Sub-zero temperatures always get me thinking about how wildlife copes with severe winter conditions.
Black bears have developed an admirable strategy: They gorge themselves in the fall, sleep through winter’s worst and give birth to cubs before emerging in the spring. It sounds odd, but black bear reproduction is timed to minimize its effect on eating. Bears mate in early summer when food is abundant, but the embryos do not implant on the uterus until late fall. This enables adults to concentrate on finding and eating food — everything from fruits, nuts, and roots to insects, bird eggs, and small mammals.
Eat, then sleep
Bears accumulate up to four inches of subcutaneous fat during the fall feeding frenzy. In late fall, bears find a hollow log, a hollow tree or even a large brush pile. There they sleep on a bed of leaves and grasses, but it’s not the deep sleep characteristic of true hibernators.
During a bear’s winter dormancy, its resting heart rate drops from about 66 beats per minute to 8 to 22 beats per minute. Its body temperature drops several degrees, but does not approach ambient temperature. They do not eat, drink, urinate, or defecate while dormant. Bears lose 25 to 40 percent of their body weight over winter.
In just a few weeks, black bears will begin giving birth; litter size usually ranges from one to four cubs, but two are typical. Because the sow’s body temperature is only a few degrees below normal, they can rouse fairly quickly if disturbed, and they can care for their cubs after giving birth.
Newborns are helpless
By any standard, newborn black bears are tiny. Blind, helpless, and covered with just a coat of fuzz, cubs measure six to eight inches long and weigh just seven to 12 ounces at birth — about the size of a guinea pig. By the time cubs reach six weeks of age, their eyes open, they are well furred, and weigh about two pounds. At eight weeks, they may leave the den with their mother for short periods, weigh about five pounds, and begin learning how to be a bear.
Cubs stay with their mother well into fall and may even den with her during their first winter. After that, young bears become independent. Though six to seven months pass from the time of mating to giving birth, the functional gestation period is only eight to 10 weeks. This, coupled with the small size of the cubs, is another adaptation for reproducing during winter dormancy. Even during this stressful time, sows have plenty of fat to nourish both themselves and their cubs.
Not true hibernators
Though black bears are often called hibernators, they simply don’t qualify physiologically. In fact, few mammals are true hibernators.
Let’s compare a true hibernator to what we now know about bears. Meadow jumping mice are common, seldom seen, tiny mammals (they weigh less than one ounce). They have huge rear feet and a tail that’s a good bit longer than the body. When hurried or frightened, they hop in zig-zag fashion and can jump six to 10 feet.
Jumping mice are true hibernators. Like bears, they feed furiously during the fall frenzy when they devour seeds, berries, roots, nuts, underground fungi, and invertebrates — up to half their body weight each day. By late October or early November, they’re ready for the big sleep. Though they live and nest above ground during the spring and summer, fall sends them underground.
They dig a burrow that may extend three feet below the surface and build a grapefruit-sized nest of grass and leaves. When the nest is complete, they plug the tunnel and assume a distinctive sleeping position. Jumping mice curl into a tight ball by placing the nose between the hind legs and wrapping the tail around the body. Hibernation can last up to six months. Body temperature drops to 35 to 40 degrees Fahrenheit, and the heart and breathing rates slow drastically.
It’s not for the weak
These extreme changes in metabolism qualify jumping mice as true hibernators. True hibernation is physiologically traumatic. More than half of hibernating jumping mice die during the experience, so breeding to replenish the population dominates the rest of the year.
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