The bears, skunks, raccoons, possums and chipmunks store up thick layers of fat and go to sleep in their dens when the severe cold comes. But they are light sleepers and on mild winter days are likely to awaken, come out, and move about. Other mammals, however, truly hibernate. All but the bats curl up in a tight ball. Breathing is faint, at long intervals.
Body temperature drops to near that of the surroundings, sometimes close to freezing.
Most of the stored fat is slowly used up. No one known just what awakens them. Temperature changes perhaps. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.