How can bears hibernate for 6 months and still have minimum loss in muscle strength?

Spring is finally here and nature is coming back to life after wintertime with snow and little sunlight. Although humans are active and live normal lives during wintertime, some animals sleep the winter away – in hibernation. The hibernation strategy is based on reducing core temperature and go into “winter sleep” in order to reduce energy consumption (metabolism) when food supply in nature is sparse. One animal that uses the hibernation strategy is the bear – in fall it builds up body fat stores so it can hibernate for approximately 6 months without eating and drinking.

In a study where scientists captured bears in Alaska and transferred them to artificial dens, it was shown that during hibernation heart rate dropped from 55 to 9 beats per minute and the metabolism was reduced by 75 percent with only a 4°C reduction of core temperature (from 37-33°C).  This is in great contrast to hibernating rodents that reduce their core temperature to only a few degrees above 0°C. The bears remained mainly inactive during hibernation but got up twice a day to rearrange bedding. So for those who have had a tempting thought to visit a bear during hibernation – you never know when it is rearranging the bedding…

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Another challenge of being inactive over long periods of time is loss of muscle mass (atrophy) and strength. In humans this is seen in for example confined bed rest, weightlessness (in space) or limb immobilization. Hibernating bears do not show the same muscle deterioration as in humans – a study showed that after 130 days of hibernation bears lost only 23 percent of their muscle strength. This is in big contrast to humans who are predicted to become 90 percent weaker after the same time period. Bed rest and simulated space flight studies show a daily loss of strength of 0.7 percent.

Why bears barely have muscle atrophy after prolonged inactivity is not completely understood, but muscle samples (biopsies) from bears show no loss of skeletal muscle cell number or size, in contrast to what is seen in great numbers in humans experiencing muscle atrophy. It is speculated that bears avoid muscle protein degradation and hence strength by synthesizing amino acids and proteins from storages of urea nitrogen, (since bears do not urinate during hibernation) or by shivering and muscle contractions.

Svein Erik Gaustad. Foto: Berre AS

Svein Erik Gaustad. Foto: Berre AS

Read also: What do you know about hearts?

A more comprehensive understanding of the mechanisms preventing muscle atrophy in bears may provide new insight in treatments of muscle disorders, muscle loss from long hospital-bed confinements and antigravity induced muscle loss on long-distance space missions.

Svein Erik Gaustad, postdoktor at CERG

References:

1.     Hibernation in black bears: Independence of Metabolic  Suppression from Body Temperature. Tøien, Ø, et. al 2011. Science.

2.     Muscle strength in overwintering bears. Harlow, J. et. al 2001. Nature.

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About CERG

The Cardiac Exercise Research Group (CERG) at the Norwegian University of Science and Technology (NTNU) seeks to identify the key mechanisms underlying the beneficial effects of physical on cardiac health in the context of disease prevention and treatment. Named the K.G. Jebsen Center for Exercise in Medicine under Professor Ulrik Wisløff's leadership in 2011, CERG uses both top-down and bottom-up approaches to combat lifestyle-related disease.

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