What Hibernation Means for Black Bears

Most animals have a tough time surviving the winter. In the Interior of Alaska, food is limited, arctic winds howl, snow falls, and severely cold temperatures persist. Some animals have adapted to hibernation as a means of survival throughout the winter. Black bears have the highest survival rate of any hibernating animal during a typical winter. To save energy and slow their metabolism, black bears enter a state known as hibernation. People used to believe that bears spent the winter in their dens and then emerged refreshed in the spring.

Contrary to popular belief, hibernation is not characterized by one extended stretch of sleep but rather by shorter stretches of sleep interspersed with more active intervals. Winter is the period of the year when people sleep the longest, as opposed to the beginning or conclusion of the season. Black bears, in preparation for the long winter, gorge themselves from the middle of summer until the end of fall, consuming as much as 20,000 calories in a single day. Blueberries, which are abundant here in the Interior and are rich in carbohydrates, account for a sizable amount of this calorie intake. As omnivores, bears will also consume flesh such as ground squirrels, carrion, and whatever else they may find. A black bear’s pelt’s insulating properties are increased by more than twice as much at the end of the fall as they were at the beginning.

The bear’s metabolic rate, including its heart rate, breathing rate, and core body temperature, drops when it enters hibernation. Contrary to popular belief, bears do not maintain abnormally low body temperatures. They enter hibernation at a temperature of 88 degrees and emerge at 100 degrees Fahrenheit. Black bears maintain a body temperature that permits them to become completely attentive whenever aroused, maybe so that they can protect themselves from predators and other hazards without using too much energy. It is estimated that black bears burn off over 4,000 calories per day when hibernating, leading to a weight drop of around 20% by spring.

Black bears don’t always go back to the same home year after year, and even when they do, the den is usually too tiny for the animal’s body size. A bear’s primary defense against the cold is its thick layer of fat and hair, as dens are often just a few degrees warmer than the soil. When the day becomes shorter, the weather turns colder, and food becomes scarce, the bear’s internal clock sends a signal for hibernation. When the snow starts to fall heavily, almost all bears in the area will retreat to their dens, where they will spend the winter alone or with their young.

Black bears have a long breeding season, from May to July. Bear cubs aren’t born until January or February, after a gestation period of roughly three months. A delay in the implantation of the fertilized eggs is to blame for the long gestation period before the cubs are born. The development of an egg ceases after it has been fertilized and has undergone a few rounds of cell division. This cluster of fertilized cells (known as a blastula) hangs out in the uterine cavity until the fall.

Assuming the mother bear had a successful summer in terms of food acquisition, the blastocyst will implant in the uterine wall and continue growing until a cub is born. This delay in embryonic development has various life-saving benefits. Having cubs in the summer when they are vulnerable to predators is a bad idea. Newborn young would also prevent the mother bear from gaining enough weight in the summer to survive the colder months.

Cubs are born weighing between 0.5 and 0.7 pounds, blind, naked, and totally reliant on their mothers. Females often sleep through the birthing process, but their cubs know to seek out their mother’s milk, which contains around 25 percent fat (human milk has 4 percent fat). The cubs are considerably more mature and ready for the world when the longer days of spring signal the mother and her cubs to leave the den, but they will still remain near their mother for at least another year.

As the human population grows, more and more of the land that wild animals call home is used for human activities like building and pleasure. Although conflicts are unavoidable, it is in our best interest to figure out how to coexist peacefully with these fascinating animals.

By Blaine Anthony.