Researchers from the University of Toronto discovered that Arctic ground squirrels, both male and female, have levels of androgens, including testosterone and other male hormones, in their bodies that are 10 to 200 times those of other ground squirrels during the summer. The androgens are produced by the adrenal glands on top of their kidneys.
The high levels of hormones allow the Arctic ground squirrels to build muscle to survive their eight-month hibernation in their frozen environment. The squirrels can increase their muscle mass by up to 30 percent.
Taking steroids can have serious side effects for humans, including aggression, high blood pressure, heart and liver problems, and infertility. The Canadian researchers wanted to find out how the Arctic ground squirrels could have such high levels of androgens without any of these side effects.
The researchers tested samples of muscle and lymph nodes, which are part of the immune system, from 13 Arctic ground squirrels when they were building up muscle mass to prepare for hibernation. They compared the samples to those from Columbian ground squirrels.
They discovered that the Arctic ground squirrels had four times as many androgen receptors in their muscles as the Columbian ground squirrels, which allowed their muscles to detect the androgens and respond by increasing in size. However, neither species of squirrels had androgen receptors in their lymph nodes, which meant that the lymph nodes could not “see” the androgens in their blood. The Arctic ground squirrels’ muscles respond to the androgens, but the rest of their bodies do not. Humans, on the other hand, see and respond to androgens throughout the body, which causes side effects if they take anabolic steroids.
The researchers believe the Arctic ground squirrels evolved this ability to gain muscle from steroids without experiencing ill effects because they are the only mammal known to hibernate in frozen ground. They live in the tundra, where the ground is too hard for them to dig through to a layer of soil that does not freeze. Their burrows get so cold that burning fat alone would not provide enough energy to keep them warm enough to stay alive. They must generate more energy by burning muscle.
The researchers’ findings were published in the journal Biology Letters.