative intensity of the encounter. These results suggest that circulating levels of testosterone and corticosterone in male arctic ground squirrels are the result of a combination of seasonal and social factors.