They also produce a sound by grinding their teeth. Woodchucks can also give low barks, but the function of this particular sound is unknown. In preparation for their long winter sleep, or hibernation, woodchucks grow enormously fat towards the end of the summer. They begin hibernation with the onset of freezing weather, the adults before the young ones, who probably need extra time to put on sufficient fat to see them through the winter. The first adults to hibernate disappear late in September, and all woodchucks are underground in October.
Hibernation is a process of deep comatose sleep.
Bodily functions are greatly retarded, allowing the accumulated body fat to nourish the animal throughout the winter. The breathing rate and consequent consumption of oxygen are also much reduced. When the animals emerge in the spring, they generally still have a good deal of body fat left, which is necessary. Emerging in March, as many of them do, they find little food about them. They may even burrow up through snow to reach daylight.