Unit 1: Hibernation
W: Some animals in cold climates hibernate. This means that they spend the winter months in a very long and deep sleep. Many animals find shelter underground. They dig out shelters to sleep in. Animals that cannot dig find cracks or holes at the base of trees and bushes. If they like the place they find, they might use it for years and years.
Animals that hibernate include cold-blooded animals, such as lizards, frogs, and snakes. Many warm-blooded animals also hibernate, such as mice, bats, and squirrels. When these animals are hibernating, they seem like they are not alive at all. Warm-blooded animals seem colder to the touch. However, their blood is still very warm. Hibernating animals have a very slow heartbeat. They almost stop breathing. Extra blood sugar and fat in their bodies keep them alive. They eat lots of food just before they hibernate.
Winters that do not stay cold are dangerous for hibernating animals. They can sometimes wake up in their shelters when it gets a little warm. Then they use energy by moving around. During winter, there is very little food. These animals can get very thin and weak. If they move around too much and do not eat, they can die.
Animals hibernate to escape the cold. There are also animals in hot climates that escape the heat. During very hot or dry weather, they sleep underground. This is called aestivation.