Arctic ground squirrels pump up for the long winter

Anabolic steroids are known to build muscle but also produce negative side effects like aggressive behavior and compromised immune systems. Arctic ground squirrels have found a way to pump up with steroids without the risks, according to a new study, published today in Biology Letters, by University of Toronto Scarborough Professor Rudy Boonstra together with University of Toronto Mississauga Professor Douglas Ashley Monks and Research Technician Dr. Kaiguo Mo.

"We had previously used genetic engineering to make mice who similarly have too many receptors for anabolic steroids in muscle, and found that their muscles produce and store more energy than those of normal mice,” says Monks, an associate professor in the Department of Psychology. “It is amazing to learn that natural selection arrived at the same solution in arctic ground squirrels long ago."

Most hibernators only add layers of fat in preparation for winter. Arctic ground squirrels hibernate in frozen ground at subzero temperatures, so they also need to burn muscle mass as well as fat to get through the harsh winter. Muscle, but not fat, can provide sufficient glucose needed by key tissues like that of the brain and the heart.

A previous study showed the squirrels’ level of anabolic steroids spiked in late summer, although the animals didn’t exhibit the behaviours typically associated with testosterone and other steroids.  The timing was unusual, since late summer is post-breeding time, when testosterone production is virtually absent for most hibernators preparing for winter and all other seasonally breeding mammals.

The study showed the elevated anabolic steroid levels help the Arctic ground squirrels add muscle mass before winter. “We also found they can do this safely because they have more steroid receptors within their muscle cells, and fewer in other types of cells, like immune cells, that could be negatively affected by the steroids,” says Boonstra.  “The Arctic ground squirrel has evolved this trait to make muscle to cope with hibernating in a deep freeze.”

Thus, unlike humans, these squirrels only have receptors where they need them, to give them the benefits, but not the costs, of high anabolic steroid levels.

This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada.