What is hibernation?

Hibernation, aestivation, torpor and denning explained.

A hibernating dormouse. © Sasha Fox Walters/iStock

Hibernation is a way for many creatures – from butterflies to bats – to survive cold, dark winters without having to forage for food or migrate to somewhere warmer. Instead, they turn down their metabolisms to save energy.


Animals in hot climates also undergo a form of hibernation called aestivation. This works in a similar way and enables them to survive extreme heat, drought or lack of food.

Hibernating is much more profound than simply sleeping, though. Depending on the species, it can vary from long, deep unconsciousness to light spells of inactivity.

But hibernation carries risks as the dormant animal is vulnerable to predators and the unpredictable climate.

A baby hedgehog born too late to have enough fat reserves for hibernation.
A baby hedgehog born too late to have enough fat reserves for hibernation. © lorenzo104/Getty


Which animals hibernate? Small mammals, such as chipmunks, dormice, hamsters, hedgehogs and bats. Also, many insects, amphibians and reptiles.

Just one bird is known to be a true hibernator: North America’s common poorwill. This beautifully camouflaged nocturnal bird is a relative of the nightjar found in Britain, and in winter often hibernates among rocks. It can slash its oxygen intake by 90%, while its body temperature plummets to 5°C, barely registering signs of life.

How does it work? A hibernating animal’s metabolism slows and its temperature plunges – in ground squirrels it can fall to -2°C. Breathing slows and, in bats, the heart rate can fall from 400 to 11 beats per minute. Some cold-blooded animals, such as wood frogs, produce natural antifreezes to survive being frozen solid.

How do they prepare? Mammals feed heavily in summer and autumn, storing fat to see them through the winter.

What are the dangers? Animals may die during hibernation from lack of fat, severe weather or premature awakening.

Common dormouse (Muscardinus avellanarius) curled up asleep in nest with hazel nuts
Common dormouse (Muscardinus avellanarius) curled up asleep in nest with hazelnuts. © Nature Picture Library/Getty


What is aestivation? This is the equivalent process to hibernation, but for animals in hot climates that are trying to escape extreme heat or drought.

Which animals aestivate? Many terrestrial and aquatic animals, including lungfish, earthworms, snails, amphibians and reptiles, including Nile crocodiles.

How does it work? Most animals bury themselves in the ground, which protects them from the heat. Here, they wait for the wet season or cooler temperatures. Some land snails climb trees to escape the heat of the ground, sealing themselves into their shells using dried mucus.

What are the dangers? Large numbers of aestivating animals perish in periods of prolonged drought.


What is torpor? It is a brief bout of suspended animation, usually lasting less than a day, when an animal’s breathing, heartbeat, body temperature and metabolism are reduced.

How does it work? Torpor conserves energy in the short term and often helps the animal survive a brief bout of poor conditions, such as cold nights.

Which animals enter torpor? Birds such as hummingbirds and frogmouths, or small mammals such as bats, can go into torpor every day.

What are the dangers? One of the problems with torpor is that the animals are too sluggish to react to predators. And, if the cold spell is unusually long, the animal may die if its body temperature drops too low.

A hummingbird in torpor.
A hummingbird in torpor. © Hailshadow/Getty


What is denning? This is a light form of dormancy typical of bears, where the animal is groggy, but easily roused.

How does it work? A bear’s body temperature only drops a few degrees, but it loses up to 40 per cent of its body weight – more than true hibernators. Amazingly, many female bears give birth and suckle young while denning.

How do they prepare? Bears eat a lot of high-energy food to build-up fat reserves that will last all winter.

What are the dangers? Bears can be woken easily during a mild spell of weather, but may not have enough energy to survive the rest of the winter.

Polar bear (Ursus maritimus) cubs looking out of the den
Polar bear cubs looking out of the den. © Thorsten Milse/Robert Harding/Getty



An aestivating primate

The dwarf fat-tailed lemur of Madagascar is the only known primate to aestivate, using up fat reserves in its tail during a long dry season.


A hibernating bird

The common poorwill, a small species of nightjar, is the only bird known to hibernate. It conceals itself among piles of rocks to escape winter.

Common Poorwill camouflaged on rock
Common Poorwill camouflaged on rock. © Jared Hobbs/Getty

A dormant fish

The Antarctic cod Notothenia coriiceps can enter a state of dormancy by lowering its metabolism. Its blood also contains ‘antifreeze’.


A sneaky snake

Male garter snakes are the first to emerge from their winter dens in order to mate with females as they wake up. Some males emit female pheromones to trick their rivals.