Hibernation

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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.