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These arctic squirrels recycle pieces of their own body to survive the winter

Arctic ground squirrels can survive harsh winters with temperatures below freezing by being eight months without food. These hibernators “live at the most extreme limit of existence, they barely plan on death, and we don’t fully understand how this works,” says Sarah Rice, a Fairbanks biochemist at the University of Alaska.

By exploring what is going on inside these squirrels, researchers now have a better idea. Recycled nutrients from muscle breakdown help animals overcome during hibernation, Rice and colleagues reported Dec. 7 in Nature Metabolism.

From autumn to spring, Arctic ground squirrels (Urocitellus parryii) hibernate in times of deep torpor. In a state similar to suspended animation, squirrels breathe only once per minute and their heart beats five times per minute. Every two or three weeks, the squirrels revive a little for about 12 to 24 hours; their body temperatures rise and the animals shudder and sleep, but do not eat, drink or defecate.

To monitor the body chemistry of the animals, “I worked in cold, dark, totally silent cameras, surrounded by hibernating squirrels,” Rice says. Periodically, he carefully removed the blood from a tube inserted into his blood vessels.

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During the awkwardness of the squirrels, Rice and his team observed a chemical sign showing that the skeletal muscle was slowly breaking. That process would release nitrogen-containing compounds, an important element in making the proteins found in muscle. But hibernators, including these squirrels, are known to cling to muscle mass as they hibernate (SN: 17/02/11). Thus, scientists have wondered whether squirrels accumulate new reserves of protein during hibernation and, if so, how.

Monitoring the flow of nitrogen in the body of animals provided clues. The researchers gave the critters a cocktail of chemicals labeled with isotopes, shapes of elements with different masses. This revealed that nitrogen enters amino acids, the building blocks of proteins, which form in the muscles of animals and also in the lungs, kidneys and other areas of the body during those brief periods between awkward episodes.

By recycling nutrients from their muscles, squirrels are maintained and also prevent a toxic consequence of muscle rupture, says team member Kelly Drew, a neurochemist also from Fairbanks at the University of Alaska. During hibernation, nitrogen would otherwise end up in ammonia, which could build up to life-threatening levels. Instead, squirrels are able to incorporate that nitrogen into new molecules, she says.

Other studies have pointed to a role of the microbiome, the microbes that live inside and inside animals, in nitrogen recycling while animals hibernate, says James Staples, an environmental physiologist at Western University in London, Canada, who was not part of the work. Typically, the breakdown of proteins eventually creates urea, a nitrogen-containing chemical that is excreted. Microbes can remove that urea and release its nitrogen into the blood. But in squirrels, muscle "is being broken down and then recycled directly into these amino acids … the gut microbiome may not be as important as we thought it was."

Sandy Martin, a biochemist at the University of Colorado School of Medicine in Aurora who did not participate in the study, could one day have help from hibernators. “Hibernators are so extraordinary” in their abilities to withstand conditions to which humans are extremely sensitive, she says (SN: 12/19/17). For example, animals like these squirrels are much more resistant to the damage they can cause when the organs do not receive the necessary blood flow and oxygen. And taking advantage of hibernation-like approaches could prove advantageous in cases where a slower metabolism would be helpful, from routine surgery to long space trips, she says.

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