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A Mother Bear’s Long Nap

A Mother Bear’s Long Nap

 A Mother Bear’s Long Nap


MIGRATING birds unerringly herald the approach of autumn in the North. To escape the freezing cold, gaily fluttering flocks of starlings and noble wedges of cranes fly southward across the sky. At the same time, the brown bear lumbering along the ground also faces cold winter weather. How will it survive when vegetation withers, the ground freezes, and everything becomes covered in a blanket of snow? It is easy for those with wings to escape, but the brown bear cannot just rush through backwoods and wilderness to a warmer climate.

The solution is quite practical. The bear will eat its nourishment for the winter during the summer and then hibernate until spring. This, however, is not as easy as it may sound. Just imagine what sort of shape you would be in if you did not eat or drink for half a year. Let us consider some amazing phases of a mother bear’s winter sleep.

Busy Summer

To be able to fast for several months, a mother bear must store energy in advance. Therefore, she does not worry about her figure. Her main goal is to have a lot of fat under her skin, in some places up to three inches [8 cm] of it. Although sugar-rich berries are her favorite menu item, she is not picky​—anything goes. She eats roots, small mammals, fish, and ants. Finally, by autumn her weight may have increased from 280 pounds [130 kilos] to 350 pounds, [160 kilos] about  one third of which is fat (by then the male may weigh up to 600 pounds [300 kilos]). Before it is time to retire to the land of dreams, she stops eating and empties her intestines. She does not eat, urinate, or defecate until about half a year later.

A good place for a den is a cave, an abandoned anthill, or a cavity beneath tree roots, as long as the place is peaceful​—after all, no one likes to be disturbed while asleep. The bear gathers spruce boughs, moss, turf, and other bedding material to make her den as cozy as possible. The den is not much bigger than the stocky bulk of the bear. When winter arrives, snow will cover the den, and only the breathing hole may be visible to a sharp-eyed observer.

The Rest

Certain small mammals, such as hedgehogs, bats, and dormice, are called true hibernators because they spend most of the winter in a state similar to death, their body temperature approximating that of the environment. However, the bear’s body temperature decreases only about 10 degrees Fahrenheit [5 degrees Celsius], so its sleep is not very deep. “It is not as if the bear were knocked out. It raises its head and changes its position basically every day,” explains Professor Raimo Hissa, who has studied the winter sleep of bears for many years at the University of Oulu, in Finland. However, the bear rarely emerges from its den in the middle of the winter.

During its winter dormancy, the bear’s life processes are in economy mode. Its heart rate drops to less than ten beats per minute, and the metabolic rate declines. Once the mother bear is snoring contentedly in her sleep, an essential process of burning fat begins. Fat tissue breaks down and supplies the bear’s body with the necessary calories and water. In spite of reduced vital functions, however, metabolism results in a certain amount of waste products. How will she get rid of them and yet at the same time keep her den clean? Instead of disposing of her waste, her body recycles it!

Professor Hissa explains: “The nitrogen urea products are reabsorbed from the kidneys and the bladder and are transported through the circulatory system into the bowels, where bacteria hydrolyze urea to ammonia.” What is even more amazing is that this ammonia goes back to the liver, where it is used to form new amino acids, the essence of proteins. So by turning waste products into building blocks, the bear’s body is nourished during the long denning period!

In the old days, people used to hunt bears in their dens. Indeed, a dozing bruin was an easy catch. First the den was located and then gradually surrounded by skiers, who formed a ring around the site. Then the bear was awakened and killed. However, nowadays winter hunting of bears has been forbidden in practically all of Europe because of its cruelty.

New Life

The male bear lounges all winter, turning easily from one side to the other, but the mother bear is up to something else. Bears mate in early summer, but the fertilized eggs inside the mother remain dormant until she goes for her winter sleep. Then the embryos implant themselves in the wall of the uterus and start growing. After only two months, in December or January, the mother bear’s body temperature rises a little, and she gives birth to two or three cubs. Then her body temperature goes down, although not as low as before giving birth. The father of the cubs does not witness their birth, but if he did, it could be quite a disappointment to him. The robust male would hardly recognize these tiny weaklings, weighing less than a pound each, [350 grams] as his own offspring.

Mother bear feeds the cubs with her nutritious milk, which strains her energy resources even more. The cubs grow fast, and in the spring the  furry little bears already weigh about ten pounds [5 kg]. That means that there is quite a bit of activity in the mother bear’s small apartment.


March. The cold of the winter is past, the snow melts, and the birds migrate back from the south. At the end of the month, male bears crawl out of their winter dens. The female bears, however, rest a few weeks longer; perhaps the minors have taken a toll on their strength.

After this long nap, mother bear is nothing but skin and bones compared with her sturdy appearance in the fall. The snow has melted, and so has the fat. Otherwise, she is amazingly agile​—no bedsores, no cramps, no osteoporosis. She emerges from the den and after a while defecates a fecal plug, a “pitchy plug” of metabolic waste. Normally the bear does not begin to eat until after about two or three weeks, as the recovery of the organism takes some time. Then it naturally is “as hungry as a bear.” However, as nature is still just awakening to the spring, there is at first nothing much to eat in the forest. The bear munches on larvae and beetles, strips old carcasses, and may even chase some reindeer.

Naturally, the mother bear has to rear the cubs so that they learn to behave like bears, and they are the apple of her eye. An ancient proverb says: “Let there be an encountering by a man of a bear bereaved of its cubs rather than anyone stupid in his foolishness.” (Proverbs 17:12) In other words, neither encounter is desirable. “The mother bear has a lot to do in caring for her cubs. If a male bear approaches, the mother immediately sends the cubs up a tree, as the male could harm them even [if] he is their father,” Hissa explains.

The mother bear takes the cubs with her into her lair the next winter. The year after that, the weaned cubs will have to find their own dens, as it will be time for the mother bear to give birth to a new brood of tiny little cubs.

Much has been learned about the complex and ingenious phenomena of the winter sleep of the bear, but many amazing aspects of it are still a mystery. Why does the bear become drowsy in the autumn and lose its appetite? Why does it not develop osteoporosis? It is not easy to learn the bear’s secrets, and that is understandable. Everybody is entitled to some privacy!

[Box on page 20]

Study of Bears’ Winter Sleep

The Department of Zoology at the University of Oulu has for several years done physiological research on the mechanisms by which animals adapt to the cold. The study of the European brown bear began in 1988, and altogether, 20 bears have been studied since then. A specific research den was built for them in the zoological garden of the university. Computers, laboratory tests, and a video camera have been used to reveal their body temperature, metabolism, and activity as well as their blood and hormonal changes during the winter sleep. There has been cooperation with universities in other countries, even as far away as Japan. The researchers hope that the results can provide information that may even be helpful in solving problems of human physiology.

[Picture on page 18]

Mother bear’s den

[Picture on page 18]

Sugar-rich berries