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.
Highlights
The black bear (Ursus americanus) spends 4–7 mo each winter hibernating in its den (Beck 1991)
We address three questions: (1) How do muscle strength and associated contractile characteristics change between early and late hibernation? (2) Do skeletal muscles of bears maintain fatigue resistance, indicative of slow oxidative fiber retention, between early and late hibernation? (3) How does the bear compare to traditional disuse atrophy models and to small vertebrate species engaged in deep hypothermia? Data from this study will help us to understand the extent of strength retention in hibernating bears and to draw a closer association between fiber morphology, contractile properties, and fatigue resistance of an animal exposed to prolonged periods of dramatically reduced activity and food intake while in a state of mild torpor
Muscle strength measured in vivo based on maximum twitch force in the tibialis anterior of denning bears was 29% greater (Fig. 2) in bears in early hibernation (24.08 ע4.4 newtonmeters) than in bears in late hibernation (17.11 ע2.33 newtonmeters; P p 0.04)
Summary
The black bear (Ursus americanus) spends 4–7 mo each winter hibernating in its den (Beck 1991). The apparent retention of locomotor function is remarkable considering that the bear is confined to a small space with limited mobility, has no access to food or water, and maintains a body temperature that is only a few degrees below its activity range for an entire winter (Nelson et al 1973; Harlow et al 2004). These observations prompted our studies on overwintering bears with a focus on comparison with traditional models of skeletal disuse atrophy and with studies on smaller vertebrate species that either hibernate or estivate
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