Studies of the metabolic and physiological changes that bears undergo
during hibernation have, for the most part, supported the paradigm tha
t bears use only fatty tissues as a metabolic substrate during hiberna
tion. 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 gas trocnemius and biceps femoris from seven free-ranging female b
lack bears on the Uncompahgre Plateau in west-central Colorado. Six of
the seven bears produced cubs during the hibernating season. Muscle s
amples were collected from the left hind limb shortly after bears ente
red their dens (fall), and additional samples were collected from the
right hind limb just prior to bears leaving their dens (spring). Prote
in concentration, fast- and slow-twitch muscle-fiber ratios and muscle
-fiber cross-sectional areas, and citrate synthase activity were measu
red in the laboratory. While protein concentration decreased in both m
uscles during the hibernation period, it was lower than predicted for
lactating females. In addition, muscle-fiber number and cross-sectiona
l area were unchanged in these muscles, suggesting only limited muscle
atrophy. In support of these observations, there was a moderate but s
ignificant increase in the proportion of fast-twitch fibers only in th
e biceps femoris, with a concomitant decrease in citrate synthase acti
vity, but no alteration of the fiber ratio in the gastrocnemius during
hibernation. These findings suggest that hibernating bears, particula
rly lactating females, do use some protein, in concert with fat catabo
lism, 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.