Hibernating Bears Stay Fit

Hibernating bears emerge from their winter dens strong and ready to take on spring. New research reveals that they get plenty of protein and exercise during their deep sleeps.

It's hard to imagine a bear deep in its winter den downing a protein shake and working a Stairmaster. But a new study shows that while these Sleeping Beauties' eyes are shut, their bodies are working hard to keep their muscles strong.

American black bears in the Rocky Mountains preserve their muscle tissue while hibernating, the study shows, by recycling nearly all of the nitrogen in their urine into amino acids, the building blocks of proteins important to repairing muscles. Scientists from the University of Washington and University of Minnesota also noted that the bears' muscles undergo periodic shivering that provide exercise, keeping them tone during this prolonged period of inactivity.

American black bear (Ursus americanus)
American black bear (Ursus americanus)
Photo: Tom Brakesfield

Many animals undergo physiological changes and enter a state of slumber to conserve energy in times of need. Hibernation occurs when temperatures are too cold; estivation, during warm, dry periods. Because of a limited ability to store slow-burning, high-energy fat, small birds and mammals such as hummingbirds and bats only undergo daily periods of torpor.



American black bear (Ursus americanus)
American black bear (Ursus americanus) in Kalispell, Montana. Photo: Gerald and Buff Corsi

This combination preserves about 80 percent of the bears' muscle strength while an inactive human would only have about 20 percent of his or her strength remaining after a similar winter slumber-too little to get out of bed. The researchers say that bears may have developed this mechanism as a defense strategy. While hibernating, they are vulnerable to large carnivores such as cougars and need to arouse quickly to fend off any intruders.

Further research on hibernating bears may eventually help battle muscle atrophy in bed-ridden people, who can lose nearly one percent of their muscle strength every day, and astronauts, whose muscles degenerate during long space flights.

map: American black bear range