| Summary: | Field observations have defined differences in hibernation phenology between sexes. Reproductive male arctic ground squirrels ( Urocitellus parryii , AGS) come to euthermy, remain in their burrows for ~3wks and then emerge above ground before the emergence of females. In late summer, females enter their burrows and begin hibernating while males remain active on the surface for at least a month. What factors control these differences between the sexes is still unknown. It has been shown that exogenous testosterone (T) inhibits torpor which implies a role for endogenous androgens in hibernation. To establish this relationship, we collected plasma samples from free‐living male AGS across their active season. The samples were analyzed for T and dehydroepiandrosterone (DHEA) concentrations which were then correlated with hibernation phenology, determined from body temperature (T b ) records. The assays determined that there is a single peak in plasma T during the spring mating and a separate, single peak in plasma DHEA in the fall coinciding with the second observed aggressive period. We also manipulated endogenous androgen levels by castration and cast+flutamide (androgen receptor blocker). Adult and juvenile males were castrated and implanted with T b loggers in late summer. A separate group of males underwent the castration and implantation surgery, but additionally were given a silastic capsule containing flutamide in the late summer. By comparing hibernation phenologies between intact and manipulated males, we were able to establish a clear role for testosterone in the timing of spring euthermy, but could not account for the differences in hibernation entry between the sexes.
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