In order to prepare for hibernation, groundhogs spend the summer months feeding and binging on a wide variety of plants such as dandelion, grasses, clover and garden vegetables. They also eat berries, insects, grubs, snails and small animals.
Groundhogs must build up layers of fat in order to survive the long winter in hibernation, and can eat as much as a pound of food a day. They reach their maximum bulk and girth in late August.
Females give birth in early spring, usually in late March or April. This enables the babies to rapidly mature during the summer so that they are capable of digging their own dens in preparation for winter. Groundhogs are skillful burrowers. They dig deep dens and line them with twigs and grasses. They usually have two to five entrances as protection against predators, and tunnels can be as large as 46 feet long and 5 feet deep.
Typically a groundhog digs two chambers: one for sleeping and the other for excrement. They normally dig their winter burrows in a wooded area, just below the frost line. Temperatures in the burrow remain stable and well above freezing. The burrow keeps the groundhog warm and protected against the cold winter months.
Once a groundhog enters hibernation, its heart rate slows significantly to just four or five beats a minute. Its body temperature can drop to as low as 39 or 40 degrees Fahrenheit. The groundhog survives on the body fat it accumulated during the summer and fall months. Hibernation lasts until around February or March, when the weather begins to warm.