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Hibernation

Hibernation

Hibernation is a state of inactivity, in which an animal's heart rate, body temperature, and breathing rate are decreased in order to conserve energy through the cold months of winter. A similar state, known as estivation, occurs in some desert animals during the dry months of summer.

Hibernation is a technique that animals have developed in order to adapt to harsh climates. When food is scarce, an animal may use up more energy maintaining its body temperature and in searching for food than it would receive from consuming the food. Hibernating animals use 70 to 100 times less energy than when active, allowing them to survive until food is once again plentiful.

Many animals sleep more often when food is scarce, but only a few truly hibernate. Hibernation differs from sleep in that a hibernating animal shows a drastic reduction in metabolism, and then awakes relatively slowly. (Metabolism is the process by which cells in an organism break down compounds to produce energy.) By contrast, a sleeping animal decreases its metabolism only slightly, and can wake up almost instantly if disturbed. Also, hibernating animals do not show periods of rapid eye movement (REM), the stage of sleep associated with dreaming in humans.

Bears, which many people think of as the classic hibernating animals, are actually just deep sleepers. They do not significantly lower their metabolism and body temperature. True hibernation occurs only in small mammals, such as bats and woodchucks, and a few birds, such as poorwills and nighthawks. Some species of insects show periods of inactivity during which growth and development cease and metabolism is greatly reduced. This state is generally referred to as diapause, although when correlated with the winter months, it would also fit the definition of hibernation.

Preparing for hibernation

Animals prepare for hibernation in the fall by storing enough food to last them until spring. Chipmunks accomplish this task by filling their burrows with food, which they consume during periodic arousals from hibernation throughout the winter. Most animals, however, store energy internally, as fat. A woodchuck in early summer may have only about 5 percent body fat. However, as fall approaches, changes occur in the animal's brain chemistry that cause it to feel hungry and to eat constantly. As a result, the woodchuck's body fat increases to about 15 percent of its total weight. In other animals, such as the dormouse, fat may comprise as much as 50 percent of the animal's weight by the time hibernation begins. A short period of fasting usually follows the feeding frenzy, to ensure that the digestive tract is completely emptied before hibernation begins.

Entering hibernation

Going into hibernation is a gradual process. Over a period of days, an animal's heart rate and breathing rate drop slowly, eventually

reaching rates of just a few times per minute. Their body temperature also drops from levels of 37° to 38°C (99° to 100°F) to 10° to 20°C (50° to 70°F). The lowered body temperature makes fewer demands on metabolism and food stores.

Electrical activity in the brain almost completely ceases during hibernation, although some areas remain active. These areas are those that respond to external stimuli such as light, temperature, and noise. Thus, the hibernating animal can be aroused under extreme conditions.

Arousal

Periodically, perhaps every two weeks or so, the hibernating animal awakes and takes a few deep breaths to refresh its air supply, or in the case of the chipmunk, to grab a bite to eat. If the weather is particularly mild, some animals may venture above ground. These animals, including chipmunks, skunks, and raccoons, are sometimes called shallow hibernators.

Arousal begins with an increase in the heart rate. Blood vessels dilate, particularly around the heart, lungs and brain, leading to an increased breathing rate. Eventually, the increase in circulation and metabolic activity spreads throughout the body, reaching the hindquarters last. It usually takes several hours for the animal to become fully active.

[See also Metabolism ]

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hibernation

hibernation (hī´bərnā´shən) [Lat.,= wintering], practice, among certain animals, of spending part of the cold season in a more or less dormant state, apparently as protection from cold when normal body temperature cannot be maintained and food is scarce. Hibernating animals are able to store enough food in their bodies to carry them over until food is again obtainable. They do not grow during hibernation, and all body activities are reduced to a minimum: there may be as few as one or two heartbeats a minute. Cold-blooded animals (e.g., insects, reptiles, amphibians, and fish) must hibernate if they live in environments where the temperature—and hence their own body temperature—drops below freezing. Some insects pass their larval stage in a state of hibernation; in such cases hibernation is closely associated with the reproductive cycle (see larva; pupa). However, most warm-blooded animals, i.e., birds and mammals, can survive freezing environments because their metabolism controls their body temperatures. Many hibernating animals seek insulation from excessive cold; bears and bats retire to caves, and frogs and fish bury themselves in pond bottoms below the frost line. Analogous to hibernation is aestivation, a dormant period of escape from heat and drought. Other methods of avoiding excessively high or low temperatures and destructive increases or decreases in the water supply are encystment and ensuing dormancy, e.g., in plant seeds and bacteria, and migration. Some animals, such as rabbits, raccoons, and squirrels, store food against scarcity and spend cold periods asleep in their burrows, though they may emerge on warm days.

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hibernation

hibernation A sleeplike state in which some animals pass the winter months as a way of surviving food scarcity and cold weather. Various physiological changes occur, such as lowering of the body temperature and slowing of the pulse rate and other vital processes, and the animal lives on its reserve of body fat. Animals that hibernate include bats, hedgehogs, and many fish, amphibians, and reptiles. See also dormancy. Compare aestivation.

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hibernation

hibernation A strategy for surviving winter cold that is characteristic of some mammals. Metabolic rate is reduced to a minimum and the animal enters a deep sleep, surviving on food reserves stored in the body during the favourable summer period. Compare AESTIVATION. See also TORPOR.

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hibernation

hibernation Dormant (sleep-like) condition adopted by some animals, such as bears, bats and squirrels, to survive harsh winters. Adaptive mechanisms to avoid starvation and extreme temperatures include reduced body temperature, slower heartbeat, breathing rate, and metabolism.

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hibernation

hibernation A strategy for surviving winter cold that is characteristic of some mammals. Metabolic rate is reduced to a minimum and the animal enters a deep sleep, surviving on food reserves stored in the body during the favourable summer period. Compare aestivation.

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Hibernation

Hibernation

Preparing for hibernation

Entering hibernation

Arousal

The importance of understanding hibernation

Resources

Hibernation is a state of inactivity, or torpor, in which an animals heart rate, body temperature, and breathing rate are decreased in order to conserve energy through the cold months of winter. A similar state, known as estivation, occurs in some desert animals during the dry months of summer.

Hibernation is an important adaptation to harsh climates, because when food is scarce, an animal may use up more energy maintaining its body temperature and in foraging for food than it would receive from consuming the food. Hibernating animals use 70 to 100 times less energy than when active, allowing them to survive until food is once again plentiful.

Many animals sleep more often when food is scarce, but only a few truly hibernate. Hibernation differs from sleep in that a hibernating animal shows a drastic reduction in metabolism, or its rate of energy usage, and arouses relatively slowly, while a sleeping animal decreases its metabolism only slightly, and can wake up almost instantly if disturbed. In addition, hibernating animals do not show periods of rapid eye movement (REM), the stage of sleep associated with dreaming in humans.

Bears, which many people think of as the classic hibernating animals, are actually just deep sleepers, and do not significantly lower their metabolism and body temperature. True physiological hibernation occurs only in small mammals, such as bats and woodchucks, and a few birds, such as poorwills and nighthawks. Some species of insect show periods of inactivity where growth and development are arrested and metabolism is greatly reduced: this state is generally referred to as diapause, although when correlated with the winter months it would also fit the definition

of hibernation. Some hibernating mammals are pregnant while hibernating, and give birth soon after awakening from sleep.

In 2004, German physiologist Kathrin Dausmann announced that fat-tailed dwarf lemurs hibernate in Madagascar for about one-half of the year. This discovery is the first tropical mammal and the first primate to be found to hibernate.

Preparing for hibernation

Animals prepare for hibernation in the fall by storing enough energy to last them until spring. Chipmunks accomplish this by filling their burrows with food, which they consume during periodic arousal from torpor throughout the winter. Most animals, however, store energy internally, as fat. A woodchuck in early summer may have only about 5% body fat, but as fall approaches changes occur in the animals brainchemistry that cause it to feel hungry and to eat constantly, which results in an increase to about 15% body fat. In other animals, such as the dormouse, fat may comprise as much as 50% of the animals weight by the time hibernation begins. A short period of fasting usually follows the feeding frenzy, to ensure that the digestive tract is completely emptied before hibernation begins.

Many hibernators also produce a layer of specialized fat known as brown fat (brown adipose tissue), which lies between the shoulder blades of the animal. Brown fat is capable of rapidly producing large amounts of heat when it is metabolized, which raises the animals body temperature and brings about the eventual arousal of the animal from hibernation.

Entering hibernation

Going into hibernation is a gradual process. Over a period of days, the heart rate and breathing rate drop slowly, reaching slow steady rates of just a few times per minute. The body temperature also plummets from mammalian levels of 101.5 to 103.5°F (38.6 to 39.7°C) to 50 to 68°F (10 to 20°C). The lowered body temperature is regulated about the new set point, and therefore makes fewer demands on metabolism and food stores.

Electrical activity in the brain almost completely ceases during hibernation, although some areas remain active. These are primarily areas that respond to external stimuli such as light, temperature, and noise, so that the hibernating animal can be aroused under extreme conditions.

Arousal

Periodically, perhaps every two weeks or so, the hibernating animal will arouse and take a few deep breaths to refresh its air supply, or in the case of the chipmunk, to grab a bite to eat. If it is a particularly mild winter day, some animals may venture above ground. These animals, including chipmunks, skunks, and raccoons, are sometimes called shallow hibernators.

Arousal begins with an increase in the heart rate. Blood vessels dilate, particularly around the heart, lungs, and brain, leading to an increased breathing rate. Blood also flows into the layer of brown fat, increasing activity there and causing a rise in body temperature. Eventually, the increase in circulation and metabolic activity spreads throughout the body, reaching the hindquarters last. It usually takes several hours for the animal to become fully active.

The importance of understanding hibernation

Scientists are interested in discovering the mechanisms that control hibernation and arousal, and the means by which animals survive such critically low metabolic activity. Many researchers hope to discover ways of placing human beings into a state of hibernation, thus allowing them to survive medical operations that cut off much of the supply of blood to the brain, or even to embark on long space voyages to planets such as Mars. Other researchers look at the changes in

KEY TERMS

Torpor A state of dormancy or inactivity.

brain chemistry of hibernators as a way of understanding obesity in humans, or as a way to unravel the mysteries of sleep and the functioning of the human brain.

Resources

BOOKS

Ganeri, Anita. Hibernation. Chicago, IL: Heinemann Library, 2005.

Perry, Phyllis Jean. Animals That Hibernate. New York: F. Watts, 2001.

David E. Fontes

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Hibernation

Hibernation

Hibernation is a state of inactivity, or torpor, in which an animal's heart rate , body temperature , and breathing rate are decreased in order to conserve energy through the cold months of winter. A similar state, known as estivation, occurs in some desert animals during the dry months of summer. Hibernation is an important adaptation to harsh climates, because when food is scarce, an animal may use up more energy maintaining its body temperature and in foraging for food than it would receive from consuming the food. Hibernating animals use 70-100 times less energy than when active, allowing them to survive until food is once again plentiful.

Many animals sleep more often when food is scarce, but only a few truly hibernate. Hibernation differs from sleep in that a hibernating animal shows a drastic reduction in metabolism , or its rate of energy usage, and arouses relatively slowly, while a sleeping animal decreases its metabolism only slightly, and can wake up almost instantly if disturbed. Also, hibernating animals do not show periods of rapid eye movement (REM), the stage of sleep associated with dreaming in humans. Bears , which many people think of as the classic hibernating animals, are actually just deep sleepers, and do not significantly lower their metabolism and body temperature. True physiological hibernation occurs only in small mammals , such as bats and woodchucks, and a few birds , such as poorwills and nighthawks. Some species of insect show periods of inactivity where growth and development are arrested and metabolism is greatly reduced: this state is generally referred to as diapause, although when correlated with the winter months it would also fit the definition of hibernation.


Preparing for hibernation

Animals prepare for hibernation in the fall by storing enough energy to last them until spring. Chipmunks accomplish this by filling their burrows with food, which they consume during periodic arousal from torpor throughout the winter. Most animals, however, store energy internally, as fat . A woodchuck in early summer may have only about 5% body fat, but as fall approaches changes occur in the animal's brain chemistry which cause it to feel hungry and to eat constantly, which results in an increase to about 15% body fat. In other animals, such as the dormouse , fat may comprise as much as 50% of the animal's weight by the time hibernation begins. A short period of fasting usually follows the feeding frenzy, to ensure that the digestive tract is completely emptied before hibernation begins.

Many hibernators also produce a layer of specialized fat known as brown fat (brown adipose tissue ) which lies between the shoulder blades of the animal. Brown fat is capable of rapidly producing large amounts of heat when it is metabolized, which raises the animal's body temperature and brings about the eventual arousal of the animal from hibernation.



Entering hibernation

Going into hibernation is a gradual process. Over a period of days, the heart rate and breathing rate drop slowly, reaching slow steady rates of just a few times per minute. The body temperature also plummets from mammalian levels of 101.5–103.5°F (38.6–39.7°C) to 50–68°F (10–20°C). The lowered body temperature is regulated about the new set point, and therefore makes fewer demands on metabolism and food stores.

Electrical activity in the brain almost completely ceases during hibernation, although some areas remain active. These are primarily areas which respond to external stimuli such as light , temperature, and noise, so that the hibernating animal can be aroused under extreme conditions.


Arousal

Periodically, perhaps every two weeks or so, the hibernating animal will arouse and take a few deep breaths to refresh its air supply, or in the case of the chipmunk, to grab a bite to eat. If it is a particularly mild winter day, some animals may venture above ground. These animals, including chipmunks, skunks , and raccoons , are sometimes called "shallow" hibernators.

Arousal begins with an increase in the heart rate. Blood vessels dilate, particularly around the heart, lungs, and brain, leading to an increased breathing rate. Blood also flows into the layer of brown fat, increasing activity there and causing a rise in body temperature. Eventually, the increase in circulation and metabolic activity spreads throughout the body, reaching the hindquarters last. It usually takes several hours for the animal to become fully active.


The importance of understanding hibernation

Scientists are interested in discovering the mechanisms which control hibernation and arousal, and the means by which animals survive such critically low metabolic activity. Many researchers hope to discover ways of placing human beings into a state of hibernation, thus allowing them to survive medical operations which cut off much of the supply of blood to the brain, or even to embark on long space voyages. Other researchers look at the changes in brain chemistry of hibernators as a way of understanding obesity in humans, or as a way to unravel the mysteries of sleep and the functioning of the human brain.


Resources

books

Stidworthy, John. Hibernation. New York: Gloucester Press, 1991.

periodicals

Kenzie, Aline. "Seeking the Mechanisms of Hibernation." Bio-Science 40 (June 1990).


David E. Fontes

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Torpor

—A state of dormancy or inactivity.

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Hibernation

Hibernation


Hibernation is a special type of deep sleep that enables an animal to survive the extreme winter cold. Hibernation lowers an animal's energy needs and allows it to live off stored fat and not have to search for scarce food. Hibernation is a form of cyclic behavior and is triggered by different cues in the animal's environment.

All animals have different survival tactics that allow them to live through difficult or life-threatening situations. The steady, severe cold that comes with winter poses a problem to animals who do not escape it (by migrating or leaving) nor adapt to it (such as by growing an extra-thick coat of fur or a layer of fat). Winter is difficult for all warm-blooded animals (those that maintain a constant internal temperature despite their environment), since they must spend most of their energy just keeping warm. When the temperature falls below freezing, these animals must eat even more than they usually do simply to produce enough internal heat to stay alive. Nature makes things even more difficult in winter, since at a time when warm-blooded animals need to increase their intake of food, it has suddenly become very scarce.

For certain animals, hibernation is a simple way to solve the particular problems posed by winter cold, since they basically sleep through winter and wake when the weather has become mild. However, hibernation is a fairly complicated physiological event. When an animal hibernates, its body processes such as breathing and heartbeat slow down, sometimes to the point that the animal appears dead. While hibernation could be described simply as a very deep sleep, it is anything but simple. True hibernators are usually small mammals like woodchucks, mice, or ground squirrels. When the time comes to hibernate, the animal responds to one of several environmental "cues," such as a certain low temperature or a reduction in the hours of daylight. These cues trigger the release of a hormone (a chemical messenger) called hibernation induction trigger (HIT) that causes major changes in the animal's body. Its heartbeat becomes slow and weak and its body temperature drops many degrees. It takes a few breaths every minute and it makes hardly any waste (urine). As a true hibernator, the animal falls into such a deep sleep that it looks dead and sometimes cannot even be awakened if picked up. All of these reactions triggered by the hormone allow the animal to maintain its necessary body processes while using far less energy than if it were awake.

Before they fall asleep for the season, hibernators usually develop huge appetites that allow them to store as much fat as possible to be burned later while sleeping. They also usually prepare the den or burrow where they sleep to comfortably insulate themselves from the cold. As spring approaches, different cues in their environment, like warmer temperatures or lengthening daylight, awaken them and they soon resume their normal level of activity. This does not happen immediately, however. As their heart rate increases, along with their blood pressure and respiration, hibernators usually begin to shiver, which slowly raises their body temperature. After readjusting to this now-high rate of metabolism (the chemical processes that take place in an organism), they are ready for normal activity.

Like their warm-blooded counterparts, cold-blooded animals (whose temperature changes with the surroundings) also hibernate. Animals like frogs, turtles, and snakes bury themselves in the mud where their slowed-down systems find just enough trapped oxygen to stay alive. Some insects like butterflies also hibernate in the open, and their systems produce chemicals that act as antifreeze.

Other animals like bears, raccoons, and skunks are not true hibernators, although they do very much the same thing. Rather, they are considered "light sleepers" since their bodily functions do not fall to such low rates as those of true hibernators. These "light sleepers" also sometimes wake up and eat something that they may have stored or even go outside to eliminate waste. Brown bears can awaken very quickly, and often give birth during a long, cold winter. Even true hibernators have built-in mechanisms to awaken them under unusual conditions. For example, if temperatures fall to such a low that even the hibernating animal is in danger of freezing to death, its body will automatically switch to actively producing heat.

Another form of hibernation occurs in the summer. This is called "estivation," and is a process similar to hibernation. Certain desert animals estivate underground when they are threatened by prolonged, extreme heat or drought. Estivation is what enables many desert dwellers to survive during the very hot summer months.

[See alsoMetabolism ]

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Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.