Hibernating Bears Conserve Muscle Strength and Maintain Fatigue Resistance

Abstract

Black bears spend several months each winter confined to a small space within their den without food or water. In nonhibernating mammals, these conditions typically result in severe muscle atrophy, causing a loss of strength and endurance. However, an initial study indicated that bears appeared to conserve strength while denning. We conducted an in vivo, nonsubjective measurement of strength, resistance to fatigue, and contractile properties on the tibialis anterior muscle of six hibernating bears during both early and late winter using a rigid leg brace and foot force plate. After 110 d of anorexia and confinement, skeletal muscle strength loss in hibernating bears was about one-half that in humans confined to bed rest. Bears lost 29% of muscle strength over 110 d of denning without food, while humans on a balanced diet but confined to bed for 90 d have been reported to lose 54% of their strength. Additionally, muscle contractile properties, including contraction time, half-relaxation time, half-maximum value time, peak rate of development and decay, time to peak force development, and time to peak force decay did not change, indicating that no small-scale alterations in whole-muscle function occurred over the winter. This study further supports our previous findings that black bears have a high resistance to atrophy despite being subjected to long-term anorexia and limited mobility.

Extreme Respiratory Sinus Arrhythmia Enables Overwintering Black Bear Survival—Physiological Insights and Applications to Human Medicine

“…Clinical applications of a reduced metabolic state typical of overwintering bears include treatment for human heart failure, stroke, myocardial infarction, and extending the duration of whole organ ischemia prior to transplantation [4]. We previously demonstrated that bears have limited loss of skeletal muscle protein and strength during winter, which has applications in preventing muscle disuse atrophy [12,23,37]. It is also advantageous to conserve cardiac muscle to maintain sufficient perfusion of all body tissues during winter and to enable a response to predatory threats.…”

Decrease in the red cell cofactor 2,3-diphosphoglycerate increases hemoglobin oxygen affinity in the hibernating brown bearUrsus arctos

“…A hibernating brown bear (Ursus arctos) routinely spends 5-7 mo per year in continuous dormancy with no food or water intake, no urination, and no defecation (19,36). During this time, bears appear to be resistant to loss of muscle mass, strength, or bone density (18,24,29,43,14,44). During hibernation bear body temperature is downregulated only slightly, fluctuating from ϳ37°C to a minimum of 30°C, as found in brown and in black bears (Ursus americanus) (17,26,27,36,43), whereas O 2 consumption rate is downregulated by 75% (43).…”

Scaling matters: incorporating body composition into Weddell seal seasonal oxygen store comparisons reveals maintenance of aerobic capacities

“…These findings suggest that Weddell seals may be physiologically "programmed" to withstand periods of reduced activity while maintaining muscle integrity. Wild animals may need to forage effectively and escape predation after such periods of reduced activities, and indeed, similar patterns of atrophy resistance have been observed in hibernating bats and rodents, and winter lethargy in bears (Lohuis et al 2007;Hershey et al 2008;Lee et al 2008;Nowell et al 2011). Despite being composed of primarily slow-oxidative fibers that are particularly vulnerable to atrophy, Weddell seal muscles maintained aerobic MHC profiles.…”