A woodchuck can allow its body temperature to drop more than 30 degrees Celsius. The jumping mouse's heart rate is about 500 to 600 beats a minute. When hibernating it can slow to about 30 beats a minute. These animals are known as true hibernators because their normal body functions change dramatically. Some animals, such as the raccoon and skunk, conserve energy by 'denning up' or sleeping for long periods of time in the winter.
Raccoons often sleep together in winter dens to keep warm.
Raccoons and skunks are not considered true hibernators because they awaken during the winter to feed and have only slight changes in their body functions. Scientists use the term 'dormancy' to describe this kind of adaptation to winter. The term 'light sleepers' is also used to describe these kinds of changes in behaviour. Scientists have discovered that hibernating animals have a special substance in their blood. This substance becomes active when the days become shorter or there is less sunlight.