In temperate zones -- the regions between the Arctic Circle and the Tropic of Cancer in the Northern Hemisphere and between the Tropic of Capricorn and the Antarctic Circle in the Southern Hemisphere -- winters get cold. Animals in these areas have adapted in different ways to cope with the lower temperatures and lack of food during the winter months. Some migrate to warmer tropical areas while others have thick fur and insulating fat to help them stay warm and active. Others survive by hibernating in sleeplike states of dormancy in which the animals' metabolic rates slow to conserve energy.
Many warm-blooded animals hibernate during the winter months when food is scarce or buried under snow. Animals such as chipmunks and ground squirrels are considered "true" hibernators. Their body temperatures decrease dramatically, though they wake up every few days to eat and drink -- this is why squirrels spend the fall storing food for the winter. In contrast, bears do not eat or drink while hibernating, depending on their bodies' fat stores. Skunks, raccoons and opossums hibernate similarly to bears. Woodchucks and bats also hibernate during the winter months.
Being cold-blooded, amphibians rely on the external environment to regulate body temperature. Amphibians in the temperate zone hibernate during the cold winter months to keep from freezing. When cold-blooded animals hibernate, the behavior is called "brumation"; it is roughly analogous to the hibernation behavior of warm-blooded animals. All amphibians brumate, including the wood frog, which lives further north than any other cold-blooded animal in North America. Wood frogs live as far north as Alaska and spend winters buried underneath leaves or under rocks. Their bodies are hard and cold, and they don't move. But they aren't dead -- the glucose in their blood keeps them from actually freezing while they winter in a dormant state.
Like amphibians, reptiles are cold-blooded creatures who brumate to survive the winter months in temperate climates. Snakes and lizards find sheltered spots in rock crevices and caves, or bury themselves underground, and enter a state of suspended animation until the soil warms in spring and they can come out again. During brumation, reptiles don't eat and their heart and respiratory rates drop to the minimum level necessary to sustain life. The Russian tortoise brumates in burrows up to 6 feet underground to escape the brutal Russian winters.
Not all insects hibernate, but some do. Insect hibernation is known as diapause and is more similar to reptilian brumation than to mammalian hibernation. These insects enter a sort of biological stasis in which their metabolic rate stays just high enough to keep them alive. Ladybug beetles and mourning cloak butterflies are just two of many insect species that spend the winter in diapause, a state believed to extend these insects' life spans relative to other insects that migrate or stay active all winter. Diapause is possible because the insects have high concentrations of the same chemicals used as antifreeze in cars to keep their body tissue from freezing.
- National Park Service: Yellowstone National Park: Denning and Hibernating Behavior
- Wisconsin Department of Education Environmental Education for Kids: Snug in the Snow
- Riverwest Currents: Where Do Bugs Go in the Winter?
- PBS Nature: Christmas in Yellowstone -- Animals that Hibernate
- Merriam-Webster Dictionary: Temperate Zone
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