During winter sleep (or hibernation), bears have a greatly reduced metabolism and all metabolic processes slow down, so that they do not expend energy on metabolism and movement. However, their metabolism does not slow down completely, as during hibernation of gophers or other rodents, so in bears this is called winter sleep.
They use up the accumulated fat reserves that they accumulate during the fattening period, and in the spring they come out of winter sleep very thin: during the winter they lose up to 30% of their body weight. The level of basal metabolism is greatly reduced, protein metabolism decreases, all processes slow down, including pulse and respiration. Naturally, bears do not feed during the winter sleep period, they do not urinate, and only in the spring, when they begin to leave their den, these processes start gradually, step by step.
At the same time, during winter sleep, bears continue to grow bones, wool grows. Bears survive the winter without food because of their nature. During the period of winter sleep, their physiology changes very much, and then, in the spring, they leave the den and come to their senses for a long time - this period is called walking hibernation. At the first exits, they almost do not eat, they have a cork, that is, all the remnants of vegetation accumulated in the intestines, which they ate before sleep. Bears sleep for 5-6 months and leave their dens in spring, when the first grass appears.
Everyone leaves their dens at different times: first, adult males, then single females and young individuals, and the last - lactating females with cubs (approximately in April-May). In Kamchatka, some large males do not sleep at all, because there fish spawn in winter, too, all year round, so they can afford not to sleep, but to continue to feed.In winter, an important event occurs in the life of bears: cubs are born during the winter sleep of a mother who sleeps weak sleep and immediately after childbirth begins to lactate and lick the cubs.
Usually they are born in January-February, rarely at the end of December. When the hunt takes place in the den and the hunters kill the bear, the female is often killed, next to which they find tiny newborn cubs weighing 300–400 grams. Hunters often take them to themselves, somehow nurture them, feed them with human food, and then these orphaned bears often grow into huge impudent uncontrollable bears, and the owners have to kill them.