The edible dormouse may hold the most remarkable natural hibernation record documented in a wild mammal. Researchers have recorded free-living edible dormice hibernating for 7.8 to 11.4 months, with some individuals remaining underground for more than 11 months in years when they skipped reproduction. Arctic ground squirrels, bats, marmots and dwarf lemurs can also spend much of the year in hibernation. However, exact rankings are difficult because species differ in climate, sex, age and what scientists classify as hibernation, torpor, brumation or freeze-tolerant overwintering.
Arctic ground squirrels can spend six to nine months in their hibernation season and reach subzero body temperatures without freezing.
Which Animal Hibernates the Longest?
Among naturally hibernating wild mammals studied in detail, the edible dormouse, Glis glis, has one of the strongest claims to the record.
In a field study from the Vienna Woods, researchers continuously monitored free-living edible dormice for about a year.
During a year when beech trees produced very little seed, some dormice skipped reproduction and returned underground exceptionally early.
Eight of 17 animals began hibernating in June or July, and five remained in hibernation for more than 11 months.
The recorded range for these exceptionally long hibernators was:
7.8 to 11.4 months.
This is a natural wild record, making it much more meaningful than extreme laboratory experiments in which animals were artificially kept cold.
Longest-Hibernating Animals Compared
| Animal | Scientific Name | Approximate Hibernation Period | Important Note |
|---|---|---|---|
| Edible dormouse | Glis glis | 7.8–11.4 months in extreme wild cases | Some individuals exceed 11 months in non-reproductive years |
| Arctic ground squirrel | Urocitellus parryii | 6–9 months | Can supercool below 0°C without freezing |
| Golden-mantled ground squirrel | Callospermophilus lateralis | About 7.5 months | Spends nearly two-thirds of the year in its hibernation season |
| Hazel dormouse | Muscardinus avellanarius | Up to about 7 months | Usually hibernates from autumn into spring |
| Fat-tailed dwarf lemur | Cheirogaleus medius | 3–7 months | A hibernating primate from Madagascar |
| Alaskan wood frog | Rana sylvatica | About 6 months frozen in the wild | Freeze-tolerant overwintering rather than classic mammalian torpor |
| Alpine marmot | Marmota marmota | About 6 months | Families can huddle together underground |
| Little brown bat | Myotis lucifugus | More than 6 months in northern populations | Cycles between deep torpor and brief arousals |
| Grizzly bear | Ursus arctos horribilis | About 4–6 months | Duration varies greatly with sex and latitude |
| Common hamster | Cricetus cricetus | Roughly November–March in some populations | Food stores and sex strongly affect torpor use |
This is not a perfectly fixed biological ranking. Hibernation duration varies between populations, years, sexes and individuals, and researchers use different definitions for seasonal dormancy.
Edible Dormouse: More Than 11 Months
The edible dormouse, Glis glis, provides one of the most extraordinary hibernation records ever measured in a free-living mammal.
In ordinary years, edible dormice emerge during the warmer season to feed and reproduce.
But their reproduction is closely connected to the availability of high-energy tree seeds, especially European beech mast.
If seed production is poor, reproduction may not be worth the energetic and predation costs.
Some individuals then stay underground for an astonishingly large portion of the year.
A long-term physiological study documented wild animals entering hibernation as early as June or July and remaining underground for more than 11 months.
The longest measured duration reached approximately 11.4 months.
Do They Sleep Continuously for 11 Months?
No.
Hibernation is not simply uninterrupted sleep.
Like many mammalian hibernators, dormice cycle through long periods of deep torpor interrupted by brief spontaneous arousals during which body temperature and metabolism return close to normal.
These arousals consume a surprisingly large proportion of the energy used during the entire hibernation season.
Arctic Ground Squirrel: 6 to 9 Months
The Arctic ground squirrel, Urocitellus parryii, is one of the most extreme mammalian hibernators.
Scientific studies describe hibernation seasons lasting approximately six to nine months.
What makes the species especially remarkable is not only duration but temperature.
During deep torpor, Arctic ground squirrels can allow their core body temperature to fall below the freezing point of water.
Researchers have recorded spontaneous arousal from body temperatures as low as approximately -2.9°C.
The animal does not normally freeze solid. Instead, its body fluids remain supercooled.
This is fundamentally different from the freeze-tolerance strategy used by wood frogs.
Golden-Mantled Ground Squirrel: About 7.5 Months
The golden-mantled ground squirrel, Callospermophilus lateralis, also spends most of the year below ground.
Field research has documented hibernation seasons of approximately 7.5 months.
That leaves only about four and a half months for the entire active season.
During that short window, the animals must:
mate;
raise young;
forage;
restore body reserves;
prepare for the next hibernation season.
The study estimated that although hibernation occupied almost two-thirds of the year, it accounted for only a relatively small fraction of annual energy expenditure because metabolism is so dramatically suppressed.
Hazel Dormouse: Up to About 7 Months
The hazel dormouse, Muscardinus avellanarius, is another famous long-term hibernator.
In Britain, hazel dormice normally move from trees to ground-level hibernation nests during autumn.
They may remain underground from approximately October or November until April or May.
That can amount to around six or seven months.
Their winter nests are often built beneath leaf litter, beneath logs or around the base of vegetation where temperatures are relatively stable.
During cold or food-poor periods outside winter, hazel dormice may also enter shorter bouts of daily torpor.
Fat-Tailed Dwarf Lemur: Up to 7 Months
Fat-tailed dwarf lemurs can hibernate for months during Madagascar's dry season, relying heavily on fat stored in their tails.
The fat-tailed dwarf lemur, Cheirogaleus medius, is famous because it demonstrates that long-term hibernation is not limited to animals living in freezing climates.
It inhabits Madagascar, where winter days can still become relatively warm.
Wild fat-tailed dwarf lemurs have been documented hibernating for approximately seven months.
They commonly shelter inside tree holes and rely heavily on fat accumulated before the dormant season.
Remarkably, their body temperature during hibernation can fluctuate greatly with the temperature inside the tree cavity.
If the cavity is poorly insulated, body temperature may passively rise and fall with environmental temperature rather than remaining continuously cold.
Is It the Only Hibernating Primate?
No.
Fat-tailed dwarf lemurs are the best-known example, but modern research has documented hibernation in several dwarf lemur species within the genus Cheirogaleus.
Dwarf lemurs are currently considered the only known obligate hibernators among primates.
Alaskan Wood Frog: Frozen for Around Six Months
Wood frogs in Interior Alaska can survive remaining frozen for months in shallow overwintering sites.
The wood frog, Rana sylvatica, uses a completely different strategy from a mammalian hibernator.
Populations in Interior Alaska overwinter close to the surface beneath leaves and other organic material.
During winter, much of the frog's extracellular body water freezes.
The heart stops.
Breathing stops.
Normal movement disappears.
Yet the animal can survive and resume normal activity after thawing.
Field studies recorded wild Alaskan wood frogs remaining frozen for an average of approximately 193 consecutive days — about 6.3 months — with all monitored animals surviving until spring.
The frogs use high concentrations of glucose and urea as cryoprotectants that help limit cellular damage during freezing.
Can Wood Frogs Survive -18°C?
Wild Alaskan frogs have experienced very low environmental temperatures, and research has demonstrated survival at body-freezing temperatures much colder than populations from temperate regions.
However, it is misleading to say that a frog simply remains at -18°C continuously for eight months.
Its actual body and microhabitat temperatures fluctuate throughout winter.
Alpine Marmot: About Six Months
Alpine marmots live in social groups and may spend about half the year hibernating underground.
Alpine marmots, Marmota marmota, are classic true hibernators of European mountain environments.
Scientific studies commonly describe a hibernation period of approximately six months.
During deep torpor their metabolism, heart rate and body temperature fall dramatically.
One of their most interesting adaptations is social hibernation.
Family members can huddle together inside underground burrows, reducing heat loss.
This is particularly important for juveniles, which have fewer fat reserves than adults.
Little Brown Bat: More Than Six Months
The little brown bat, Myotis lucifugus, is one of North America's well-known cave-hibernating bats.
In northern climates, individuals may hibernate for more than six months while insects are unavailable.
They do not remain in one continuous unchanging state.
Instead, bats cycle through deep torpor and short arousals.
During torpor:
metabolic rate falls dramatically;
heart rate can drop from hundreds of beats per minute to only a few or a few dozen;
body temperature approaches the temperature of the hibernaculum;
breathing may pause for long intervals.
This energy-saving strategy also explains why white-nose syndrome is so damaging: infected bats awaken too frequently and rapidly exhaust the fat reserves needed to survive winter.
Grizzly Bear: Roughly Four to Six Months
Grizzly bears are sometimes excluded from discussions of “true hibernation” because their body temperature does not fall nearly as far as that of small rodents or bats.
Modern physiological definitions nevertheless recognize bears as hibernators.
Yellowstone National Park reports that grizzly denning duration varies strongly by sex and location.
Females with cubs average about 171 days in dens in Greater Yellowstone, while adult males average around 131 days.
Farther north, bears can spend six months or more in winter dens.
During this period they can go for months without eating, drinking, urinating or defecating.
Yet their body temperature falls only moderately compared with deep-hibernating rodents.
Common Hamster: Winter Hibernation With Food Stores
The common hamster, Cricetus cricetus, often hibernates between approximately November and March, although timing varies greatly.
Unlike many hibernators that depend almost completely on body fat, common hamsters store substantial amounts of food underground.
They periodically arouse from torpor and may feed from these caches.
Research shows that sex, age, body condition and food availability all influence how much time an individual spends in deep torpor.
Adult females may enter hibernation much later than males and therefore have much shorter hibernation seasons.
This variability is another reason “longest hibernation” lists should not present one fixed number for every member of a species.
What About the Common Poorwill?
The common poorwill, Phalaenoptilus nuttallii, deserves special mention because it is one of the most remarkable birds known for prolonged torpor.
It is a nightjar native to western North America.
During periods of cold weather and food shortage, poorwills can reduce body temperature and metabolic rate and remain torpid for extended periods.
Long bouts can last days or even weeks.
This behavior is sometimes described as avian hibernation.
However, reliable modern sources do not support the simple claim that every poorwill continuously hibernates for five months.
Calling it “the only bird that sleeps for five months” therefore exaggerates what has actually been documented.
What About the 344-Day Bat Hibernation Record?
You may see a remarkable claim online that a big brown bat hibernated for 344 days.
The record is based on an artificial experiment involving Eptesicus fuscus.
The bat was maintained under constant cold conditions in a refrigerator-like environment and supplied with water.
This demonstrated how long the physiology of a bat could potentially sustain torpor under unusual circumstances.
However, the animal did not voluntarily choose such a long natural hibernation season and did not survive the extreme experiment.
For that reason, it should not be ranked above wild edible dormice as evidence of the longest natural hibernation.
Do Tortoises Hibernate?
Many temperate tortoises become dormant during cold periods, but researchers and veterinarians often use the term brumation for seasonal dormancy in reptiles.
Russian tortoises, also called Horsfield's tortoises, may spend long periods inactive underground during both unfavorable cold and very hot, dry conditions.
Warm-season dormancy is called aestivation.
These processes differ physiologically from mammalian hibernation, so combining tortoises, bats, bears and rodents into one precise ranking can be misleading.
Hibernation vs Torpor vs Brumation
| Term | Typical Animals | What It Means |
|---|---|---|
| Hibernation | Mammals such as dormice, ground squirrels and bats | A prolonged seasonal period containing repeated deep torpor bouts |
| Daily torpor | Hummingbirds, small mammals and some other birds | Short-term metabolic suppression lasting hours rather than an entire season |
| Prolonged torpor | Common poorwill and some mammals | Extended metabolic depression that can last days or weeks |
| Brumation | Reptiles | Cold-season dormancy in ectothermic reptiles |
| Aestivation | Some reptiles, amphibians, snails and other animals | Dormancy during hot or dry conditions |
| Freeze-tolerant overwintering | Wood frogs | Survival while body fluids partially freeze during winter |
Is Hibernation Just a Very Long Sleep?
No.
Sleep and hibernation are physiologically different states.
During deep torpor, an animal may experience:
dramatically reduced metabolic rate;
very low heart rate;
slow or intermittent breathing;
large decreases in body temperature;
suppressed kidney and digestive activity;
major changes in brain activity.
Many hibernating mammals periodically return to normal body temperature even though doing so consumes a large amount of energy.
Scientists are still investigating why these spontaneous arousals are necessary.
Why Do Animals Hibernate?
The main evolutionary advantage is energy conservation.
Cold winters often create two simultaneous problems:
maintaining body temperature requires more energy;
food becomes less available.
Hibernation dramatically lowers energy needs until favorable conditions return.
However, cold itself is not always the primary reason.
Fat-tailed dwarf lemurs hibernate in tropical Madagascar during a season when food and water availability decline.
Edible dormice can begin extraordinarily long hibernation during years when reproductive opportunities are poor.
This shows that seasonal food economics can be just as important as low temperature.
Which Mammal Has the Lowest Body Temperature During Hibernation?
The Arctic ground squirrel is one of the most extraordinary known examples.
Researchers have measured core body temperatures below 0°C and as low as approximately -2.9°C.
The animal remains unfrozen through supercooling.
When it spontaneously arouses, brown adipose tissue and shivering generate enough heat to raise body temperature back toward normal mammalian levels.
Do Hibernating Animals Wake Up?
Most mammalian hibernators do.
A typical hibernation season consists of repeated cycles:
deep torpor → spontaneous arousal → short period at normal temperature → return to torpor.
The length of each torpor bout varies greatly among species and environmental conditions.
Arctic ground squirrels, for example, can remain continuously torpid for more than two weeks before spontaneously warming again.
Do Bears Really Hibernate?
Yes.
The old distinction claiming bears are not “true hibernators” is largely outdated.
Bears do not cool nearly as dramatically as small rodents, but they strongly suppress metabolism, remain inactive for months and survive long periods without eating or drinking.
The U.S. National Park Service classifies grizzly bears as true hibernators while noting that they are relatively shallow hibernators compared with many small mammals.
Frequently Asked Questions
What animal hibernates the longest?
The edible dormouse, Glis glis, has one of the best documented natural records. Free-living individuals have been observed hibernating for as long as approximately 11.4 months during years when they skipped reproduction.
Can an animal really hibernate for 11 months?
Yes. Wild edible dormice have been scientifically monitored through hibernation periods exceeding 11 months. They do not remain in one uninterrupted sleep state, because hibernation includes periodic arousals.
How long do Arctic ground squirrels hibernate?
Their hibernation season can last approximately six to nine months. They are also famous for allowing their core body temperature to fall below 0°C without freezing.
How long do dormice hibernate?
It depends on the species. Hazel dormice commonly spend roughly six or seven months in winter hibernation, while edible dormice can exceed 11 months under exceptional natural conditions.
How long do wood frogs hibernate?
Wild wood frogs in Interior Alaska have been documented remaining frozen for around 193 consecutive days, or roughly six months.
How long do bears hibernate?
Duration depends strongly on latitude, sex and reproductive status. In Greater Yellowstone, grizzlies commonly spend about four to five and a half months in dens, while bears farther north may remain denned for six months or longer.
Do birds hibernate?
Most birds do not undergo mammal-like seasonal hibernation. The common poorwill is exceptional because it can enter unusually prolonged torpor lasting days or weeks and is frequently described as a hibernating bird.
What bird hibernates?
The common poorwill, Phalaenoptilus nuttallii, is the classic example. It can remain in prolonged torpor when weather is cold and flying insects are unavailable.
Did a bat hibernate for 344 days?
A big brown bat was maintained in an artificial low-temperature experiment for approximately 344 days. However, this was not a normal natural hibernation season and the animal did not survive the extreme experiment.
Do reptiles hibernate?
Many reptiles become dormant during winter, although the term brumation is commonly used because their physiology differs from mammalian hibernation.
Do hibernating animals sleep the whole time?
No. Mammalian hibernation normally consists of repeated bouts of deep torpor separated by brief spontaneous arousals during which body temperature and metabolism rise dramatically.
Scientific and Official Sources
Hoelzl et al., Journal of Comparative Physiology B — wild edible dormice hibernating for 7.8–11.4 months.
University of Alaska and published physiological research — Arctic ground squirrel hibernation, supercooling and subzero body temperatures.
Larson et al., Journal of Experimental Biology — natural freeze tolerance and approximately 193-day freezing in Alaskan wood frogs.
Dausmann et al., Nature — seven-month hibernation in the fat-tailed dwarf lemur.
Duke Lemur Center research — hibernation across dwarf lemurs.
People's Trust for Endangered Species — hazel dormouse hibernation ecology.
U.S. National Park Service — bat hibernation, common poorwill torpor and grizzly bear hibernation.
Published Alpine marmot physiology research — approximately six-month hibernation seasons.
Published common hamster physiology studies — seasonal hibernation and variation between sexes.
