Hibernating Time

This month as the polar vortex dips into New Jersey and we say goodbye to warm days, animals may not be the only ones in our state thinking about migrating south or about some winter hibernation.

The popular idea of hibernation is probably closer to a storybook version of fattened bears curled up in a cave than the ways that New Jersey's creatures are preparing to hibernate using a variety of physiological strategies.

After an animal finds or makes a living space (hibernaculum) that protects it from winter weather and predators, the animal's metabolism slows dramatically.

Little brown bats with white nose syndrome - Al Hicks, NYSDEC, Bugwood.org

Our over-wintering bats, including the federally endangered Indiana bats, are true hibernating mammals who regulate their metabolism to create a torpid, cold, inactive state. That's why White Nose Syndrome is a serious threat to them. The fungus disrupts their sleep, causes them to fly, and exhausts their fat reserves even if they only wake up in mid-winter for brief periods.

Also in that hibernation category are some rodents like woodchucks and chipmunks. They can maintain a constant body temperature of 38 degrees. But woodchucks, also known as groundhogs, will not be coming out of hibernation naturally as early as February 2 to satisfy some news crew's need for a Groundhog Day story.

For chipmunks, their den temperatures remain mostly above freezing because burrow entrances are plugged and their burrow system extends below the frost line. Their body temperature drops to within a few degrees of the burrow temperature and is often around 40°F. Every few days, chipmunks elevate their body temperature to over 94°F.

Timber rattlesnake - photo by Kris Schantz via state.nj.us/dep/

Our timber rattlesnakes in the Ridge and Valley and Highlands regions will hibernate in deep, rocky mountain crevices with a southern exposure.

In the Pinelands, lacking those rocky crevices, the rattlesnakes will hibernate in the springs and roots of cedar swamps where the water movement means it probably will never freeze.

The Northern pine snake, which also inhabits the Pinelands, will first fully digest their last autumn meal before hibernating. (Undigested food in a reptile can lead to bacterial infection and death.) Then, they will burrow into upland sands about four feet below the surface to hibernate.


Fish and many reptiles and amphibians don't go into a true hibernation but rather into dormancy. That is a period in an organism's life cycle when growth, development, and physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. This allows some species to survive extremely low oxygen conditions in the mud and deep water of ponds.

Aquatic frogs such as the American bullfrog typically hibernate underwater. They do not spend the winter like aquatic turtles which dig into the mud at the bottom of a pond or stream. If hibernating frogs did that, they would suffocate. Hibernating aquatic frogs require oxygen-rich water and spend a good portion of the winter just lying on top of the mud or only partially buried, and may even slowly swim around from time to time.

A hibernating turtle's metabolism slows down so drastically that it can get by on the mud's poor oxygen supply.

Of course, we also have terrestrial frogs that normally hibernate on land. American toads and other frogs will burrow deep into the soil, safely below the frost line. Not all frogs are diggers. The wood frog and the spring peeper will find deep cracks and crevices in logs or rocks, or, not as safely, just burrow down as far as they can in the leaf litter. Be careful raking or kicking those leaf litter piles!

Is this bear hibernation or just sleeping?

Black bears are still our number one popular poster species for hibernation, but they are not true hibernators and in New Jersey may be active all year long.

Black bears begin entering their winter dens in the fall to avoid periods of food shortage and severe weather. Impregnated females typically enter dens first, during the last week in October. Males may not enter dens until December.

Unlike smaller mammals that hibernate, black bears do not drop their body temperature appreciably. They enter a state of torpor (low metabolic activity). The small amount of urine that is produced is reabsorbed by their kidneys, and they don't have to wake up to urinate or defecate.

Bears are too big to allow their bodies to get really cold, and they need to be able to wake up quickly in an emergency. Den sites generally include ground nests, excavation sites, brush piles, hollow trees, rock cavities, and, more rarely, caves (which are not very plentiful in NJ). Sometimes they might even be found beneath houses and other buildings. The den sites are typically small in size to retain body heat and ensure that black bears stay well insulated.

They live off of their body fat, which is metabolized to produce the calories and water that they need to survive. That is why autumn is such a critical and active feeding time for them. They generally lose between 18-20% of their body fat while in their dens, and they can maintain their bone and muscle mass. While in torpor, black bears are capable of being easily awakened if disturbed, and they may leave their dens on mild winter days in search of food.