Rodents pumped up on steroids run wild across the Canadian tundra each summer. Now, new research has uncovered how they do it without succumbing to nasty side effects like "roid rage."

In the summer, Arctic ground squirrels of both sexes have levels of testosterone and other "male" steroid hormones or androgens in their blood that are 10 to 200 times that of other ground squirrels.

University of Toronto Scarborough biologist Rudy Boonstra says that's because the groundhog-like rodents need to bulk up with muscle in order to survive winter hibernation in their deep-frozen Arctic burrows. In fact, the animals can increase their muscle mass by 30 per cent in the weeks before their eight-month annual hibernation. That's with the help of androgens produced in huge quantities by the adrenal glands on top of their kidneys.

But a nagging question remained.

"How do they get the benefits but not pay the costs? There are enormous costs to humans taking synthetic steroids," said Boonstra, citing problems such as a depressed immune system and mood disturbances such as "roid rage" – angry, aggressive and sometimes violent outbursts. Other side effects in humans can include high blood pressure, heart problems, liver problems, shrunken testicles and infertility.

To figure out how Arctic ground squirrels sidestepped those problems, Boonstra turned to his colleagues Kaigo Mo and Douglas Ashley Monks in the Department of Psychology and Cell Systems Biology at the University of Toronto Mississauga.

Mo and Monks tested samples of muscle and lymph nodes (part of the immune system) from 13 Arctic ground squirrels collected by Boonstra at a ranch near Whitehorse, Yukon, at the time of year when they would be bulking up for hibernation. The samples were compared to those from Columbian ground squirrels captured near Barrier Lake, Alta.

Using a technique called Western blotting, the researchers found that Arctic ground squirrels had four times as many androgen receptors in their muscles as Columbian ground squirrels. The receptors allow the muscle to detect and respond to androgens by increasing their growth.

Neither kind of ground squirrel had many androgen receptors in their lymph nodes. That meant that their lymph nodes couldn't really "see" the androgens circulating in their blood.

In contrast, humans see and respond to androgens throughout their bodies, resulting in all kinds of negative side effects when they take anabolic steroids.

Rudy Boonstra and Tim Karels with Arctic ground squirrels

University of Toronto Scarborough biologist Rudy Boonstra, is seen here with former Ph.D. student Tim Karels, with a bunch of hungry Arctic ground squirrels. Boonstra discovered the squirrels' unusually high testosterone levels by accident while measuring their hormone levels to test how stressed they were. (Tim Karels/University of Toronto Scarborough)

In other words, Boonstra said, the Arctic ground squirrels have "jury-rigged their genetic system in a certain way" so that when they are getting ready to hibernate, their muscle sees and responds to the steroids, but the rest of their body generally does not.

The team published their results in the journal Biology Letters this week.

Frozen burrows

Boonstra said his research suggests that the reason Arctic ground squirrels evolved this unique ability to bulk up on steroids is they are the only mammal known to hibernate in frozen ground – other hibernating mammals keep their winter burrows just below the frost line, where the temperature hovers close to zero all winter.

Arctic ground squirrels, which range from Hudson Bay to Alaska, live on the tundra, where it's impossible to dig through the permafrost to a layer of soil that never freezes.

"It's like concrete," Boonstra said.

The squirrels' burrows can get as cold as -23 C while they're hibernating, forcing them to burn huge amounts of energy to keep their body temperature above freezing. Under those conditions, fat alone can't generate enough energy in the form of glucose to keep their brain and heart alive.

"And so what they do is they burn muscle."

While Arctic ground squirrels have made this possible by evolving a neat way to tolerate rampant steroid use, Boonstra doesn't think it's a trick that humans could or should learn.

"I think the salient point from the perspective of humans is you do not have the [right] genetic machinery… so don't take the bloody stuff," he said. "You're not an Arctic ground squirrel."