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AS ICY WINDS HOWL outside my attic office, the idea of
tucking inside a warm den and sleeping until spring seems appealing. Several
familiar animals spend winter in some form of hibernation, surviving thanks to
their amazing physical adaptations.
Northern leopard frogs and wood frogs await the return of
warm weather in quite different quarters. From October until March, leopard
frogs hunker down in small hollows in the mud at the bottom of deep ponds and
streams. They must select bodies of water that have enough oxygen and will not
freeze solid. Wood frogs, on the other hand, hibernate underground, with soil
and snow cover for insulation. Even if the temperature drops to -6 C, freezing
the frogs solid, they will survive! Their bodies produce a type of
"antifreeze" that protects cells from damage.
The bodily processes of hibernating mammals drop to an
astonishingly low threshold. Looking at a hibernating woodchuck, you would
swear it was dead. Its body feels cool to the touch, although its normal body temperature
is close to a human's. Its heart rate slows from 100 beats per minute to four,
and the woodchuck breathes only once every six minutes. The woodchuck and other
true hibernators, such as ground squirrels, can't stay in this state all
winter, however. For several reasons, including the need to release body
wastes, they must awaken every week or so and rev their metabolism up to
normal. These speed-up periods are costly in terms of energy expended. For a
ground squirrel, one such awakening uses as much energy as 10 days of
hibernation.
Perhaps the most remarkable winter slumber is that of the
black bear. Throughout time and across cultures, people have found the bear's
yearly life cycle miraculous. Suppose you know nothing about hibernation. You
observe that bears disappear into the ground in early winter. You do not see
them again through the long, cold season, often a time of starvation and death
for your people. In spring, the bruins emerge from the earth alive, often
accompanied by young. Did the bears die in their dens, only to be reborn? If
not, how did they survive those many months without food?
The miracles accomplished by a black bear's body each winter
are as fantastic as any myth. Unlike a true hibernator, the bear stays warmer;
its body temperature falls by only three to seven degrees C. Its metabolism
slows by half, and its heart beats only eight to 12 times per minute, rather
than the normal 40 to 70.
However, once the bear slips into hibernation, it stays
there. For four to six months, the bear does not eat, drink, defecate or
urinate. A human deprived of food, water, and the ability to eliminate wastes
would probably die within 10 days.
The bear lives off its body fat, losing up to a third of its
weight over the winter. Its body recycles the wastes generated during this time
by building new proteins from them. The bear's metabolic processes produce
water as a byproduct, enough water to offset that lost as vapour when the bear
breathes, and to keep fluid levels in its body balanced without the bear taking
a drink all winter. Despite the fact that it hardly moves for months, the bear
neither develops osteoporosis nor loses much muscle tone.
Most astonishing of all, while their bodies perform these
feats, some female bears give birth to tiny cubs - only about 340 grams each -
and produce enough rich milk to feed them until it is time to leave the den.
Then, as the earth begins to awaken from its long, cold slumber, the bears
emerge: a hungry mama trailed by several playful, black bundles of curiosity.
Spring is truly a season of wonders.
- REBECCA L. GRAMBO
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