Why do some animals hibernate?

Hibernate - some animals in cold climates escape the severest weather by hibernating. That is they spend the winter months in a very long deep sleep. The word "hibernate" comes from the Latin hibernare, which means "to winter". Many animals find sheltered places underground or at the base of trees and hedges in which to hibernate. Hibernating animal include frogs, newts, toads, lizards, dormice, bats, snails, tortoises, hedgehogs and squirrels. During hibernation the animal appears to be lifeless. Breathing almost stops and the heartbeat is slow. The feet tails and snouts of warm blooded animals are much colder than usual although the blood in their hearts remains at a high temperature. The animals are nourished by sugars stored in the liver and by fat that has been built up during the summer. Mild winters are bad for hibernating animals because they wake up during warm spells and use energy in moving about. But they do not feed normally and, by the end of the winter, are very thin. Creatures which cannot burrow find cracks and holes in which to shelter. Some have been known to return to the same place year after year. Just as animals in cold climates escape winter by hibernating so some in the tropics avoid hot dry spells by sleeping underground. This is known as a estivation from the Latin word meaning heat.