Hibernation

V

Arousal from Hibernation

Scientists do not know what makes an animal periodically arouse from hibernation. Some scientists propose that torpid animals may be aroused by warmer temperatures, the need to excrete accumulated waste products in the body, or to replenish lost body water. An animal’s circadian rhythm, an internal biological clock with a daily cycle, may periodically arouse an animal so it can check its surroundings.

Some animal studies indicate that as body temperature drops during hibernation, the brain waves normally present during sleep gradually diminish. The sleep brain waves are absent altogether in a prolonged bout of hibernation. In addition, animals emerging from a bout of hibernation appear to be sleepy. These observations have led some scientists to hypothesize that animals may actually become sleep deprived during hibernation and that they arouse periodically in order to catch up on their sleep.

Ectothermic animals may emerge from torpor in response to warmer outside temperatures, which raise their body temperature and metabolic rate. An endothermic animal may use violent shivering and muscle contractions to increase its metabolism and shift its heat production to a maximum rate. These movements require an increase in heart rate and oxygen consumption, and soon body temperature begins to rise. Vital organs, such as the heart and brain, warm first, followed by rising temperatures in the extremities.

In some mammals a specialized tissue known as brown fat also helps warm the body. Brown fat is found in patches along the neck and between the shoulders of newborn mammals, including humans, and in adults of hibernating animals. Cells that contain ordinary fat burn fuel and use the energy to power various life processes. But brown fat cells burn fuel and release the energy directly as heat in a process known as nonshivering thermogenesis.

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The time required for an animal to arouse from hibernation depends on its body mass. Small animals less than 10 g (0.4 oz) in weight can warm their bodies at maximum rates of 1 Celsius degree (2 Fahrenheit degrees) per minute. Larger animals of more than 5 kg (10 lb) manage rewarming around only 0.1 Celsius degrees (0.2 Fahrenheit degrees) per minute. Animals may spend periods of a few hours to a few days in an active state before entering another bout of hibernation.

VI

Other Types of Torpor

The term hibernation is often used to describe any state of winter dormancy, and it is sometimes applied to bears, shrews, deer mice, and other animals that are inactive in winter. But the dormancy experienced by these animals differs from hibernation. Internal body activity decreases sharply in hibernating animals while body temperature and metabolism drop only slightly in other animals experiencing winter inactivity.

In one type of animal dormancy, known as daily torpor, an animal may become inactive at any time of the year for periods lasting less than a day. During daily torpor, an animal’s body temperature, metabolic rate, and other physiological functions do not decrease as much as in hibernation. Daily torpor is widespread among many species of shrews, skunks, badgers, deer mice, gerbils, lemurs, and tenrecs, a group of primitive, insect-eating mammals that inhabit the island of Madagascar. Among birds, daily torpor is found in nighthawks, nightjars, pigeons, hummingbirds, swifts, martins, swallows, manakins, and sunbirds. Unlike hibernation, which often involves significant preparation to help the animal avoid an energy shortfall, daily torpor is often triggered at any time that food is scarce or internal fuel reserves become depleted.

A type of dormancy known as estivation occurs in certain animals for extended periods during the summer, when temperatures are high and water is scarce. Various species of desert-dwelling ground squirrels, insects, and snails experience this type of dormancy. Although little is known about estivation, scientists have established that estivating animals decrease their metabolism and breathing rate and are able to conserve body water.

Many birds exhibit nocturnal hypothermia, in which body temperature drops by about 3 to 5 Celsius degrees (5 to 9 Fahrenheit degrees) during the night and the metabolic rate falls to about half the rate of resting birds during the day. Nocturnal hypothermia has been observed in chickadees, finches, sparrows, silvereyes, honeyeaters, and many other small birds.

Some mammals, such as brown bears, spend several months during the winter sleeping in dens without eating, a condition that resembles hibernation. But in these animals, body temperature and metabolism drop only slightly. On warmer days, they may awaken, move about in a coordinated manner, and feed. Some bears even give birth during the winter.

Scientists find it difficult to establish clear-cut boundaries between hibernation and these other types of dormancy. Many animals, including poorwills, some bats, and a few rodents, can enter either hibernation or what appears to be daily torpor. As metabolism slows, the body temperature of hibernating animals falls sharply. While active, most mammals have an average body temperature around 37° C (99° F). In hibernation, the body temperature generally falls below 10° C (50° F) and, in many species, may drop as low as 5° C (41° F). The body temperature of a hibernating Arctic ground squirrel may be as low as –3° C (27° F), depending on the season of the year and other environmental conditions. Some desert ground squirrels that estivate during the hottest part of the year remain inactive throughout the winter, so it is difficult to mark when estivation ends and hibernation begins in these animals.

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