Do Bats Hibernate? Where Bats Go and What They Do in Winter

Why bats hibernate

Almost all bats in temperate regions eat insects, and insects become scarce once nights turn cold. A small bat burns energy fast: a common pipistrelle weighs only 3.5–8.5 g, and it could not survive many days without food if it kept its body warm. Hibernation solves the problem by switching the body into energy-saving mode until spring.

Bats prepare in late summer and autumn by feeding heavily and building up fat, sometimes increasing their weight by a third or more. That fat must last the whole winter, often five or six months in the north.

Hibernation is not the same as sleep. A sleeping bat keeps its body warm, while a hibernating bat lets its temperature drop and becomes cold and stiff to the touch. Many bats also use shorter bouts of torpor in summer, cooling down for part of a day in their roost to save energy when the weather is cold or wet.

What happens to a bat’s body?

A hibernating bat is in deep torpor, a controlled state of very low activity. In the little brown bat:

• Body temperature falls from about 35–40 °C (95–104 °F) when active to just above the air temperature of the cave, often 2–10 °C (36–50 °F).
• Heart rate drops from up to about 210 beats per minute at rest, and much higher in flight, to as few as about 8–20 beats per minute.
• Breathing slows to a few breaths a minute, sometimes with pauses of several minutes.
• Energy use falls to a tiny fraction of normal, less than 5% of the resting rate in some studies.

Even so, a little brown bat loses around a quarter of its body weight over the winter. Bats wrap their wings around their bodies and choose spots with the right temperature and humidity; many hibernate in clusters, while others tuck themselves alone into cracks.

Waking up to drink

Bats do not sleep straight through. Every week or few weeks they rouse for a short time, warming up from near 5 °C to about 35 °C within roughly an hour using heat from brown fat and shivering. During these arousals they may drink condensation from cave walls, urinate, move to a better position or, on mild nights, fly outside.

Arousals are very expensive. A classic study of little brown bats in Canada found that the brief arousals used most of the fat burned during the whole winter, even though the bats spent most of their time in torpor. This is why every unnecessary wake-up matters.

Where bats hibernate

A good hibernation site, called a hibernaculum, stays cold but above freezing all winter, usually about 2–10 °C (36–50 °F), with high humidity so bats do not dry out. Common choices include:

• Caves and old mines: the classic sites, used by little brown bats, Indiana bats and many European Myotis species.
• Tunnels, cellars, ice houses and bunkers: important in much of Europe.
• Buildings: cracks in walls, cavity walls and roof spaces, used by pipistrelles and big brown bats.
• Trees: thick-walled tree hollows, used by noctules and other woodland bats.

Some bats return to the same site year after year and travel long distances to reach it. Female little brown bats may fly hundreds of kilometers from their summer roosts. Read more in Where do bats live?.

Bats that migrate instead

Not all bats hibernate locally. Some species fly south in autumn to warmer regions. In North America, hoary bats, eastern red bats and silver-haired bats migrate, and many Mexican free-tailed bats leave the southwestern US for Mexico. In Europe, Nathusius’ pipistrelle travels between the Baltic region and western or southern Europe, with recorded journeys of more than 2,000 km (1,240 mi). Even migrants may enter torpor in cold weather along the way.

Bats in the tropics, including most fruit bats, do not hibernate at all, because food is available year-round. For a broader look at winter sleepers, see animals that hibernate.

Why disturbance is harmful

Because every arousal burns fat, people can unintentionally kill hibernating bats. Noise, lights, body heat and touching can all wake a bat, and a few extra arousals may leave it without enough fat to survive until spring.

In North America, the danger is made worse by white-nose syndrome, a fungal disease that grows on hibernating bats and makes them wake far too often. Millions of bats have died, and many caves are closed in winter. Learn more in Why are bats dying in North America?.

For these reasons, bats and their roosts are legally protected in the UK and the European Union, and several US states protect bat hibernation sites. Surveys of hibernating bats are carried out by licensed experts. If you visit caves, follow closure notices and decontamination rules, and never disturb bats you see.