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In the weeks before hibernation, Arctic squirrels are able to increase their muscle mass by 30 percent thanks to a massive spike in their anabolic steroid levels.

Most hibernators only add layers of fat in preparation for winter but Arctic ground squirrels hibernate in frozen ground at subzero temperatures, so they need to burn muscle mass as well as fat to get through the harsh winter and supply key tissues in the brain and heart with the glucose they need.

But unlike humans, that muscle-building spike in steroids doesn’t come with aggressive behavior—known as ‘roid rage—or compromised immune systems. In a new study published in Biology Letters, researchers explain why.

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The answer, they say, lies in the squirrels’ androgen receptors.

The scientists compared Arctic squirrels to Columbian ground squirrels found in Alberta and discovered the Arctic squirrels were able to bulk up safely because they had four times as many androgen receptors in their muscles than the Columbian ground squirrels.

“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 Rudy Boonstra, a professor at the University of Toronto Scarborough.

“The Arctic ground squirrel has evolved this trait to make muscle to cope with hibernating in a deep freeze.”

This ensures that, unlike humans, Arctic squirrels can receive the benefits of high anabolic steroid levels without the costs.

The Natural Sciences and Engineering Research Council of Canada supported the research.

Source: University of Toronto