I 
KEEPING WARM 
A Bear in Its Lair 
Although its den provides little protection 
from the elements of a Minnesota winter, a 
black bear has a 99 percent chance of seeing spring 
by Lynn Rogers 
Winter is the season of highest mor- 
tality for most northern mammals. But 
not for black bears, which are masters of 
winter survival. Their winter death rate 
of less than one percent is their lowest of 
the year. 
Once considered not true hibernators 
because of their high body temperatures 
in winter, black bears are now known to 
be highly efficient hibernators. They 
sleep for months without eating, drink- 
ing, urinating, or defecating. Hiberna- 
tors with lower body temperatures, such 
as chipmunks, woodchucks, and ground 
squirrels, cannot do this. These other 
mammals must awaken every few days, 
raise their temperatures to over 94°F, 
move around in their burrows, and uri- 
nate. Some of them must also eat and 
defecate during arousals. Black bears, 
however, develop far more insulative 
pelts and have lower surface-to-mass 
ratios than the smaller hibernators. As a 
result, the bears’ body heat is lost very 
slowly, enabling them to cut their meta- 
bolic rate in half and still make it 
through winter, maintaining tempera- 
tures above 88° — within 12 degrees of 
their normal summer temperature. This, 
in turn, means that a black bear can 
react to danger faster than most other 
hibernators whose body temperatures 
may be less than 40°. New knowledge of 
hibernation processes has led biologists 
to redefine mammalian hibernation as 
simply a specialized, seasonal reduction 
of metabolism concurrent with the envi- 
ronmental pressures of food unavailabil- 
ity and low environmental temperatures. 
In northeastern Minnesota, where I 
have studied black bear behavior and 
ecology since 1969, bears commonly go 
for seven months without eating — from 
mid-September until mid-April. To do 
that and remain in good condition re- 
quires changes in physiology, and some 
of these changes create conditions in 
bears that would cause problems in peo- 
ple. How bears remain healthy in win- 
ter, and how their adaptations may be 
used to improve human medicine, have 
recently become subjects of study at 
some of the leading medical research 
centers in the country. For example, 
bears that are living off their fat have 
cholesterol levels more than twice as 
high as their summer levels and more 
than twice as high as the cholesterol 
levels of most humans. Yet bears have 
no known problem with hardening of the 
arteries or with the formation of choles- 
terol gallstones. Medical studies have 
shown that bears in winter produce a 
bile juice, ursodeoxycholic acid, that 
may help them to avoid problems with 
gallstones. When given to people, this 
acid dissolves gallstones, eliminating the 
need for surgery. Black bears also 
greatly reduce their kidney function in 
winter. They do not urinate for months 
but still do not poison their bodies with 
waste products such as urea. The urea is 
somehow broken down and the nitrogen 
from it is reused to build protein. This 
ability to build protein while fasting 
allows the bears to maintain their mus- 
cle and organ tissue throughout the win- 
ter. They only use up fat. Evidence is 
accumulating that the physiological 
changes that occur in hibernating black 
bears are controlled by hormonelike sub- 
stances. These substances also produce 
hibernationlike effects when injected 
into other species — both other hiberna- 
tors and nonhibemators, suggesting pos- 
sible uses in human medicine. 
Hibernation for the black bear, as for 
other mammals, is primarily a mecha- 
nism to conserve energy through seasons 
of no food or water. The process, how- 
ever, does not work every year. If there 
is insufficient food in summer, bears 
lose weight and starve, much as we 
would. In northeastern Minnesota, sum- 
mer is practically the only season with 
enough of the right foods for bears to 
gain weight. In other seasons, the right 
foods are either lacking or so scarce that 
Lynn Rogers 
64 
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