The answer to whether groundhogs hibernate is an unqualified yes; they are one of the few mammals classified as a true hibernator. Known scientifically as Marmota monax, the groundhog is a member of the squirrel family, Sciuridae, and is the largest ground squirrel in its North American range. This lowland marmot must undergo a profound biological shutdown to survive the long, cold months when food sources are unavailable. This process allows them to conserve energy by dramatically altering their internal biology, which is far more extreme than simple winter rest.
Defining True Hibernation
True hibernation is a state of metabolic suppression known as torpor, involving extreme physiological changes to sustain life using minimal energy. During this time, a groundhog’s heart rate plummets from its normal rate of about 80 beats per minute down to as few as four or five beats per minute. This reduction is paired with a significant drop in body temperature, falling from approximately 99°F to near the ambient temperature of its burrow, often as low as 35°F to 37°F.
Their breathing also slows considerably, decreasing from around 16 breaths per minute to just one or two shallow breaths per minute. The overall metabolic rate can be suppressed by up to 90%, allowing the groundhog to subsist almost entirely on stored body fat. Despite this deep torpor, groundhogs experience periodic arousals, briefly warming their bodies back to a near-normal temperature before re-entering the hypothermic state. These short waking periods, which may occur every week or two, are necessary for certain physiological maintenance.
The Annual Hibernation Timeline
The groundhog’s hibernation cycle is tied to the availability of food and the onset of cold weather, typically beginning in late fall. Most groundhogs enter their winter burrows, called hibernacula, around October or sometimes as early as late September in northern latitudes. This timing ensures they are secured before the ground freezes and food becomes scarce.
The duration of their deep sleep can last anywhere from three to six months, depending primarily on the local climate and geography. Groundhogs inhabiting colder, northern ranges hibernate for longer periods, sometimes not emerging until March or even April. Those in milder, southern ranges may have a shorter dormancy period, occasionally skipping the process entirely if winter conditions are not severe enough.
The first groundhogs to emerge in late winter are usually the males, who wake up a few weeks earlier than the females to establish territories and prepare for the breeding season. This early emergence is why groundhogs are observed stirring in February, often coinciding with the traditional date of Groundhog Day. The females follow shortly after, with the full population typically becoming active again as the weather warms and vegetation begins to grow.
Preparing for the Long Sleep
Preparation for the months-long fast begins in late summer and early fall with a period of intense feeding known as hyperphagia. During this time, groundhogs gorge themselves on vegetation, accumulating significant fat reserves that can double their body weight. This stored fat provides the necessary energy to fuel their suppressed metabolism throughout the winter.
A specialized form of fat, called brown adipose tissue, is built up and is particularly important for the survival strategy of true hibernators. This brown fat is positioned near the upper body and is metabolically active, generating heat without shivering to power the periodic arousal cycles.
Before entering torpor, the groundhog constructs a winter chamber, the hibernaculum, typically dug below the frost line to maintain a stable, insulated temperature. The groundhog then seals the entrance with soil and bedding materials, creating a secure, isolated environment for its long sleep. This construction prevents temperature fluctuations and protects the animal from predators while it is in its most vulnerable, unresponsive state.
Upon emerging in spring, the groundhog will have lost a substantial portion of its body mass, sometimes up to half of its pre-hibernation weight.