Groundhogs enter obligate hibernation in autumn after laying down substantial fat. Core body temperature falls to roughly 5 degrees Celsius, close to burrow temperature, and heart rate drops from around 80 to 100 beats per minute when active to under ten. Oxygen consumption falls to a small fraction of the resting rate.
This torpor is not continuous. Like other hibernators the groundhog periodically arouses, rewarming to near-normal temperature for a period of hours before dropping back. These arousals consume a large share of the winter energy budget, and why they are necessary remains one of the standing puzzles in hibernation physiology. The winter burrow is separate from the summer one and is dug below the frost line.
Groundhog Day is folklore rather than forecasting. Long-run comparisons of ceremonial predictions against subsequent weather records show no skill above chance, and wild groundhogs emerge on a schedule set by latitude, temperature and, in males, the search for mates.