The animals that truly hibernate include ground squirrels, groundhogs, chipmunks, hedgehogs, bats, and one remarkable primate — while bears do something related but gentler, and deer don’t hibernate at all. True hibernation is far stranger than a long nap: a hibernating ground squirrel’s body temperature can drop below freezing, its heart slows from hundreds of beats a minute to a handful, and it may breathe once a minute or less. Here’s which animals hibernate, which only visit the edges of sleep, and how everything else — deer included — tough out the winter awake.
What hibernation actually is
Hibernation is a controlled, drastic turndown of the body’s thermostat and metabolism, lasting days to weeks at a stretch. A true hibernator’s body temperature falls to near the temperature of its burrow — sometimes within a degree or two of freezing — and its metabolic rate can drop by more than 90 percent. The animal isn’t asleep in any normal sense; brain activity nearly flatlines, and hibernators must periodically rewarm just to sleep normally for a few hours before going back under. The champion is the Arctic ground squirrel, which lets its core temperature fall slightly below freezing — the coldest body temperature ever measured in a mammal — without turning to ice, thanks to supercooled body fluids.
The true hibernators
| Animal | How deep? | How long? | Notable details |
|---|---|---|---|
| Arctic ground squirrel | Body temp to about 27°F | Up to about 8 months | Coldest hibernating mammal known |
| Groundhog (woodchuck) | Body temp to about 37–40°F | 3–6 months | Heart rate falls from ~80–100 to about 5 beats/min |
| Chipmunk | Deep torpor bouts | Off and on all winter | Wakes to eat from its seed larder |
| Little brown bat | Near-ambient body temp | Up to 6+ months | Hibernates in caves and mines called hibernacula |
| Hedgehog | Near-ambient body temp | 2–5 months | Britain’s classic hibernator |
| Fat-tailed dwarf lemur | Tropical torpor | Up to about 7 months | The only primate known to hibernate |
| Common poorwill | Body temp to about 41°F | Weeks at a time | The only bird known to truly hibernate |
| Wood frog | Frozen solid | Months | Freezes and thaws using natural antifreeze |
| Box turtles, snakes, frogs | Cold-blooded dormancy | All winter | Technically called brumation in reptiles |
Bats are among the most committed hibernators in North America — most of our bat species hibernate, packing into caves and mines where temperature and humidity stay stable, while a few species migrate south instead; where bats go in winter maps both strategies. A hibernating little brown bat breathes so seldom and cools so completely that it can look dead — one reason you should never touch one found in winter, since each accidental wake-up burns weeks’ worth of stored fat. And the wood frog performs the most extreme trick on the list: it genuinely freezes — no heartbeat, no breathing, ice crystals through its body cavity — then thaws out and hops away in spring, protected by glucose antifreeze in its cells.
Do bears really hibernate? The honest answer
Bears are the famous borderline case. A denned black bear’s body temperature drops only about 10–12 degrees — nothing like a ground squirrel’s plunge — and a disturbed bear can rouse and defend itself within moments, which is why some textbooks long insisted bears were “just in torpor.” But modern bear biologists mostly call it hibernation anyway, and for good reason: a black bear can go up to 100 days without eating, drinking, urinating, or defecating, recycling its own wastes into protein while pregnant females somehow give birth and nurse cubs in the den. Many researchers now describe bears as highly efficient hibernators rather than shallow ones. We take the question apart in do bears really hibernate? and walk through the timeline in when and how black bears hibernate.
A tier below the bears sit the drowsy part-timers. Skunks, raccoons, and opossums den up and go torpid through cold snaps, then emerge to forage on mild nights — do skunks hibernate? covers the pattern. Even hummingbirds use torpor nightly in cold weather, dropping their body temperature to save fuel until morning.
What about deer, elk, and moose?
Deer don’t hibernate — no deer species does, anywhere. A deer’s winter strategy is the opposite: stay awake, spend less, and endure. Whitetails grow a hollow-haired winter coat that insulates so well snow doesn’t melt on their backs, cut their metabolism and activity by up to about half, and shift to browsing woody twigs and buds. In northern forests they gather in traditional wintering areas called deer yards, packing trails through the snow under sheltering conifers. Our guides to whether deer hibernate and where deer sleep in winter cover the whole survival playbook. Elk and moose follow the same endure-it strategy, with moose so well insulated that warm winters stress them more than cold ones.
The stay-awake club has one more famous member: squirrels. Gray squirrels and red squirrels do not hibernate — they cache food all autumn and live off it between naps in leaf nests and tree cavities, as our guide to where squirrels sleep explains. If you see a bushy tail bounding across February snow, that’s normal; only the ground squirrels and their groundhog cousin are missing from the winter woods.
Getting ready: the strangest eating contest in nature
Hibernation is won or lost in autumn. In the weeks before denning, hibernators enter hyperphagia — a biologically driven eating frenzy in which a black bear can consume up to about 20,000 calories a day, ten times its spring intake, and a groundhog can add a third of its body weight in fat. Little brown bats nearly double their weight before entering their caves. That fat is the entire winter budget: a hibernating bat spends it at a miserly trickle, and every disturbance is a budget crisis. This is why white-nose syndrome, a fungal disease that irritates hibernating bats awake again and again, has killed millions across North America — the bats don’t die of the fungus so much as of running out of winter fuel. It’s the biggest threat facing our bat species today, and a sad proof of how finely tuned hibernation budgets are.
Waking up has a cost even in a normal year. Rewarming from near-freezing to full operating temperature burns more energy than days of torpor, so hibernators schedule their arousals carefully — and an early spring thaw or a midwinter warm spell can trick animals into spending fuel they can’t replace. It’s one reason biologists watch hibernators closely as climates shift: the calendar their bodies evolved to trust is drifting.
Three hibernation myths worth retiring
Myth one: hibernation is a long sleep. It’s much stranger — deep torpor suppresses brain activity so thoroughly that hibernators become sleep-deprived, and part of why they periodically rewarm is, remarkably, to sleep. Myth two: hibernators snooze straight through until spring. Almost all of them surface repeatedly; chipmunks wake every week or two to raid their seed stores, and even deep hibernators arouse on a schedule. A groundhog seen in a February thaw isn’t a failed hibernator — brief midwinter walkabouts are normal, whatever Groundhog Day implies about weather forecasting. Myth three: denned bears are out cold. Mother black bears give birth in the den in January, then nurse and tend cubs for months while eating nothing at all — arguably the most impressive feat in all of hibernation, and proof the bears are far from unconscious.
A bonus myth for gardeners: hibernation doesn’t mean the wild goes quiet. Most of the animals in your winter landscape never hibernate at all — deer, elk, moose, rabbits, foxes, coyotes, squirrels, and nearly all birds stay active straight through, which is why winter is prime time for tracks in the snow. If the woods seem empty in January, it’s because the residents are being careful with their energy, not because they’re asleep underground.
Hibernation timing also runs on cues you can watch for. Shortening daylight and falling temperatures start the countdown, but food is the real trigger for many species: bears den earlier in poor acorn years and stay out foraging deep into December in good ones, and groundhogs disappear within days once frost kills the green forage they depend on. In spring the order reverses — males of most hibernating species emerge first, staking out territory weeks before the females wake.
Frequently asked questions
Which animals truly hibernate?
The deep hibernators are mostly small mammals: ground squirrels, groundhogs, chipmunks, hedgehogs, dormice, marmots, and most temperate-zone bats, plus the fat-tailed dwarf lemur and one bird, the common poorwill. Reptiles, amphibians, and many insects go dormant too, though in cold-blooded animals it’s technically brumation or diapause. Bears hibernate in their own distinctive, warmer-bodied way.
Do bears wake up if you disturb them?
Yes — and much faster than a true deep hibernator can. A denned bear keeps its body temperature high enough to rouse within moments, which is one of the strongest arguments that bear hibernation works differently. Never approach a winter den: a groggy bear is still a bear.
Why don’t deer hibernate?
Hibernation only works if you can shelter in an insulated den and survive on stored fat — practical for a 10-pound groundhog, impossible for a 150-pound deer that has no burrow and burns fat too fast at that size. Instead, deer evolved superb winter coats, lowered winter metabolism, and yarding behavior. It works, though late winter is genuinely the hungriest, most dangerous season of a deer’s year.
Do any animals hibernate in summer?
Yes — it’s called estivation, and desert animals use it to escape heat and drought rather than cold. Desert tortoises, some ground squirrels, snails, and lungfish all estivate, with African lungfish able to wait out a dry season sealed in a mucus cocoon in dried mud for months or even years.
Winter is a whole subject of its own: see the longest-living animals — hibernating bats are on that list, and hibernation is part of why — check where bats live the rest of the year, or browse every species profile in the Animals A–Z index.
