The Big Sleep: How Animals Use Hibernation To Survive the Winter

How does hibernation work and which animals use it?

Hazel Dormouse (Muscardinus avellanarius) hibernating in burrow
(Photo: .

Winter can be tough in the animal kingdom. Colder temperatures, shortened daylight hours, and lack of food can make it pretty rough for many animals to survive. So how do they do it?

Some animals adapt to winter by hibernating. Hibernation is a state of inactivity that allows animals to conserve energy by slowing their metabolism and reducing their body temperature for days, weeks or even months at a time.

Hibernation is characterized by low body temperature, slow breathing and heart rate, and low metabolic rate. Hibernating species usually build up large fat reserves before they begin to hibernate, and subsist on this during their sleep.

In the past, the term, "hibernation," was reserved for "deep" hibernators such as rodents that experience an absolute body temperature decline. But today, the term is used as an umbrella to include animals such as bears, skunks, and raccoons that use torpor, or a lighter form of hibernation to suppress their metabolism. Torpor is like "hibernation lite." Animals still experience a decrease in body temperature to conserve energy but it is not quite as great that noted for hibernators.

Most animals create a special nest, tunnel, or den called a hibernacula that they can use to remain safe from predators during hibernation.

What's The Difference Between Torpor and Hibernation

Both torpor and hibernation are tactics that animals use to survive the winter months. Both involve periods of deep sleep, but torpor is a short-term reduction of body temperature on cool days, while hibernation is an extended form of torpor.  Torpor is generally considered involuntary. It is driven by outside temperature and the availability of food sources.

Hibernation, on the other hand, is a voluntary state entered into in response to day length and hormone changes. 

Typically, the internal body temperature and the metabolic rate of an animal drops a lot lower during hibernation than in torpor. For example, the heart rate of an active hedgehog is 200 to 280 beats per minute, but only 5 bpm in hibernation.

Top 10 Hibernators

So which animals use hibernation to survive the winter?  Here are some of the world's most common hibernators:

  1. Ground Squirrels. Most types of ground squirrels hibernate. In fact, some hibernate for as many as nine months out of the year. Some ground squirrels also use torpor for a few days at a time during the rest of the year to conserve energy. Ground squirrels dig elaborate underground tunnels to use as hibernaculums, with different rooms for food storage, sleep, and even bathrooms.
  2. . The Common Poorwill is the only bird species that is known to hibernate. They hibernate by picking a spot under a shallow rock or a rotten log when the temperatures are really cold or really hot (also called estivation,) or when food is scarce. 
  3. . When most people think about hibernating animals, they think about bears. But bears actually use torpor to survive the winter months. 
  1. Bats. When left alone, bats experience some of the longest hibernation lengths on record. In the wild, big brown bats have spent 64-66 days in hibernation while in captivity one lasted 344 days! The heart rate of a hibernating bat drops from 1000 beats per minute to only 25 and some bats only take a breath every 2 hours. Bats hibernate in large trees, caves, and even in people’s attics. 
  2. Hedgehogs. Hedgehogs are some of the deepest hibernators. During hibernation, their heart rate drops almost 90 percent. But if their temperatures drop too low, they will wake briefly to increase their heart rate and warm up before going back into hibernation.
  3. Snails. Snails carry their hibernacula right on their backs. When the time comes, they retreat into their shell and seal up the hole with a skin made of chalk and slime. There is a small hole in the opening to let air in while keeping predators out.
  1. . Hibernating box turtles make their hibernacula in well hidden holes and dens. During hibernation, their heart beat drops to just one every 5-10 minutes and they don’t have to breathe in air at all (but they do absorb oxygen through their skin.)
  2. Skunks. Like bears, skunks go into a lighter form of hibernation than "true hibernators." They hibernate in dens with their families and occasionally wake up to find food.
  3. Deer Mice. Another example of light hibernators, door mice torpor from morning to late afternoon bundled up in groups. Then they wake at night to search for food. This daily energy conservation helps them survive the winter months.
  4. Bumblebees. When winter hits, males and worker bees die off but the queen survives by hibernating. She makes her hibernaculum in a hole in the soil, in rotten tree stumps or under leaf litter. After several months, the queen will emerge, warm-up and then find a nice spot to build a nest and create a new team of bees.