Winter is a time of hardship for most mammals. Food is scarce, icy winds howl, snow falls, and bitter cold temperatures presist, at least here in the Interior of Alaska they do. For some mammals the strategy evolved for coping with this harsh season is hibernation. Among the hibernators, black bears have an extremely good survival rate in an ordinary winter. Hibernation is the mechanism that black bears use to conserve energy and reduce their internal fires of metabolism. For a long time people thought that the bears slept through the winter in cozy dens and emerged in the spring fully charged.
However, far from being a long, uninterrupted sleep, hibernation consists of periods of sleep punctuated by periods of arousal. Sleep time is long during the dead of winter but is shorter at the beginning and end of the season. To prepare for this long season black bears feed ravenously from midsummer through the end of autumn, gleaning up to 20 thousand calories in a day. Here in the Interior, carbohydrate-laden blueberries contribute a large portion to this caloric intake. Bears are omnivores and will eat meat too, including ground squirrels, carion and whatever they can find. By the end of autumn, a black bear will have added about 4 or 5 inches of body fat and more than doubled the insulation provided by its pelt.
As the bear enters hibernation, its metabolic processes such as body temperature, heart rate, and respiratory rate are reduced. But bears do not lower the body temperature as much as once thought. Their hibernation temperature is around 88 degrees and waking temperature is 100 degrees F. This relatively high sleeping temperature allows black bears to become fully alert if aroused, perhaps to enable the bear to protect itself from predators and other dangers without unnecessarily taxing their energy reserves. Over the course of a hibernating season it is thought that black bears use approximately 4 thousand calories a day, which results in a weight loss of about 20 percent of it body weight by spring.
When it is time to den, black bears do not return to the same place every year and the size of the den is relatively small for the size of the animal. Also, dens usually are only a few degrees warmer than the soil, so the bear’s main protection from the cold comes from the insulative quality of its fat and fur. Bears enter their dens when signaled by an internal clock responding to day length, regional weather patterns and most importantly, a decrease of food supply. Most all bears go into their dens by the time there is a heavy snowfall in the region and generally are solitary except for females with cubs.
The mating period for black bears extends from May to July. It takes about three months for the embryo to develop but the bear cubs are not born until January or February. The lapse of time between fertilization of the eggs and the birth of the cubs is due to delayed implantation of the eggs. After the egg is fertizlied and undergoes some cell division it stops development. This tiny ball of fertilized cells (called a blastula) remains suspended in the uterus until autumn. If the female bear was able to acquire enough food over the course of the summer and is in good shape when she goes into hibernation, the blastocyst will implant in the uterine wall and growth continues until a cub is born. The survival advantages to this embryonic delay are several. It would be dangerous for the cubs to be born during summer months and exposed to predation. Also, the mother bear must put on sufficient body fat during summer to make it through the winter and newborn cubs would interfere with this.
At birth the cubs weigh about one-half to three-quarters of a pound, are blind, naked and completely dependent on their mother. Even though the female may sleep through the birth process, the cubs know instinctively to snuggle close to her for warmth and find her milk which is approximately 25% fat (human milk is 4% fat). When the longer days of spring signal the mother and her cubs to leave the den, the cubs are much more developed and ready for the world, although they will stay close to their mother for at least another year.
As the human population continues to increase, development and recreation continues to encroach into wild animal habitats. Conflicts are inevitable, however, it will be to our own benefit to learn how to live in harmony with these wonderful creatures.
