When Arctic ground squirrels are getting ready to hibernate they not only add fat, they also build up muscle. They do it without suffering the harmful effects that high levels of testosterone and other anabolic steroids usually cause.
Arctic ground squirrels increase their anabolic steroid levels and keep them high not just during the spring mating season, but during the summer and fall. These high levels of androgens help both males and females to increase lean body mass (i.e., muscle) by about 25 percent in the months leading up to winter hibernation – mass which is then consumed as they hibernate. To avoid the damaging effects of these high levels, they seem to suppress androgen receptors in all tissues except muscle.
"We propose that AGS have solved the problem of hibernating in sub-zero temperatures by ramping up the adrenal production of key androgens. We hypothesize that at the tissue level it must ultimately be T [testosterone], as only T stimulates muscle growth (Mooradian, Morley & Korenman 1987) and muscle protein is required for gluconeogenesis to supplement that coming from the glycerol when lipid is catabolized." (Boonstra et al. 2011:1358)
It is not yet known how the Arctic ground squirrel mitigates the cost of high androgen levels.