HIBERNATION

1. What is the favored definition of hibernation? Is a greatly reduced core temperature a necessary component of hibernation?

2. Is the bear considered to be a true hibernant?

3.

4. What prevents hibernants from freezing? Is there periodic awakening from hibernation?

5. What is brown fat?

Hibernation is the act of resting in a dormant state in a protected burrow. This definition has recently returned to favor. Formerly, it was proposed that

Hibernation is the assumption of a state of greatly reduced core temperature by a mammal or a bird which has its active body temperature near 37 °C, meanwhile retaining the capability of spontaneously rewarming back to the normal homeothermic level without absorbing heat from its environment. (Menaker, 1962)

According to the former definition, bears were not considered to be true hibernators because their core body temperature was not greatly reduced. The core body temperature of bears is reduced by only 6.8 °C during their dormancy, as opposed to a reduction of 20 °C to 30 °C by animals that were considered to be true hibernators. The lesser reduction in body temperature of the bear is now believed to be a biologic protection for hibernating bears; accordingly, they are considered true hibernants. Because of their large body mass, it is thought that too much time would be involved in their revival to activity if their body temperature were lowered by 20 °C to 30 °C. The longer revival time would make them easy victims to another cannibalistic bear that had revived.

The characteristics of hibernation are as follows:

1. Hibernation is a process of warm-blooded animals.

2. The process is autonomous - the animal induces and reverses it by some self-contained mechanism.

3. The process is radical - changes involve not only overt physiologic functioning, but also cellular and subcellular changes.

4. All physiologic functions continue, but at a reduced rate.

5. During the process, body temperature is lowered significantly to a level compatible with survival for the species.

Awakening from Hibernation

Hibernating animals awake from their dormant state periodically. For example, the kidneys continue to form urine and the animal has a need to urinate. A protective mechanism against profound cooling also exists in winter hibernants. If the body temperature declines to levels near freezing, the animal awakes and rapidly rewarms.

Brown Fat versus White Fat

Brown fat is a connective tissue with a color that results from cytochrome pigments and a high density of mitochondria. It is typically found in hibernating animals and in smaller species. It is also present in the newborn of many species and disappears within the first few months of life. Its usual location is in the subcutaneous region between the scapulae (shoulder blades) and in the region of the kidneys as well as within the myocardium. The ability of hibernators to elevate their body temperature from reduced levels to the temperature necessary for arousal (nonshivering thermogenesis) is facilitated by their depots of brown fat. Brown fat differs from white fat not only in color but also in metabolic characteristics. When brown fat cells are stimulated, they consume oxygen and produce heat at a high rate.

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