Bears are probably the most notorious animals for taking season-long naps every winter. During that time, they experience very limited muscle loss, a phenomenon that has largely intrigued researchers. Now, a new study has found a set of unique factors that are triggered in bears’ blood every winter, preventing overall muscular atrophy.

Hibernation is when animals “sleep” during the winter season. During that time, the animal’s heart rate, body temperature and breathing rate drop to lower levels. Animals do this to survive the winter as the weather is cold and food is scarce. They quite literally shut themselves off for weeks instead of trying to survive the harsh winter.

Bears don’t technically hibernate. They undergo a similar process called torpor. Like hibernation, torpor also involves a lower body temperature, breathing rate, heart rate, and metabolic rate, but unlike hibernation, torpor seems to be an involuntary state rather than a voluntary one.

Bear muscles

In bears, this state can last from five to seven months, a period of time they spend in their dens. While three weeks of inactivity would be enough to cause muscle mass loss in humans, that’s not the case with bears. They can go through this pseudo-hibernation with very limited muscle loss, unharmed physical functions and minimal metabolic dysfunction. So how do they do it?

“The ‘use it or lose it’ phenomenon is a well-accepted physiological principle for the skeletal muscle, which is highly plastic in response to functional demands,” study author Mitsunori Miyazaki said in a statement. “Disuse typically leads to skeletal muscle loss and metabolic dysfunction in many animal species, including humans.”

Miyazaki worked alongside a team of researchers from Hokkaido University, specifically studying Japanese black bears (Ursus thibetanus japonicus). They found muscle gain in cultured human skeletal muscle cells infused with serum from hibernating bears, which confirms that a set of unique factors are activated in the animal’s blood.

Muscle mass is usually measured by the balance between the synthesis and degradation of proteins. In the study, the researchers found protein growth in cultured muscle cells infused with the bear’s serum. However, the increase only occurred when using serum collected during the bear’s hibernation period, and not during their active summer season.

Gain like a bear

The researchers believe this reduced capacity of the muscles’ destruction mechanism happens because of the suppressed expression MuRF1 (Muscle RING-finger protein-1), which is what triggers the shredding of unused muscles. They also observed higher levels of the growth factor hormone IGF-1 in the hibernating bear serum, something seen in previous studies.

“We have indicated that ‘some factor’ present in hibernating bear serum may regulate protein metabolism in cultured human skeletal muscle cells and contribute to the maintenance of muscle mass. However, the identification of this ‘factor’ has not yet been achieved,” Miyazaki said in a statement.

The researchers believe that this is just the start, as they plan to keep exploring more of hibernation-related secrets. By being able to identify the specific factors and the mechanisms behind muscles that don’t weaken without use, it could be possible to develop more effective rehabilitation strategies in humans, they conclude.

The study was published in the journal Plos ONE.