Squirrel 2
Image: Colleen/Flickr

When the earth entered an ice age millions of years ago, Arctic ground squirrels had a problem. As the planet cooled, so too did their northern habitat. Permafrost set in and temperatures in their dens began plummeting to below -20 degrees Celsius during the winter months. In this chilly environment, burning fat stores like other hibernators just wasn’t enough for the squirrels to survive.

The solution? Hormones. In the summer months prior to hibernation, high levels of androgen, a type of anabolic steroid, allow both male and female Arctic ground squirrels to pump up their muscle mass, says Professor Rudy Boonstra, a researcher in the Department of Biological Sciences at the University of Toronto.

“Essentially, these animals are crawling into a freezer to hibernate, so they [had to evolve a way] to kick up their metabolism significantly to survive those cold temperatures,” says Boonstra. The extra muscle mass allows Arctic ground squirrels to burn both muscle and fat during hibernation.

Boonstra explains that the fat provides fuel mainly for staying warm, whereas the muscle provides the protein to produce glucose, which is required by the brain and heart when the body’s engine (metabolism) is working hard and fast at extremely cold temperatures.

The problem is that running around in a state of steroid superabundance comes with a whole set of problems, like possible heart strain, a compromised immune system and that infamous steroid-induced side effect – extreme aggressiveness (a.k.a 'roid rage). 

At least that's the case with most animals. In a 2011 study, Boonstra noticed none of these side effects in Arctic ground squirrels ... but what allowed the rodents to sidestep the perils of all this steroid exposure remained a mystery. 

In his latest work, he compared Arctic ground squirrels with their cousin Columbian ground squirrels – a species in which only males produce anabolic steroids (primarily testosterone) during the breeding season, when a small bravado boost is needed. For the rest of the active season when these ground squirrels are not hibernating, steroids are normally very low. Many other hibernating mammals follow this same pattern.

Squirrel 1
Both male and female Arctic ground squirrels undergo a steroid spike in the months prior to hibernation. Image:NPS Photo, Tim Rains/Flickr

But Arctic ground squirrels are an exception. During the summer prior to hibernation, anabolic steroid levels spike in both males and females.

What Boonstra found is that Arctic ground squirrels have evolved a greater number of androgen receptors in their muscle cells (four times as many as their Columbian cousins!), a nifty evolutionary trick that directs muscle-building steroids where they are needed and away from other cells (like the cells of the immune system), where high levels would be detrimental.

Thanks to this 'receptor restructuring', Arctic ground squirrels seem to reap the benefits – but not pay the costs – of high anabolic androgen levels in a way that we humans (and other mammals) just cannot match.  

So, what is Boonstra’s advice to humans thinking about using steroids to ‘get pumped’? It’s simple: “Don’t take the bloody stuff. You are not an Arctic ground squirrel!”

Top header image: DenaliNPS/Alex Vanderstuyf