Hibernation
Some animals handle winter by nearly switching themselves off.
By Greg Goodwin · Published October 16, 2026 · Updated October 24, 2026
Cheat Sheet
- What it is: a prolonged state of torpor, with sharply reduced body temperature, heart rate, and metabolism, used to survive seasons of scarcity.
- True hibernators: ground squirrels, marmots, hedgehogs, many bats, and the fat-tailed dwarf lemur, the only known hibernating primate.
- Extreme case: the Arctic ground squirrel can let its body temperature drop to about -3 C (27 F) without freezing solid.
- Bears: winter dormancy with a milder temperature drop (to roughly 30-35 C), so they can wake more easily and some give birth during it.
- Cost of waking: periodic arousals every one to three weeks use a large share of a hibernator's winter energy.
- Related states: daily torpor in hummingbirds and some bats, and estivation (summer dormancy) in some desert animals.
The 60-Second Version
In regions where winter means little food and brutal cold, staying active would burn more energy than an animal can find. Hibernators solve the problem by dramatically slowing down. A ground squirrel's heart rate can fall from about 200 beats a minute to under 10, and its breathing may pause for minutes. Body temperature drops close to that of its den. The animal lives off fat built up in autumn. Not all winter sleepers are alike: bears enter a lighter dormancy with a smaller temperature drop, while some small mammals and birds use brief torpor on a daily basis. Scientists study hibernation for clues to protecting organs, preventing muscle loss, and perhaps even long-distance space travel.
The Long Version
Why Slow Down
Metabolism rises as temperature falls for warm-blooded animals trying to keep warm, which is costly when food is scarce. By letting body temperature fall, hibernators cut energy use by as much as 95 percent. Hibernation is therefore a strategy for saving energy rather than a long sleep. Small mammals gain the most, since they lose heat quickly. Many species also depend on stable, insulated shelter.
Getting Ready
Hibernators gorge in the late summer and autumn, sometimes doubling their body weight. Fat is the main fuel because it stores a lot of energy per gram. Some species, like chipmunks, also cache food and wake to eat. Hormonal and genetic changes shift the body's metabolism, and appetite and activity patterns change on an internal annual cycle that persists even in captivity.
Deep Sleep, Not Quite Dead
During deep torpor, a hibernating Arctic ground squirrel can cool to below freezing, the lowest known body temperature for a mammal, while keeping its blood from freezing. Its brain activity drops dramatically, yet the animal remains alive and responds to extreme conditions. Despite this, hibernators periodically rewarm to normal temperature for a day or so, shivering and burning brown fat. This consumes large energy reserves, and researchers think arousals may allow sleep, immune function, or clearing metabolic waste.
Bears and Other Variations
American black bears and brown bears sleep for months without eating, drinking, or defecating, yet their temperature falls only a few degrees. They can wake and defend themselves. Females give birth in the den and nurse cubs while dormant. Bears lose little muscle or bone despite months of inactivity, which interests medical researchers. Meanwhile, hummingbirds enter torpor each night, and the common poorwill is the one bird known to hibernate for weeks.
Lessons for Medicine
Hibernators avoid problems that harm humans lying still for long periods, such as bone loss, muscle wasting, and blood clots. Studying the molecular signals that trigger hibernation could help preserve organs for transplantation, protect the brain after stroke, or treat metabolic disorders. Space agencies have also considered induced torpor for deep-space missions, though turning these ideas into treatments for people remains a long way off.
Why People Care
Hibernation shows how flexible biology can be, and it offers inspiration for medicine. It also highlights how climate change may disrupt animals that depend on seasonal cues, such as bats and marmots.
Glossary
- Torpor
- A short-term state of lowered metabolism and body temperature, lasting hours to days.
- Estivation
- Dormancy during hot or dry periods, as seen in some snails, lungfish, and desert tortoises.
- Brown adipose tissue
- Heat-producing fat that helps hibernators rewarm.
- Supercooling
- Cooling below freezing without turning to ice.
- Hibernaculum
- The den, burrow, or cave where an animal hibernates.
Go Deeper
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- Mammals
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- Wolves
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- Sharks
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- Symbiosis
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