Grizzly Bear
Denning
A
bear’s instinct to den is an adaptation to life in places where winter
conditions would otherwise threaten survival.
The problems of reduced food supply, decreased mobility and increased
energy needs in winter are solved by denning.
Denning time coincides generally with the first inclement winter weather.
In
the Mackenzie Mountains, most bears are in dens by the middle of October.
On Richards Island, dens may be occupied from late September onward.
In all areas, female bears probably den first and are the last to emerge
in spring.
Grizzlies
in mountainous areas usually den on steep alpine slopes or in subalpine terrain,
often between elevations of 1500 and 1800 m.
The Richards Island grizzlies on the Tuktoyaktuk Peninsula den in the
steep banks of rivers or lakes. Some
barren-ground grizzlies may move south to treeline to den but many find suitable
sites on the tundra.
Bears
are rather selective about denning sites. The
entrance is often below a stand of willow or alder so that hanging branches
partially obscure the opening and act as a snow catch.
The den might also be found on a north-facing slope where snow drifts
deeper.
Dens
are usually located in soil that is soft enough for digging but not prone to
slumping. Most dens are egg-shaped
with a small entrance tunnel. The
den itself is quite small (the average in the Mackenzie Mountains was 259 cm
long, including the tunnel, 134 cm wide and 94 cm high), yet large enough to
allow the bear some movement and stretching room during dormancy.
It must also be small enough so that the air space will be efficiently
warmed by the bear.
Scientists
differ as to whether or not bears enter a state of true hibernation, but the
controversy may be largely a matter of terminology.
Many smaller mammals enter a deep sleep from which they cannot be easily
roused. While they are dormant,
their body temperatures are considerably lower than normal.
A bear’s temperature, in contrast, does not drop more than about 5°C.
Bears can be easily awakened, but if undisturbed, may sleep for as long
as a month without changing position. Smaller
mammals awaken periodically to eat and expel body wastes.
Bears cease all such functions during their hibernation.
During
dormancy bears may lose up to 40% of their body weight and are lean and hungry
when they emerge in spring. The
time of emergence varies with locality and weather.
Richards Island grizzlies leave their dens in late April or early May,
while bears in the Mackenzie Mountains are not generally seen until May and
June. In the barren-grounds, bears
are frequently seen from late May onwards.
As
well as winter dens, bears sometimes construct day beds.
The bed may be simply an area trampled in bushes, or it may be similar to
a winter den, differing only in the lack of an entrance tunnel.
Day beds are often located near food sources and are used for resting
after a heavy meal.
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