What's in a Name?
Hibernation
Means Different Things to Different Animals
by Mark D. Jones
Black Bear Project Leader
April, 1999
“Do bears really hibernate”? This is a question
I often hear from the general public and even other wildlife biologists. When
I answer yes, many people next ask, “do they really hibernate
in North Carolina, or is this just a northern behavior”? Well,
the answer is that bears hibernate throughout North America and certainly
in North Carolina. However, the process is very different from
what is considered “normal” hibernation exhibited by rodents
and bats.
To understand this issue, we first need to define the process of hibernation. In
its simplest definition, hibernation is a specialized reduction in
metabolism brought about by low food availability and/or low temperatures. Several
body changes can occur during hibernation. These include lower
heart rates, constriction of blood vessels, suppressed shivering, reduced
breathing, and lower oxygen consumption. Another process common
during hibernation is known as adaptive hypothermia. This is
a process by which an animal lowers its body temperature in order to
conserve energy. Different species may utilize different degrees
and combinations of the previously mentioned metabolic processes. Many
species of rodents and bats drop their body temperatures very dramatically,
some to near freezing. Bears on the other hand only drop their
body temperatures by 10-15 degrees in most cases. As a result,
bears are somewhat wakeful sleepers and are capable of abandoning a
den if seriously disturbed whereas many rodents are so deep in hibernation
that they can be handled without waking.
Most hibernating mammals, such as woodchucks (ground hogs to most
North Carolinians), must arouse from hibernation periodically to feed,
urinate, and defecate. Herein lies the main difference in bear
and rodent hibernation. Bears have the ability to remain stationary
for longer periods than rodents without feeding or eliminating waste. In
northern areas of the U.S. and Canada, bears hibernate as long as 8
months without moving from their den. In the South, bears exhibit
the same characteristics, only for shorter time periods. The
build-up of waste that bears somehow process would kill most animals
if they did not arouse from hibernation to handle normal bodily functions. Furthermore,
the long periods of inactivity would result in degradation of bones
and muscles for non-hibernating animals. Bears however exit their
winter dens strong and healthy after long periods of inactivity. Whatever
you call the process in bears, it is a truly remarkable physiological
achievement. Medical doctors are attempting to understand bear
hibernation better in order to help patients suffering from kidney
dysfunction and bone degradation.
What about bears in North Carolina? Based on hundreds of radio-collared
black bears studied across the state, we know that the vast majority
of our bears hibernate. Females typically hibernate longer than males. North
Carolina’s bears just do it for shorter time periods than their
northern cousins. Bears studied in eastern North Carolina by
radio-telemetry entered dens as early as November and as late as January. These
same bears exited dens as early as February and as late as April. This
results in the possibility of bear sightings and roadkills in all months
and the misconception that coastal bears do not hibernate. Only
human disturbance interrupts these periods of hibernation in North
Carolina’s bears.
Female bears give birth during hibernation and nurse their cubs through
a period of helplessness. The female “hibernates” throughout
the entire process, and unless disturbed, she will not leave the den
until she brings her cubs out of the den in the spring. By that
time, the cubs can walk and follow the mother as she feeds.
The next time someone says black bears do not “hibernate”,
please tell them that whatever you wish to call the process, bears
do enter a long period of physical inactivity and exhibit some amazing
physiological responses to low food availability and temperatures. In
many ways, their ability to hibernate for long periods without feeding
or eliminating waste is more advanced and remarkable than what has
been considered “normal” hibernation involving periodic
arousal to move about and feed.
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