Do Groundhogs Hibernate? A True Hibernator's Winter

What makes the groundhog a true hibernator?

Many animals slow down in winter, but scientists reserve “true hibernation” for a deep, long-lasting drop in body temperature and metabolism. The groundhog, or woodchuck, fits that definition well.

During hibernation, a groundhog’s body changes dramatically:

These changes cut its energy use to a small fraction of normal, which is what allows a 4–6 kg (9–13 lb) animal to survive four months or more without eating.

When and how long do groundhogs hibernate?

The length of hibernation depends mainly on climate. In much of the northeastern United States, groundhogs go underground in October or November and stay until February or March. In Canada and Alaska, hibernation can last five months or more. In the southern part of the range, where winters are mild, hibernation may last only around three months, and some animals stay active through the year.

Hibernation is driven by internal body clocks as well as by temperature and day length. A groundhog’s appetite and metabolism change with the seasons even in captivity, where food is always available.

Getting ready: the autumn feast

Groundhogs do not hoard food. Instead, they store energy as fat. From early summer, they feed heavily on clover, alfalfa, grasses and other green plants, and their weight can increase by half or more. By early fall they are at their heaviest, often around 5 kg (11 lb) or more for a large male.

At the same time, a groundhog prepares a winter burrow. This is usually dug in woodland or brush, often a short distance from its summer burrow in an open field. The hibernation chamber lies below the frost line, so its temperature stays above freezing all winter. The groundhog lines it with dry grass and leaves and often plugs the tunnel with soil after it settles in.

What happens inside the burrow?

Hibernation is not one long, unbroken sleep. A groundhog spends most of the winter in deep torpor, a state of very low body temperature and metabolism. From time to time, typically every several days to a couple of weeks, it rewarms to a normal temperature for a short period, then sinks back into torpor. Scientists are still studying why hibernators make these costly arousals, but they may help the immune system, the brain and the kidneys to function.

Over the winter, the groundhog burns its fat reserves steadily. By the end of hibernation it may have lost a third to a half of its body weight. It still needs some fat left when it emerges, because fresh plants may not appear for several more weeks.

Why do male groundhogs wake up in February?

One of the most surprising discoveries about groundhog hibernation is that males often wake up early on purpose. Researchers in southeastern Pennsylvania who fitted wild groundhogs with temperature sensors found that males ended deep hibernation in February, when there was still snow on the ground and little to eat.

The males used this time to travel around their territories and visit the burrows of nearby females, apparently to establish relationships before the real mating season. After these visits, they often returned to their own burrows and spent several more weeks resting underground. Mating then followed in March and early April.

Some people have pointed out that this February wake-up happens around the time of Groundhog Day, but the timing varies from year to year and place to place. Whether a groundhog sees its shadow has no effect on spring, as explained in Is Groundhog Day accurate?.

Groundhogs vs other hibernators

Groundhogs are often compared with bears, but their winters are quite different. Bears enter a lighter state of winter dormancy in which their body temperature drops only a few degrees, as explained in Do bears hibernate?. A groundhog’s temperature falls by more than 30 °C (54 °F).

Close relatives of the groundhog, such as other marmots and ground squirrels, hibernate in a very similar way. Some rodents take a different approach: the eastern chipmunk stores a large food cache and wakes regularly to eat, as described in Do chipmunks hibernate?. For more species, see animals that hibernate.

Why groundhog hibernation interests scientists

Because groundhogs can slow their heart and metabolism so far and then recover fully, researchers study them to learn how the body copes with cold, low blood flow and long fasting. Groundhogs gain and lose large amounts of fat every year without apparent harm, so they are also studied in research on metabolism and obesity. Scientists hope that understanding hibernation may one day help protect human organs and tissues during surgery or after injury.