In winter, a brown bear sleeps in a den. Some details of a bear's life. Melbet technical support
Nature holds many mysteries, many things are still unsolved and inexplicable in it. And yet, with the passage of time, as a result of observations and experiments, humanity was able to answer some of them.
For example, why does a bear sleep in winter and do all species of these animals hibernate? How, under conditions of absolute lack of food, does an animal manage to preserve the vital functions of the body at the same level and continue to actively hunt after a long period of starvation? Why do brown bears fall into winter sleep, but their white relatives do not? This article will answer these and other questions.
Why does a bear sleep in winter?
As you know, brown bears are fairly large animals. Therefore, in order to feed themselves, they need a decent amount of food. And although they are omnivores, in winter the plant part of the diet disappears, and it is quite difficult to survive only by eating other living creatures - birds, small mammals, eggs, carrion, insects, fish. Yes, and it is impossible to find frogs, ants, slugs in winter, and hunting hares and wild boars is problematic, since they simply run away from the clubfoot, which falls into the snow under its own weight and is not able to move quickly.
Note: It is because of the inability to eat fully that these predators hibernate. Hibernation is understood as a period of slowing down vital processes during a time of low availability of food, when the animal cannot maintain activity and the same level of metabolism.
Characteristic features hibernation called: lowering body temperature, slowing down work respiratory system and heart inhibition nervous activity. So, during winter sleep, the body temperature of a brown bear drops from 37-38 to 31-34 degrees, and its metabolic processes slow down. However, this sleep is not very deep, because at the slightest danger the animal wakes up and can leave the den (Figure 1).
Figure 1. As winter approaches, bears become lethargic and begin to prepare for hibernation. Hibernation is preceded by the appearance of lethargy, slowness of movements, and a decrease in the animal’s appetite. In this state, the animal does not defecate or urinate, because all waste products are processed into proteins necessary to maintain vital processes. The duration of winter sleep can be from 2.5 to 6 months, depending on weather conditions and the amount of nutrients accumulated by the animal.
Hibernation ends in the spring with the appearance of the first grass. At the same time, bears leave their shelters at different times: adult males leave first, then young individuals. Females with cubs leave the dens last - in April-May. This is due to the fact that the female gives birth to offspring in January-February, therefore, with the arrival of spring, the cubs are still too small to be released into the external environment, full of dangers. Already at the beginning of autumn, animals begin to feed intensively, picking up berries and fruits, eating insects and oats. In this way, they accumulate subcutaneous fat, which they need so much for hibernation, and in females, also for feeding their babies.
Features of bear hibernation in winter
The food consumed by animals is the source of energy on which they exist. Therefore, the more active your lifestyle, the more more The body needs energy, the more food you should consume. Therefore, with an insufficient amount of feed, it becomes necessary to reduce the intensity of all metabolic processes, which can be achieved by being at rest (Figure 2).
Note: It is for this reason that bears fall into winter sleep, when plant food, which makes up 80% of their diet, disappears.
However, even during hibernation, the animal can wake up in case of danger and show sufficient activity. This is explained by the fact that energy expenditure during winter sleep is minimal, and the cells receive exactly as much energy as is necessary to maintain the vital functions of the body. The reserves of fat and glycogen accumulated during the active period of the year are consumed gradually, so they last until spring. On the contrary, an animal that has not stored enough fat has a much lower chance of sleeping until spring. A hungry animal leaves its den ahead of time and wanders in search of food, remaining aggressive and dangerous to humans. The connecting rod bear can attack dogs or livestock, search for food in landfills or beg from people when going out onto highways.
Figure 2. During hibernation, all life processes slow down In addition to fat and glycogen, another source of energy is oxygen. During winter sleep, the body is inactive, its tissues need a small amount of oxygen and nutrients, so the blood that carries them moves much more slowly, the heart rate decreases, the breathing rate decreases significantly, and accordingly, energy costs are reduced. And although after hibernation the animal can lose up to half its mass own body, he still finds the strength to leave the den and begin active life even after a 3-month hunger strike.
By observing animals in a den, scientists learned that predators do not suck their paws, as is commonly believed, but lick them to relieve the itching that occurs as a result of the change of skin on the pads of the limbs. Thus, hibernation is a genetically determined defense mechanism, allowing the bear’s body to adapt to the lack of nutritional resources.
How a bear sleeps in a den in winter
In a warm and safe den, a bear can sleep all winter. Most often, the animal is located on its side, curled up in a ball, sometimes on its back, less often - in a sitting position, with its head lowered between its paws. Males and young sexually mature individuals sleep alone, and females who have young yearlings sleep with them (Figure 3).
Note: Unlike other animals, which become numb during hibernation and show no signs of life, their body temperature drops slightly, by only 3-5 degrees, their heart beats rhythmically, although it slows down, and their breathing becomes somewhat less frequent. Therefore, the animal easily awakens from winter sleep in case of alarm, and often leaves the den itself during prolonged thaws, returning to it when there is a noticeable cold snap.
If the temperature in the den becomes very low, the dormant animal wakes up, burrows deeper and falls asleep again. During winter sleep, the animal’s body does not remove waste products, but recycles them into useful proteins and water.
Figure 3. Types and structure of a den It took more than one thousand years natural selection so that such a a complex system animal adaptations to harsh conditions climatic conditions. Winter hibernation of a brown bear usually lasts about four months (from the second half of November to the first half of April), which depends on weather conditions, age and health of the animal.
Why don't polar bears sleep in winter?
Brown and polar bears, descended from common ancestors only 150 thousand years ago, and often interbreeding wildlife, are strikingly different in habits and way of life. Thus, a brown bear falls into a state of winter sleep in the cold season, but its white counterpart hardly sleeps in winter. He sleeps more sensitively and for short periods of time, usually in the spring and winter. The only exceptions are females who are pregnant or nursing newborn babies.
Figure 4. Polar bears are very different from their brown relatives The peculiarities of this behavior are explained by the fact that the polar bear’s diet consists mainly of seal meat and fish, which are available almost all year round, even during the winter cold, when he has the opportunity to hunt them strong ice. Predators snatch seals from the holes through which they breathe, or grab seals on the ice while resting. By the end of summer, when the ice has almost completely melted, it becomes more difficult for the bear to hunt, since prey easily swims away from him or runs away overland. Then the animal has to be content with the carcasses of dead whales or walruses found on the shore, and sometimes even starve.
Note: During such periods of temporary starvation, animals seem to “sleep on the move.” In other words, their body has all the signs of hibernation. Thus, the concentration of urea in their blood drops sharply, which causes lethargy, drowsiness and loss of appetite in the brown bear.
The polar bear does not hibernate, and in the presence of food it is able to raise the concentration of urea to a normal level:
- The polar bear's body uses urea to synthesize amino acids and blood plasma proteins, which ensure the maintenance of the required level of metabolism in the body.
- The lower the urea content, the less frequently it needs to be removed, which means that the need to quench thirst is also reduced, which is energetically justified in conditions of food shortages, because in order to obtain water from snow in Arctic conditions, a lot of energy is needed to warm it. Therefore, as soon as ice appears, polar bear goes out hunting, because the well-being of the animal next year depends on it.
- Females nursing babies have to spend the winter in a den. This is due to the fact that the cubs of the white subspecies are born very small, blind and helpless. Their body is not covered with hair, but with short fluff, which is unable to protect the animal from the northern cold.
- Polar bears make dens on the shore, in snow drifts, and if there is insufficient snow, even in a hole dug in frozen ground.
- Typically, females go into dens when hunting becomes problematic due to melting ice.
Before babies are born, they mostly sleep. Cubs (usually two) are born, as a rule, in November-January and remain in the den until spring. The female who is with them is in a state of winter sleep, that is, she does not eat, does not drink, and does not defecate, while feeding her offspring with milk (Figure 4). All these processes are possible due to the fact that immediately after mating, which occurs in April-May, pregnant females begin to eat intensively in order to accumulate the necessary supply of nutrients. Often, female bears manage to increase their body weight by 200 kilograms, while the development of the embryos is suspended for early stage and resumes only in the fall, closer to the time the female lies in the den, which depends on several factors, for example, weather conditions or the rate at which the animal accumulates nutrients. Another interesting fact is that during the period of winter sleep, the mother bear manages not only to feed her babies, but also not to lose mass of bones and muscles, because during hibernation only fat deposits are consumed. From everything described above, it turns out that it is polar bears that are most adapted to winter sleep.
In the video you can see what a bear’s hibernation den looks like.
Hibernation in bears
We all love animals since childhood. The bear is no exception. This animal has always captivated with its clumsiness and cuteness. appearance. The bear became the hero of Russian fairy tales and has always remained a symbol of Rus'. This animal has an impressive size and very thick fur. This animal is always associated with hibernation. It's hard to believe, but a bear actually spends a third of its life sleeping. When children ask about the reason for such behavior, it is sometimes difficult for us to answer the simple question “why does a bear sleep in winter?” The explanation for this phenomenon is very simple.
As you know, bears are omnivorous animals, in other words, they consume both animal and healthy plant foods in their diet. Hibernation always occurs during the cold season, when nature goes into economical mode and plant food for a bear practically none. Certainly? This animal could, with grief, eat only animals, but they are not so easy to catch.
Causes of hibernation in bears
Average weight the bear is half a ton. These are very impressive dimensions. The bear's clumsiness limits its movement; it is sometimes incredibly difficult for it to catch a hare or other animal. You can try to find vegetation. But in winter there is practically no food left and therefore the bear has to choose hibernation. Why the bear sleeps in winter is explained in detail in popular science magazines. This is the so-called protective mode, which allows him to save his life and not die from the cold.
The main reason for bears is simple Then you may have a completely logical question: why does a bear sleep in winter, but other animals do not? Everything is quite simple. As they say, he can afford it. During the summer and early autumn it is capable of accumulating a huge amount subcutaneous fat, which will become his salvation from hunger in sleep mode. Since it is very cold in winter, energy consumption increases several times. A bear cannot afford such luxury. His fat is not enough for active image life and therefore hibernation is the only thing correct solution In current situation.
Bears sleep in winter, a video that confirms this can be found in any program about nature. It is worth saying a few words about hibernation itself. By its nature, this is a unique process that allows the animal to survive in harsh winter conditions. Sometimes it seems so incredible that it’s even hard to believe. It’s worth starting with the fact that hibernation is a certain mode of existence that is familiar to a bear. Hibernation can last even six months. But how to survive during this time and not die of hunger. Everything is very simple.
Features of hibernation in bears
Bears sleep in winter in the video; you can see how the dozing animal just lies there. But in fact, this is a very complex biochemical process in which the functioning of all vital organs of the animal slows down to such a state that one might think that the animal is no longer breathing. This is the so-called energy saving mode. This is the bear's only chance to survive a hungry winter. His breathing is very slow.
Bear's den in the forest To ensure that all organs continue to function, fat is used as energy. During winter hibernation, an animal can spend half its body weight. It could be 200 kg. Enough big losses, which will be replenished with interest again over the summer. Bears often give birth to cubs during hibernation. They are born in a litter. Usually there are several of them. The most interesting thing is that their weight is so small compared to an adult that it is difficult to believe in their ability to survive.
The average weight of a bear cub is about 200 g. You know how bears sleep in winter, but you don’t yet know how cubs manage to survive in winter. The answer to this question is the natural feeding of mammalian offspring. Like a human, a bear cub feeds on its mother's milk. It is so fatty that it guarantees its survival in winter.
In popular science magazines you can see how a bear sleeps in winter; taking such photos is very risky, since, despite the animal’s hibernation, its sleep is very light.
Teddy bear in hibernation There are cases when packs of animals woke up a bear, and he could no longer fall into his natural state in winter - hibernation. This is very bad for a bear. He can't feed himself. There is very little food in the forest and so he begins to destroy the villages. Before hibernating, the animal carefully arranges its den. This is very important, since he will have to live in it for six months.
Very wonderful animal bear
A bear is a very large animal and that is why when you see it on a walk with its offspring. You should be extremely careful. The bear reacts very sharply to people, especially when her fragile offspring are nearby, so an attack can become a defensive reaction and then you will have no chance of survival. You should know that an attack on a person by a bear is in most cases the person’s fault.
Bears are very funny and lively The animal simply protects its offspring. This is an unconditioned reflex. Bear is main character numerous fairy tales, films and legends. It is customary to depict this animal paw sucking. Not everyone can answer with complete confidence whether such behavior is really observed in the specified animal. In fact, this is true, but the explanation for this behavior lies in the fact that upper layer The animal's paw is very rough.
Why does a bear suck its paw?
For a bear, the molting process is accompanied by itching, and therefore sometimes he simply chews off the skin on his paw, and from the outside it looks as if he is sucking it. It is worth saying a few more words about hibernation. You should know that not all types of bears hibernate. The most beautiful view The polar bear has always been considered a bear. It has snow-white fur that perfectly camouflages it White snow, representatives of this species can do without hibernation, but this only applies to males. Females still go into hibernation.
Sucking a paw is the same as sleeping This is due to the fact that they are responsible for caring for the offspring. Mother bears must give their cubs fatty milk so that they can survive. harsh winter. That's the only reason they hibernate. Male polar bears spend the winter well, eating fish. This is an excellent diet for this animal. Bears are unique animals. This is the most popular animal that is found in Russian folk art.
The bear loves his hut The song the bear sleeps in winter is a work that has long been loved by everyone. The lyrics to the song can be found on the Internet. This great option for karaoke performance. No one will deny that the bear is a very beautiful animal, which sometimes itself needs human protection from extermination by poachers. You must always remember that during hibernation this animal is most vulnerable, so you should not take advantage of this and kill a defenseless animal. Love nature, and it will love you back.
Video
Ksenia Kondrasheva answers,
Candidate of Biological Sciences:
They prepare in advance
Bears prepare for hibernation thoroughly, already from the end of summer - when daylight hours begin to shorten, and there is still plenty of food. The process can be compared to fattening a pig: the bear consumes up to 20,000 kcal per day, gaining up to 15 cm of fat per season. In addition to the usual fat - white - there is also brown fat, in which unsaturated fatty acids predominate. Fats will help you survive a long period without food.
Technically, hibernation looks like this:
- tocopherol, which accumulates in adipose tissue and in the liver, inhibits the level of metabolism;
- the level of serotonin increases in the brain, which, having a powerful vasoconstrictor effect, sharply suppresses the activity of the central nervous system, endocrine and other organs, slows down heat formation, which leads to a decrease in body temperature and also a decrease in metabolism;
- in the den where the bear goes before hibernation, the oxygen content decreases, the concentration of carbon dioxide increases and the temperature decreases environment. And each of these reasons also contributes to the animal’s hibernation.
They fall into stupor, but not into suspended animation
zivot.cas.sk
During hibernation, serious changes occur in the bear's body: due to a decrease in body temperature, the concentration of carbon dioxide in the blood increases, and an increase in blood acidity in combination with other factors causes a slowdown in breathing and heart rate. Under the influence of hypoxia (lack of oxygen. - Ed.) and hypothermia (cooling. - Ed.) complete muscle relaxation and slight numbness occur.
This state of reduced functional activity of living organisms, caused by factors external environment, is called hypobiosis (and this is not the same as suspended animation; suspended animation is the suspension of the body’s vital functions with subsequent restoration under favorable conditions - Ed.).
The breathing and heart rate of a hibernating bear decreases four to five times, body temperature drops by 2-7 °C, and general level metabolism - by 50-70%. Due to fat reserves large bears can exist in a state of sleep for several months. All this time, they maintain a constant high body temperature and basic physiological processes, maintain the activity of most tissues (wounds received before hibernation heal and new hair appears where the hair was damaged) and organs, as well as the constancy of the internal environment.
They don't eat or urinate
A hibernating bear uses very little energy to maintain life. Instead of getting it from food, it slowly burns the fat accumulated over the summer. When fat is completely burned, it forms carbon dioxide and water. During hibernation, the bear does not urinate, that is, practically does not lose water. Therefore, even without water, the animal is able to maintain its water balance tolerably by burning fat. Losses in a bear’s body weight over the entire hibernation period range from 15 to 25%.
They are not alone
Just like bears, in winter the badger, raccoon dog and striped raccoon fall into a long sleep. But marmots, gophers, hedgehogs, the bats, some amphibians and reptiles plunge into deep hibernation: their body temperature can drop to sub-zero values, and their heart beats only once or twice a minute.
It’s good for those who have wings - they flew away and that’s it. well and brown bear through the thickets and wild forest can’t get to places where the climate is warmer.
And he finds a rather practical solution. In the summer, the bear eats its food and then goes into hibernation until spring. But not everything is as simple as it might seem at first glance. Imagine what you would be like if you didn’t drink or eat for six months. Let's get acquainted with some amazing processes that occur in the body of a bear during hibernation.
Busy summer
To prepare for the six-month “fast,” the she-bear needs to make energy reserves.” So she doesn't worry about her figure. Its main goal is to accumulate more subcutaneous fat (in some places its thickness reaches eight centimeters). Although she likes sweet berries best, she is not picky about food. She eats everything: roots, small mammals, fish and ants. By autumn, she can gain weight up to 130-160 kilograms, a third of which is fat. (The male can weigh up to 300 kilograms.) Before plunging into the world of dreams, she stops eating and emptys her intestines. For the next six months she does not eat anything, does not urinate or defecate.
Bears choose a place for a den in a cave, an abandoned anthill or a depression under the roots of trees. The main thing is that it is quiet there and no one disturbs your sweet sleep. Bears collect fir branches, moss, peat and other materials to make a warm and cozy bed. The den is not much larger than the massive body of the bear. When winter comes, snow will cover the den and only an attentive observer will be able to see the hole through which air enters there.
Hibernation
Some small mammals, such as hedgehogs, bats and dormouse, fall into real hibernation, that is, they carry out most winters in a state similar to death. Their body temperature approaches the ambient temperature. But a bear's body temperature only drops by 5 degrees Celsius, so its sleep is not that deep. “You can’t say that a bear ‘sleeps without its hind legs.’ The bear raises its head and turns from side to side almost every day,” says Raimo Hissa, a professor at the University of Oulu in Finland, who has spent many years studying bear hibernation. Yet the bear rarely comes out. from her den in the middle of winter. During hibernation, the animal’s body works “in economy mode.” The heart rate drops to 10 per minute, and the metabolic process slows down. When the bear sleeps sweetly, fats begin to be burned in her body. Fatty tissues are broken down by enzymes and supply the animal's body with the necessary calories and water. Although the processes that support life in the body slow down, a certain amount of waste is generated as a result of metabolism. How can a mother bear get rid of it and at the same time keep her den clean? Instead of removing waste , the body processes them!
Professor Hissa explains: “Urea from the kidneys and Bladder reabsorbed into the blood and transported circulatory system into the intestines, where it is hydrolyzed by bacteria into ammonia.” Even more surprising is that this ammonia goes back to the liver, where it participates in the formation of new amino acids that form the basis of proteins. Converting waste products into Construction Materials, the bear's body feeds itself during a long period of hibernation!
In the old days, people hunted bears sleeping in dens. Sleepy Toptygin became easy prey. First, hunters on skis found a den, then surrounded it. After this, the bear was woken up and killed. Today, winter bear hunting is considered a cruel activity, and it is banned almost throughout Europe.
Studying bear hibernation
Research has been carried out for several years at the Department of Zoology of the University of Oulu physiological processes, with the help of which animals adapt to the cold. Brown bears began to be studied in 1988, and a total of 20 individuals were observed over these years. A special den was created for them in the zoological garden of the university. To measure body temperature, study metabolism, vital activity, as well as changes that occur during hibernation in the blood and hormones, scientists used computers, video cameras, and did laboratory tests. Biologists collaborated with specialists from other universities, even Japanese ones. They hope that the research results will be useful for solving problems related to human psychology.
New life
The bear sleeps all winter, turning over from side to side, but what happens in the life of the bear is an important event. Bears mate in early summer, but the fertilized cells inside the expectant mother's body do not develop until the mother hibernates. The embryos then attach to the wall of the uterus and begin to grow. After just two months (in December or January), the body temperature is expectant mother rises a little, and she gives birth to two or three cubs. After this, her body temperature drops again, although it does not become as low as before childbirth. Papa Bear does not see his children being born. But the sight of newborns would probably disappoint him. It would be difficult for a huge dad to recognize these tiny creatures weighing less than 350 grams as his offspring.
The she-bear feeds the cubs with nutritious milk, this depletes her already weakened vitality. The cubs grow quickly, by spring they become fluffy and already weigh about five kilograms. This means that the bear’s small “apartment” is full of excitement.
Spring
March. Cold winter has passed, the snow is melting, the birds are returning from the south. At the end of the month, male bears emerge from their dens. But the mother bears remain in their shelter for several more weeks, perhaps because the babies take a lot of their energy.
After long hibernation All that remains of a well-fed bear is skin and bones. The snow melted, and with it her fat melted. With all this, the bear is surprisingly mobile - no bedsores, seizures or osteoporosis. Some time after leaving the den, she cleanses the intestines. Typically, bears begin to eat only two or three weeks after waking up, since the body does not immediately get used to the new conditions. But then they develop a remarkable appetite. But since nature itself has recently awakened from winter sleep, at first there is not much food in the forest. Bears chew larvae and beetles, eat old carcasses, and sometimes even hunt reindeer.
The care of raising cubs falls on the shoulders of the mother bear, and she protects her cubs like the apple of her eye. An ancient proverb says: “It is better for a man to meet a mother bear without children than a fool with his foolishness” (Proverbs 17:12). In other words, it is better not to date either one or the other. “Mama bear has a lot on her plate. If a male bear approaches, she immediately forces the cubs to climb a tree. The point is that the male can harm them, even if he is their father,” explains Hissa.
The cubs spend another winter in the den with their mother. Well, next year they have to look for their own den, since the bear will have new tiny cubs.
We already know a lot about the complex and unusual phenomenon of bear hibernation, but much still remains a mystery. For example, why does a bear become sleepy in the fall and why does it lose its appetite? Why doesn't he have osteoporosis? Uncovering a bear's secrets is not easy, and that's understandable. Everyone has their own secrets!
Only pregnant females and those nursing newborn cubs fall into winter sleep. The rest of the polar bears, if they lie in dens, do so only for a short time and not every year.
Surprisingly, they barely sleep! That is, they sleep normally, just like in the summer (only in the summer they usually sleep more). But they do not fall into winter sleep. (“Hibernation” of bears is more correctly called winter sleep; bears do not have real hibernation, since their body temperature hardly drops, and they can wake up at any moment.) Only females who are pregnant and nursing newborn cubs fall into winter sleep. The rest of the polar bears, if they lie in dens, do so only for a short time and not every year.
And this is surprising because the polar bear is the closest relative of the brown bear. They descended from common ancestors who lived only 150 thousand years ago (this is very recent for the evolution of species). Even in the wild, these species sometimes interbreed. At the same time, in their way of life, polar bears differ sharply from brown bears.
The main food of polar bears is seals. These are such seals. Polar bears hunt them on the ice. They either snatch the seal with their paw from a hole in the ice through which the seal is breathing, or they lie in wait and grab seals that have climbed out onto the ice to rest. In many areas of the Arctic where polar bears live, the ice almost completely melts by the end of summer. They can no longer hunt seals. On land, most Arctic animals are able to outrun a polar bear, and in the sea, they can swim away from it. It’s good if you can find the carcass of a dead whale or walrus on the shore. And if not, then at the end of summer and autumn the bears sometimes starve for several months. So in winter they do not sleep, but start hunting again as soon as ice appears.
But the females have nowhere to go - they have to lie down in dens. After all, polar bear cubs, like other bears, are born small (their weight is less than a kilogram) and blind; they are covered only with short fluff. Usually females make a den on the shore, sometimes 50 km from the seashore. As a rule, a bear makes a den in a snow dune, but if there is not enough snow, she can dig a hole in frozen ground. The female lies down in a den just when the ice melts and hunting becomes difficult. Cubs are usually born in November-January, and remain in the den until February-March. Before the cubs are born, the mother bear actually sleeps mostly, but during childbirth she wakes up, and after giving birth she has to sleep less. However, before leaving the den, she is still in a state of winter sleep: she does not eat, does not drink, does not pee or poop.
How does a female manage to accumulate nutrients for long sleep and for feeding cubs (and there are usually two of them)? It turns out that polar bears mate in the spring - in April-May. Immediately after mating, pregnant females begin to feed so intensely that by autumn they become 200 kg heavier - their weight sometimes almost doubles! At the same time, the development of embryos in the female bear’s abdomen stops at an early stage in the spring and continues only in the fall; Before this, they are in a state of rest (this is scientifically called embryonic diapause). Apparently, this allows female bears to “adjust” the onset of embryo development to the time they enter the den; after all, this time greatly depends on the conditions in a given area and even on the weather in a given year.
It’s not very clear why all polar bears shouldn’t eat their fill in the same way. But for some reason they don't do this.
It is interesting that, apparently, at any time of the year, during prolonged fasting, polar bears seem to “sleep on the move.” The concentration of urea in their blood drops sharply, which is typical for other species of bears during hibernation. Bears are able to use urea to synthesize amino acids and proteins in the plasma (liquid part) of the blood. (The concentration of proteins in plasma should be as constant as possible, otherwise various problems with fluid transport and metabolism in the body arise.) In addition, the lower the urea content, the less it needs to be excreted in the urine, which means the less you need to drink. Although water in the form of snow is usually readily available in the Arctic, drinking (or rather, eating) it is energetically unprofitable - a lot of energy is lost to warm it.
If a brown bear's urea concentration has decreased, it becomes lethargic, no longer wants to eat, and falls asleep. But when food is available, the polar bear begins to eat again and raises the urea concentration to normal levels.
It is interesting that during the period of winter sleep, the polar bear somehow manages to lose almost no mass of bones and muscles. Usually in humans and other animals their weight decreases sharply with prolonged immobility, even when there is food; The mass of bones and muscles also decreases in other species of bears during sleep. But the polar bear uses almost only fat. It turns out that in some respects polar bears are better adapted to winter sleep.