Hibernation

I

Introduction

Hibernation, state of reduced activity that occurs in some animals during the winter. In cold weather most animals must eat large quantities of food to obtain the energy needed to carry on normal body activities. In winter, however, food often becomes scarce, so many animals cannot survive unless they hibernate. A hibernating animal greatly reduces normal body activities that expend energy. It survives on energy reserves, such as fat, stored in the body.

During hibernation, an animal lowers its metabolic rate—the rate at which an animal uses energy and stops generating the heat necessary to keep its body temperature above that of the environment. As body activities slow, the animal becomes less and less capable of coordinated movement, gradually slipping into a state of dormancy, or torpor. If, however, the animal’s body temperature slips below a certain range, the animal will generate heat to boost body temperature to a safe range. Animals may hibernate for several months, but they do not remain completely inactive during this time. Hibernation typically occurs in bouts, or episodes, lasting from a few days to a few weeks depending upon the animal, body size, outside temperature, and time of year. These bouts of inactivity are interspersed with brief periods of activity, when the animals increase their body temperature to a normal level.

Although hibernation helps animals survive adverse environmental conditions, hibernating animals can still freeze to death, and their lack of mobility and coordination makes them vulnerable to predators. To help protect themselves, many animals hibernate in protected areas, such as caves or underground burrows. These sites often remain several degrees above freezing even when the outside temperature is far colder. Animals usually choose sites that are inaccessible to predators.

II

Animals That Hibernate

Hibernation is primarily associated with animals that live in cold climates, but scientists have learned that hibernation occurs in many animals found from the Arctic to the tropics. Many of these animals are mammals. Mammals are endotherms—warm-blooded animals that generate heat internally and maintain a high, relatively constant body temperature regardless of the surrounding environment. Hibernating mammalian endotherms include rodents such as ground squirrels, hamsters, chipmunks, marmots, and dormice; bats; hedgehogs; pygmy possums; small, South American marsupials called colocolos; and echidnas, primitive egg-laying mammals from Australia also called spiny anteaters. The only species of bird known to hibernate is the common poorwill, an insect-eating bird that lives in North America. (Birds are also endotherms.)

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Most hibernating animals tend to be small. The smallest weigh only 10 g (0.4 oz) or less while the largest weigh up to 10 kg (22 lb), although most are not more than 1 kg (2 lb). Smaller animals benefit from hibernation because they lose body heat to the environment more rapidly than larger animals and therefore must have higher metabolic rates. For these animals to maintain a high body temperature in cold weather, they require plenty of food to boost their already high metabolic rate. Acquiring sufficient food is impossible when food is scarce. Hibernation permits an animal to drop its body temperature, reducing the energy expended to keep itself warm. In addition, a cold body uses energy more slowly than a warm one, so the animal’s stored fuel reserves last longer. Endotherms arouse from hibernation by increasing their body temperatures using internally generated heat.

Hibernation is not restricted to endothermic mammals and birds. Some ectothermic animals—cold-blooded animals that depend upon heat from the environment to maintain their body temperature—also hibernate. Hibernating ectotherms include certain species of lizards, snakes, turtles, frogs, toads, fish, snails, and shrimp, and some insects. Unlike endotherms, when ectotherms arouse from hibernation, they are unable to rewarm themselves using internal heat production. Instead they rely on rising outdoor temperatures to warm their bodies.

III

Body Changes During Hibernation

Although the physiological changes that occur during hibernation vary from one animal to the next, all animals entering hibernation undergo certain similar changes. In the earliest stages of hibernation, the animal’s metabolic rate slows considerably. Dormant snails can reduce their metabolic rate to 10 percent of their active rate, and some brine shrimp drop their metabolic rate to less than 1 percent of their active rate.

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, on average drops to 6° C (43° F). The body temperature of a hibernating Arctic ground squirrel may be as low as –3° C (27° F).

A hibernating animal’s heart rate drops substantially, often to just five to ten beats per minute. Its breathing rate also decreases. Many mammals do not breathe continuously during hibernation. They have periods of apnea (absence of breathing) that can last an hour or more.

Hibernation is a controlled physiological state—the animal regulates all internal body changes even during the deepest torpor of hibernation. For instance, hibernating mammals continue to regulate their body temperature even as it falls, making sure that it does not drop below a certain level, known as the set point. When the set point is reached, the animal increases its metabolic rate, generating heat to keep its body temperature at or above that point. The set point differs widely among animals, and it appears to be related to both body size and the surrounding temperature.

IV

Entering Hibernation

Hibernation is controlled by a set of complex cues that vary greatly from one group of animals to another and are not well understood by scientists. In many animals, such as dormice, seasonal changes in day length, or photoperiod, trigger hibernation. Shortening days in the late summer and autumn signal many animals to prepare for hibernation. In many species of ground squirrels, a circannual rhythm, an internal biological rhythm with an approximately yearly cycle, cues hibernation. Other animals, including pygmy possums, exhibit opportunistic hibernation—that is, they enter prolonged torpor at any time of the year when environmental conditions deteriorate, regardless of the photoperiod.

To prepare for hibernation, many animals eat large quantities of food, which is stored in the body as energy-providing fat. Hibernating mouse lemurs may live entirely off these stored fuel reserves for the entire hibernation season, up to seven months. Some animals, such as chipmunks and ground squirrels, store caches of food in their hibernation site to nourish them through the hibernating season.

Many animals begin hibernation with a series of short bouts of torpor in which their body temperature stays relatively high. Other animals fall immediately into a long and deep bout of torpor in which their internal body activities drop sharply. Bouts tend to be relatively short early and late in the hibernation season and longest in the middle of the season. For most animals, torpor is deepest—that is, metabolism and body temperature are lowest—during the long, mid-season bouts of hibernation.

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