Preparing For Hibernation
Animals prepare for hibernation in the fall by storing enough energy to last them until spring. Chipmunks accomplish this by filling their burrows with food, which they consume during periodic arousal from torpor throughout the winter. Most animals, however, store energy internally, as fat. A woodchuck in early summer may have only about 5% body fat, but as fall approaches changes occur in the animal's brain chemistry which cause it to feel hungry and to eat constantly, which results in an increase to about 15% body fat. In other animals, such as the dormouse, fat may comprise as much as 50% of the animal's weight by the time hibernation begins. A short period of fasting usually follows the feeding frenzy, to ensure that the digestive tract is completely emptied before hibernation begins.
Many hibernators also produce a layer of specialized fat known as brown fat (brown adipose tissue) which lies between the shoulder blades of the animal. Brown fat is capable of rapidly producing large amounts of heat when it is metabolized, which raises the animal's body temperature and brings about the eventual arousal of the animal from hibernation.