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Home » Animals » Species » Hibernation

Animal Hibernation

Prairie Dogs

Some animals, especially small species conserve energy during the cold months by hibernating, just as some reptiles do. Their temperature falls, their breathing slows, their metabolism drops to almost imperceptible levels, and they fast, drawing on stored fat reserves. According to a new study published in The American Naturalist, mammals that hibernate or that hide in burrows are less likely to turn up on an endangered species list. The study's authors believe that the ability of such "sleep-or-hide" animals to buffer themselves from changing environments may help them avoid extinction.

When hibernating, the animal is torpid and difficult to rouse. A Western European hedgehog, for example, begins hibernating when the outside temperature falls below 59°F (15°C), and in midwinter its body temperature lowers to about 43°F (6°C). In some bats, rectal temperatures of 32°F have been recorded during hibernation.

Larger mammals, such the American Black Bear, do not truly hibernate; they sleep and become cold, but are roused more easily.

Estivation and Torpidity

Related to hibernation is estivation, which is torpidity during the summer. Like hibernation, estivation saves energy when food is short. The actual main event of hibernation, the condition of torpidity, results from changes in an animal's thermoregulatory system, which affect the mechanisms of heat production. This occurs when hibernation triggers (cold, shorter day length, lack of food, internal clock, etc) influence the secretion of hormones by endocrine glands, which initiate the process of hibernation.

However, in the mammals hibernation is a controlled condition and they regulate their body temperature even when in deep torpor. They do not just shut down and lie sluggish at the mercy of the environment. Their temperature drops only to a prearranged level called the "set point," below which the animal arouses and shivers violently to raise temperature, a safety mechanism to ensure that it does not freeze.

Differences Among Species

The set point differs among species and is related to body size and the environment. When the mammal warms up it must use self-generated heat, burning fat to provide this energy. Similarly, endotherms, which practice daily torpor have control of their metabolism at all time. Cold-blooded animals have no such control and cannot raise their body temperature to avoid freezing, but those that are able to move, such as garter snakes in their limestone sink-hole, or frogs on the bottom of the pond beneath the ice, are known to relocate to safer places, if they are able, if there is a risk of freezing. Those that cannot move will die if they are not one of the few species that are freeze tolerant.

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