No abstract

Studies of the metabolic and physiological changes that bears undergo during hibernation have, for the most part, supported the paradigm that bears use only fatty tissues as a metabolic substrate during hibernation. This study was performed to document the extent of protein loss and alteration of muscle-fiber characteristics of selected muscles in black bears during winter dormancy. Muscle biopsies were removed from the gastrocnemius and biceps femoris from seven free-ranging female black bears on the Uncompahgre Plateau in west-central Colorado. Six of the seven bears produced cubs during the hibernating season. Muscle samples were collected from the left hind limb shortly after bears entered their dens (fall), and additional samples were collected from the right hind limb just prior to bears leaving their dens (spring). Protein concentration, fast- and slow-twitch muscle-fiber ratios and muscle-fiber cross-sectional areas, and citrate synthase activity were measured in the laboratory. While protein concentration decreased in both muscles during the hibernation period, it was lower than predicted for lactating females. In addition, muscle-fiber number and cross-sectional area were unchanged in these muscles, suggesting only limited muscle atrophy. In support of these observations, there was a moderate but significant increase in the proportion of fast-twitch fibers only in the biceps femoris, with a concomitant decrease in citrate synthase activity, but no alteration of the fiber ratio in the gastrocnemius during hibernation. These findings suggest that hibernating bears, particularly lactating females, do use some protein, in concert with fat catabolism, as a metabolic substrate and as a source of water. However, the extent of this protein use is moderate and is associated with limited alteration of muscle structure, characteristic of disuse atrophy.

No abstract

Polar bears (Ursus maritimus) summer on the sea ice or, where it melts, on shore. Although the physiology of "ice" bears in summer is unknown, "shore" bears purportedly minimize energy losses by entering a hibernation-like state when deprived of food. Such a strategy could partially compensate for the loss of on-ice foraging opportunities caused by climate change. However, here we report gradual, moderate declines in activity and body temperature of both shore and ice bears in summer, resembling energy expenditures typical of fasting, nonhibernating mammals. Also, we found that to avoid unsustainable heat loss while swimming, bears employed unusual heterothermy of the body core. Thus, although well adapted to seasonal ice melt, polar bears appear susceptible to deleterious declines in body condition during the lengthening period of summer food deprivation.

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.