What happens to the bear during hibernation? Specialist Comment

Each fall, bears of temperate and polar latitudes (in particular, brown and black) begin to prepare for hibernation. Throughout spring, summer and autumn, these animals actively fed, feeding their fat stores for the winter. And now, when the cold comes, they are looking for suitable shelter in order to winter. After shelter is found, the bear hibernates.

Hibernation of bears in some cases lasts up to six months. During hibernation, some species, such as the black bear (Ursus americanus), reduce heart rate from 55 beats per minute to about 9. The metabolic rate decreases by 53%. Naturally, all this time the bears do not eat, drink or produce waste products. How do they do it?

To understand what happens in the bear’s body during hibernation, you must immediately clarify what hibernation itself is. And why is it not "suspended animation" in the truest sense of the word. In the literal sense of the term "suspended animation" is a process of complete inactivity of the animal. At this time, the level of metabolism decreases to indicators that for most higher animals are incompatible with life.

Some species of amphibians (some newts and frogs) freeze in the cold, thawing without harm to themselves when the warm season sets in. This “freezing” is literally through and through for them in connection with the development of a specific substance having the properties of antifreeze, which prevents the freezing of water in their body.

Bears do not freeze. The temperature of their body during hibernation remains high enough, which allows them to wake up in case of any danger, leaving the den. By the way, bears that woke up ahead of time are called "connecting rods." They pose a significant danger to humans, because in winter the bear cannot find enough food, and is always hungry and aggressive.

Some researchers claim that bears do not fall into suspended animation, as mentioned above. But there are scientists who call the bears “super-suspended” because they do not eat, drink or defecate for six months, while remaining able to quickly get out of hibernation - this is a unique phenomenon in the animal kingdom.

“In my opinion, bears are the best suspended animals in the world,” says Brian Barnes of the Institute of Arctic Biology, University of Alaska (Fairbanks). This scientist spent three years studying the features of hibernation of black bears.

“Their body is a closed system. They can spend the whole winter using only oxygen for breathing - that's all they need, ”says Barnes.

Why don't bears defecate while hibernating? In short, it is because in their body at this time a fecal plug forms . This is a special mass that researchers have long found in the esophagus of hibernated bears.

Previously, it was believed that bears before getting into a den, eat a large amount of plant material, wool of other bears and other materials that are not digested, and which then form a cork in the intestines of the animal. Scientists who came to this conclusion relied heavily on information received from bear hunters. Those claimed that the nutritional method mentioned above led to "intestinal fastening" and the animal simply could not carry out an act of defecation during sleep.

In fact, this is not so. Bears do not eat anything special before hibernation. They, like omnivores, try to consume any food available to them, including fruits, vegetables, nuts, meat, fish, berries and much more.



And during hibernation, the intestine of the animal continues to work. Not in the previous activity mode, but still it works. Cells continue to divide, intestinal secretion is carried out. All this forms a small amount of feces, which accumulate in the intestines of the animal. A “plug” is formed with a diameter of 3.8 to 6.4 centimeters.

“Fecal plugs are the same waste products that have been in the intestines of an animal for so long that the intestinal walls absorb liquids from this mass, leaving it dry and hard,” says the site of the North American Bear Research Center. Thus, the bear’s body does not lose the water it needs, the reserves of which are almost impossible to replenish in the den.

Specialists placed cameras in the bear’s dens, which recorded everything that happened during hibernation. As it turned out, plant fibers and wool are often an integral part of cork because a bear, even during hibernation, can pick up something from the ground in a den, or it can lick its wool.



After the bear leaves the den, they cleanse the intestines, which begins to function normally. Typically, bowel movements occur already at the threshold of the den. Therefore, there is no mysticism or riddle, as some hunters or even scientists say, in a bear jam. All this is a product of the vital activity of the body. By the way, the bear in the den does not suck its paw at all. The fact is that in January and February there is a change in the skin on the paw pads. The old skin breaks, itches, which causes the bear some inconvenience. In order to relieve itching, the bear licks its paws.

In order to clarify the details of the process of hibernation in bears, I requested a comment from scientists from the Krivoy Rog State Pedagogical University.

How do bears keep their body hibernated?

Each animal exists due to the metabolism and energy that are provided by the consumed food. Naturally, the more active the lifestyle and the more intense the physiological processes, the more “fuel” in the form of food must be introduced into the body. In the body, which is at rest in the form of hibernation, the intensity of all metabolic processes is reduced to a physiological minimum. That is, energy is spent exactly as much as it is necessary so that the beast remains alive and that degenerative processes in tissues and organs do not occur due to a lack of energy. In general, this condition can be compared with what happens during normal sleep, but, of course, it is more "exaggerated."

The main consumers of energy in the body are the brain and muscles (at least 2/3 of the body’s energy). But since the muscular system is inactive during sleep, the energies of its cells receive exactly as much as is necessary to maintain their existence. Therefore, at "low speeds" other organs also begin to work, which also receive very little energy. The digestive system essentially has nothing to digest (since the intestines are almost empty, as mentioned above). Where, then, does this minimum amount of energy come from, which is nevertheless necessary for the beast? It is extracted from the reserves of fat and glycogen accumulated over the active period of the year. They are spent gradually and usually last until spring.

By the way, precisely those bears that "badly ate" in the summer become quite often cranks. There are many oral stories that there are more rods in the hungry years. So, the stores of fat and glycogen are the main source of energy. Another vital substance is oxygen. But since the body is inactive, then much less oxygen is needed. Thus, the respiratory rate is significantly reduced. And if the tissues of the body during hibernation require a very small amount of oxygen and nutrients, then the blood that carries them can move much more slowly. Therefore, the heart rate decreases significantly, and accordingly, the heart also consumes less energy. Water saving is associated not only with "blocking" the intestines, but with the actual suspension of kidney activity.

Are there any other examples of hibernation among warm-blooded animals?

Such a device as hibernation in bears is very unusual for warm-blooded phenomena, but not at all unique. Hedgehogs of moderate latitudes, marmots, inhabitants of the Eurasian steppes, some representatives of the Kunih family (badger) also have it. In especially cold and hungry winters, squirrels and raccoon dogs can fall into a similar state, but not for long, and their vital processes do not slow down like bears do. In addition to hibernation (hibernation), there is also summer hibernation (estivation). Some inhabitants of hot deserts (some insectivores, rodents, marsupials) fall into the latter.

This happens during the hottest periods of the year, when forage and water production become much more energy-consuming and, in fact, inefficient. Therefore, it is easier for an animal to hibernate and wait out adverse conditions. In addition to seasonal hibernation, there is also daily allowance. It is characteristic of some warm-blooded flying animals - hummingbirds and bats. The fact is that both one and the other very quickly flap their wings during the flight. Thanks to this, their flight became more maneuverable, and forage extraction more efficient. But you have to pay for everything in nature. Their flying muscles consume a lot of energy, which is not enough for a full day (despite the fact that both hummingbirds and bats consume food weighing more than half their own weight during the active phase of the day).

As you can see, their metabolic rate is simply colossal. Therefore, during sleep (and rest in the form of sleep is necessary for each animal - this is also a normal and obligatory physiological process), their vital activity decreases to parameters comparable to those observed in bears.

How does the state of hibernation of bears differ from, for example, suspended animation of frogs?

In warm-blooded physiological processes during hibernation cannot be completely “turned off”. That's why they are warm-blooded - you need independently produced heat. A different picture can be observed in poikilothermic animals - their vital processes are almost completely stopped. That is, the cells of the body remain in a practically preserved state until the time comes, when the sun warms up and gives enough heat to warm the body. This happens in all amphibians of temperate and more northern latitudes.

It is a known fact that individuals of the caudate amphibian Siberian chubber, after being literally frozen in the ice for several decades (!) After thawing, “came to life” and felt quite normal. Wintering snakes and lizards also fall into suspended animation, but their body is not so tenacious (they will not tolerate freezing). Another example is fish living in the drying up ponds of Africa, South America and Australia, and dug in silt for a period of drought. The processes occurring in their body during this period are close to those that occur in amphibians - an almost complete suspension of life until better times.

As for the reptiles of hot countries, it must be said that although they are cold-blooded, the experience of adverse conditions is more similar to that of warm-blooded ones - a significant decrease in the intensity of physiological processes, but not a halt (there is enough solar thermal energy). Large reptiles (crocodiles, pythons and boas) thus "rest" for up to a year, digesting the eaten large prey.

Is it possible to artificially create a hibernation mode for animals that do not hibernate?

Not. This will be an abnormal condition like a coma.

How could a similar wintering mechanism appear in bears? Has such a mechanism been developed for many hundreds of thousands of years or appeared spontaneously?

All physiological processes are genetically controlled. During evolution, a certain group of individuals could have a certain physiological peculiarity, consisting in a special sleep regimen (daily, normal) in the cold season, accompanied by a slight decrease in physiological activity and a drop in body temperature by 1-2 degrees.

This feature gave these individuals a certain advantage in terms of more economical energy consumption in conditions with less feed. Moreover, it began to give such a great advantage in survival that gradually only such mutants remained in the population. In the future, selection on this basis continued - sleep became more and more long and deep, and the intensity of the body's processes decreased more and more. Finally, animals learned to equip their lairs. By the way, this feature could give a significant advantage also because just during hibernation the female gives birth to cubs and at this time they are warm and protected, hidden from prying eyes. On the whole, the evolution of the winter hibernation phenomenon continued (and may continue) for no less than several hundred thousand years, of course.

For help in preparing the article, the editorial team thanks Geektimes:
Evgeny Olegovich Broshko, Ph.D., Senior Lecturer, Department of Zoology, KSPU (Krivoy Rog State Pedagogical University),
Eduard Alekseevich Evtushenko, Ph.D., Associate Professor, Department of Botany and Ecology, KSPU