By Leah Rambadt, American Forests

How do most wildlife survive the cold, winter temperatures? A few ideas may immediately come to mind: Mammals growing another layer of fur, birds migrating south, bears hibernating, etc.

Well, it turns out bears aren’t easily classified as hibernators and not all birds migrate for the winter. Instead, some birds enter a scaled-down version of hibernation known as torpor.

There are three types of sleeping techniques wildlife may use to escape bad weather conditions: hibernation, torpor and estivation. All three allow an animal’s bodily functions to slow down, so its energy is used to sleep through periods of extreme temperature instead of looking for food.


Hibernation is a voluntary state triggered by day length and hormone changes within an animal that indicate the need to conserve energy. An animal enters hibernation when food is scarce, and to reduce the need to go out and face the winter conditions. Most animals store fat before entering hibernation, and may wake up briefly to eat, drink or excrete waste during hibernation. A hibernating animal remains in a deep sleep that mimics a death-like state.

Characteristics of Hibernation:

  • Triggered by day length and hormone changes
  • Lowered body temperature — may become as low as the surrounding air.
  • Slowed breathing and heart rate — an animal may breathe once every five minutes, and heartbeats may occur four or five times a minute.
  • Lowered metabolic rate
  • Duration: An animal tries to remain in hibernation for as long as possible. Depending on the species, an animal can remain in hibernation for several days, weeks or months at a time.
  • Waking: It takes several hours for a hibernating animal to wake up, and it uses most of its conserved energy reserve to do so.

Examples of Hibernators: hedgehogs, bats, ground squirrels, wood frogs, some species of prairie dogs

The common poorwill is the only bird known to hibernate. Credit: Doug Backlund/

So…Do Bears Hibernate?

For years it’s been debated whether bears are true hibernators. Bears can go 100 days or so without needing to wake up to consume anything or excrete waste, which is uncharacteristic for animals that hibernate. They can also be woken much more easily than other hibernators. Since bears don’t exactly meet the characteristics for most hibernators, the U.S. National Park Service suggests they are super hibernators.

Bears are unlike any other hibernator, since they stay in hibernation for longer stretches of time without needing to wake up. Credit: Ingo Arndt Minden Pictures/Corbis/


Torpor is a short-term version of hibernation. It shares many characteristics with hibernation, but it’s an involuntary state that an animal can enter at any time depending on environmental and weather conditions. Animals that enter longer periods of torpor — several months instead of several hours — may occasionally wake up and leave their dens in search of food.

Characteristics of Torpor:

  • Can occur at any time based on food availability and ambient temperature
  • Lowered body temperature
  • Slowed breathing and heart rate
  • Lowered metabolic rate
  • Duration: An animal stays in torpor for short periods of time. Depending on species and feeding patterns, an animal may stay in torpor just through the day or night instead of several days.
  • Waking: An animal in torpor can be easily woken, though depending on the animal, it can take anywhere from several minutes up to an hour for it to wake up. While waking uses energy, this loss is counteracted by the energy saved while the animal was in torpor.

Examples of Winter Torpor Users: raccoons, skunks, some species of prairie dogs

Skunks exhibit wintertime torpor. Credit: cc by 3.0 via

Birds may use torpor in order to stay in their territories year round, instead of migrating to areas with better food availability and weather conditions.

However, torpor use isn’t limited to the winter months.

Many animals use torpor under different conditions on a regular basis. Smaller bird species, marsupials and rodents that live in areas with unpredictable food availability and extreme nighttime conditions may enter torpor.

Examples of Regular Torpor Users: hummingbirds, common poorwills, ground squirrels, deer mice, bats, hedgehogs

An example of a hummingbird in torpor. Credit: Mary Ann Jacobs


Estivation (also called aestivation), like torpor, has the same characteristics as hibernation — except it’s exhibited in the summer! Many animals, both vertebrate and invertebrate, use estivation to stay cool and avoid drying out during the hottest and driest months of the year.

Characteristics of Estivation:

  • Triggered by high temperatures and low water levels
  • Lowered body temperature
  • Slowed breathing and heart rate
  • Lowered metabolic rate
  • Duration: An animal tries to stay in estivation until the temperature cools (nighttime).

Examples of Estivation Users: some species of hedgehogs, some salamanders, desert tortoises, crocodiles

So if there’s a nice, sunny day this winter and you want to head out to the woods, keep an eye out for some of the animals listed above. They might decide to wake up and enjoy the weather too!