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North American grizzly bear (Ursus
arctos) in Denali National Park, Alaska.
Photograph by Mark Chappell |
Adapting a system used for the evaluation of
neuromuscular disease progression in humans, the researchers tested
black bears from Middle Park, Colorado, both early and late in the
hibernation cycle. After sampling, bears were placed back in their
den, and the entrance was covered with pine boughs and snow.
The researchers found that after 110 days of
anorexia and confinement in the den, bears lost about 29% of their
muscle strength. In comparison, humans on a balanced diet but confined
to bed for 90 days have been reported to lose 54% of their strength.
Other studies have shown that human astronauts in a weightless
environment lose 9%–11% of their strength during a 17-day spaceflight.
Black bears also sustained only a moderate loss of
fatigue resistance, the researchers found: "After 110 days of fasting
and limited mobility imposed by denning, bears still exhibited a
fatigue profile similar to that of healthy, active, fed humans."
In contrast to other, smaller hibernating mammals
such as prairie dogs and squirrels, black bears also conserved muscle
protein and size. The researchers found no loss in the fiber
cross-sectional area of the bears’ thighs or calves, a drop of only
about 10% in the protein content of the thighs, and no reduction in
protein content in the calves or quadriceps of the bears.
"Bears in this study exhibited remarkable
conservation of muscle function," write the authors. "In spite of a
size difference of almost three orders of magnitude and a 30 degree
Celsius difference in torpor body temperature, the black bear may
conserve muscle function to the extent of or perhaps better than
small-mammal hibernators."
Since 1928, Physiological and Biochemical Zoology
has presented current research in environmental, adaptational, and
comparative physiology and biochemistry. Original research results
represent a variety of areas, including thermoregulation, respiration,
circulation, osmotic and ionic regulation, environmental acclimation,
evolutionary physiology, and metabolic physiology and biochemistry. |
