Animals That Hibernate: Bears, Bats, Hedgehogs and More

What is hibernation?

Hibernation is a way to survive a season when food is scarce and keeping warm is expensive. Before winter, hibernators eat heavily to build up fat, or, in a few species, store food in their burrows. Then they let their bodies cool and slow down. During deep hibernation, metabolism can fall to less than 5% of its normal resting level, and the heart rate may drop by as much as 95%.

Hibernation is not one long sleep. Small hibernators spend several days to a few weeks at a time in deep torpor, then rewarm to normal body temperature for a few hours before cooling again. Scientists are still studying why these costly arousals happen; they may help the immune system and the brain.

Some species, called obligate hibernators, hibernate every year on an internal schedule, even in a warm laboratory with plenty of food. Others, called facultative hibernators, do so only when cold or hungry.

Hibernation, torpor and brumation: what’s the difference?

1. Bats

Bats in Europe and North America eat insects, which disappear in winter, so most species hibernate in caves, mines, cellars, tree holes and buildings. The little brown bat of North America lets its heart rate fall from up to about 210 beats per minute when resting to as few as 8 beats per minute in torpor. Even so, it loses roughly a quarter of its body weight over the winter.

Many bats gather in large winter colonies; one little brown bat colony was counted at more than 180,000 animals. Since 2006, a fungal disease called white-nose syndrome, which strikes while bats hibernate, has killed millions of bats in North America, with losses of around 90% at many surveyed sites in the Northeast. Learn more in do bats hibernate?

2. European hedgehog

The European hedgehog is Europe’s best-known hibernator. In Britain and similar climates it hibernates for about four to five months, from around November to March or April, in a nest of packed dead leaves called a hibernaculum. Its body temperature falls from about 35 °C (95 °F) to just a few degrees above that of the nest, often around 2–5 °C (36–41 °F). Details are in when do hedgehogs hibernate?

3. Dormice

Dormice are such famous sleepers that their name probably comes from the French verb dormir, “to sleep.” The hazel dormouse of Europe hibernates for about six months, from around October to April or May, in a woven nest on or near the ground under leaves or log piles, after fattening up on hazelnuts and berries.

Its larger relative, the edible dormouse, is even more extreme. It usually hibernates from about October to May, and in years when beech and oak trees produce few seeds, some animals stay in hibernation for more than 11 months. While hibernating it can stop breathing for up to an hour at a time. Its name comes from the ancient Romans, who fattened and ate dormice as a delicacy.

4. Groundhog

The groundhog, or woodchuck, is one of North America’s classic true hibernators. From about October or November to February or March, it sleeps in a burrow below the frost line. Its body temperature drops from about 37 °C (99 °F) to a few degrees above freezing, and its heart slows from around 80 to about 4–10 beats per minute. By spring it may have lost a third to half of its weight. See do groundhogs hibernate?

5. Ground squirrels and marmots

Ground squirrels include some of the deepest hibernators known. The Arctic ground squirrel of Alaska, northern Canada and Siberia hibernates for seven months or more in burrows dug above the permafrost. Its abdominal temperature can fall to about −2.9 °C (26.8 °F), below the freezing point of water, for weeks at a time, without its body fluids turning to ice. Thirteen-lined ground squirrels of the North American prairies and the European ground squirrel, or souslik, of central and southeastern Europe also hibernate deeply.

Marmots, which are large ground squirrels, hibernate too. Alpine marmots in the Alps spend six to nine months a year underground, with whole family groups huddling together in one chamber to save heat.

6. Chipmunks

The eastern chipmunk is a lighter, facultative hibernator. Instead of getting fat, it fills its burrow with a store of nuts and seeds in autumn. Through the winter it sinks into torpor for several days at a time, then wakes up to eat from its larder. Its body temperature does not fall as low as a groundhog’s.

7. Bears: the debated case

Bears spend up to seven months in winter dens without eating, drinking, urinating or defecating. Yet their body temperature drops by only a few degrees, from about 37–38 °C (99–100 °F) to around 30–35 °C (86–95 °F), and they can wake fairly quickly, so many biologists once said bears did not truly hibernate.

Studies of black bears and brown bears changed that view. Researchers found that denning black bears reduce their metabolism to about a quarter of the normal resting rate while staying fairly warm. Because hibernation is now defined by this controlled drop in metabolism rather than by temperature alone, most scientists today count bears as hibernators. Pregnant females even give birth in the den in midwinter. Read more in do bears hibernate?

8. Common poorwill: the bird that hibernates

The common poorwill, a small nightjar of the dry hills and deserts of western North America, is the only bird known to stay in torpor for weeks or months. In 1946 the naturalist Edmund Jaeger found a poorwill hibernating in a rock crevice in the Chuckwalla Mountains of California, and his report in 1948 gave science its first clear case of a hibernating bird. The Hopi people may have known about the bird’s habit long before: their name for it, hölchko, means “the sleeping one.”

9. Hibernators in other parts of the world

Hibernation is not only a cold-climate trick. The fat-tailed dwarf lemur of Madagascar was the first primate known to hibernate. It spends up to seven months a year in tree holes, living on fat stored in its tail, even though outside temperatures can rise above 30 °C (86 °F). In Australia, the short-beaked echidna enters deep torpor in cooler months, and its body temperature can fall as low as about 5 °C (41 °F).

Animals that do not truly hibernate

Many familiar animals slow down in winter without hibernating. Tree squirrels stay active and rely on buried nuts; see do squirrels hibernate? Badgers, raccoons and skunks may stay underground for days or weeks during cold spells, but their body temperature drops only a little and they come out on milder nights. Foxes, deer, wolves and most birds stay active all winter, and many birds migrate.

Reptiles, amphibians and insects in winter

Cold-blooded animals cannot keep themselves warm, so in winter they simply become inactive, a state often called brumation. Snakes gather in underground dens: at the Narcisse Snake Dens in Manitoba, Canada, tens of thousands of red-sided garter snakes spend the winter together in limestone sinkholes. Turtles may spend the winter on the bottom of ponds, taking in some oxygen through their skin.

The wood frog of North America takes winter survival further than any mammal. It overwinters in leaf litter and can survive with much of its body water frozen, its heart stopped, protected by high levels of glucose and urea in its cells. Many insects, from ladybugs to queen bumblebees, pass the winter in a paused state called diapause.

Why hibernating animals should be left alone

Every time a hibernating animal wakes, it burns a large amount of its stored fat to warm up. Disturbing bats in a cave or hedgehogs in a leaf pile can force extra arousals that leave them without enough energy to reach spring. For this reason, all bats and their roosts are protected by law in the United Kingdom and the European Union, and in the United States many caves used by hibernating bats are closed to visitors in winter.