Biology Reference
In-Depth Information
shaped circles of grass and leaves not unlike huge birds' nests. Once this edifice is complete, the bear
curls up in it for the winter. Still farther south, bears may become torpid for a time, but don't have a
winter den.
Denning time varies greatly, depending on the food supply rather than on the weather. In years when
food is scarce, they'll den quite early. When good food is abundant, however, bears will keep on for-
aging for several more weeks, going into hibernation only when driven to it by severe winter weather.
In January or February the mother black bear gives birth to tiny cubs, usually two or three, which
weigh only one-half to three-quarters of a pound. As with kittens, their eyes are closed at first and don't
open for nearly a month.
A major misunderstanding is the belief that the female bear sleeps through the birth of the cubs,
nurses them in her sleep, and wakes up in the spring to learn for the first time that—wonder of won-
ders—she has babies. The truth is that bears den, but don't hibernate in the strict technical sense of that
word. In true hibernators, both heartbeat and body temperature drop dramatically; the temperature of
denning bears doesn't drop during what we normally refer to as their hibernation period.
All of this means that bears don't go into a comatose state of true hibernation similar to that of the
woodchuck. Instead they awaken frequently, move about in the den, and occasionally even emerge for
short periods. In any event, the female is very much aware of her cubs, and takes good care of them in
the den. She cuts their umbilical cords, nurses them, and is careful not to roll on them and crush them.
Despite the fact that they aren't true hibernators, bears have a complex assortment of wondrous ad-
aptations to see them through the winter. For example, a bear's heartbeat slows to eight beats per minute
for much of the time while it's denning. Moreover, it doesn't drink during that time, and “eats” only by
consuming its own fat. Meanwhile, it neither urinates nor defecates until it leaves the den in the spring.
The cubs grow rapidly, for their mother's milk is extremely rich. To put this richness in perspective,
consider that a black bear's milk contains at least 24 percent fat, while cows and humans produce milk
with roughly 4.5 percent fat. It's no wonder that cubs thrive on such high-energy fare while still restric-
ted to their den.
A thin mother and usually lively, well-fed youngsters emerge from their den in early spring. Black
bears only breed every other year, so the cubs remain with their mother for a year and a half, learning
how to fend for themselves. Finally, during the summer of their second year, the cubs set out on their
own.
Mortality is very high among cubs during their first spring and summer— usually 50 percent or
more. In areas where food is apt to be scarce in the spring, cubs often starve, but it also appears that
other bears, especially males, kill many cubs. In one Arizona study, older bears killed half of the cubs
that died from identifiable causes. In such cases the older bear also frequently eats the cub.
It's generally assumed that older, larger females will fight off males in order to protect their cubs, but
that small, young, and inexperienced mothers aren't up to the task of driving off a much bigger male.
