Data from: Managing anabolic steroids in pre-hibernating Arctic ground squirrels: obtaining their benefits and avoiding their costs

Androgens have benefits, such as promoting muscle growth, but also significant costs, including suppression of immune function. In many species, these tradeoffs in androgen action are reflected in regulated androgen production, which is typically highest only in reproductive males. However, all non-reproductive arctic ground squirrels irrespective of age and sex, have high levels of androgens prior to hibernating at sub-zero temperatures. Androgens appear to be required to make muscle in summer, that, together with lipid, is then catabolized during overwinter. In contrast, most hibernating mammals catabolize only lipid. We tested the hypothesis that androgen action is selectively enhanced in arctic ground squirrel muscle because of an up-regulation of androgen receptors (AR). Using Western blot analysis, we found that they have AR in skeletal muscle > 4 times that of Columbian ground squirrels, a related southern species that overwinters at ~0oC and has low pre-hibernation androgen levels. In contrast, AR in lymph nodes was equivalent in both species. Brain AR was also modestly but significantly increased in arctic relative to Columbian ground squirrels. These results are consistent with the hypothesis that tissue-specific AR regulation prior to hibernation provides a mechanism whereby arctic ground squirrels obtain the life history benefits and mitigate the costs associated with high androgen production.