Mammal Image Gallery
Mammal Image Gallery

Hazel dormouse hibernating in burrow. See more pictures of mammals.

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How Hibernation Works

Every living animal on Earth is burning energy all the time. Physical activities like walking and breathing burn energy. Pumping blood and digesting food burns energy. Even thinking burns energy. For warm-blooded animals, a lot of energy is burned just keeping our body temperature where we need it. Even when we're sleeping, we're burning energy.

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That's the whole reason animals eat -- to gain enough energy to fuel all those processes. The system works fine when there's plenty of fruit on the trees or rabbits to catch and eat (or pizzas in the freezer). But what happens when winter comes and it becomes very difficult to find food? How do animals survive with few energy sources available?

There are many winter survival strategies in the animal world, and one of the most fascinating is hibernation. Some animals enter a state of "suspended animation." Their breathing and heart rates slow and they allow their body temperature to drop, in some cases even below freezing. They stop eating and in many cases stop excreting. All of these things happen so the animal can use less energy.

Whether an animal hibernates or migrates to a warmer area is generally a quirk of evolution. Smaller animals tend to be more likely to hibernate, because migration would require an enormous amount of energy relative to their body size. Larger animals are less apt to hibernate because of the additional energy required to warm up a large body.

Hibernation is more varied than you might think. Many animals hibernate in a den all winter, but some animals hibernate in the summer. Some fish can hibernate in a waterproof mucus envelope if their lake dries up. Certain birds and bats enter a sort of daily hibernation called torpor.

On the next page, we'll discuss how hibernation is different from sleep -- and learn what happens to animals in the zoo.

Early spring sun makes it warm enough to wake up a hibernating black bear.

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What's the Difference Between Hibernation and Sleep?

Biologists love to argue about how to classify things, and hibernation is no different. A common definition of hibernation is a long-term state in which body temperature is significantly decreased, metabolism slows drastically and the animal enters a comalike condition that takes some time to recover from. By this definition, bears don't hibernate, because their body temperature drops only slightly and they awake relatively easily. Not everyone accepts this narrow definition, however. For the purposes of this article, we'll use the term hibernation to describe any long-term reduction in body temperature (hypothermia) and metabolism during winter months.

When an animal enters a hibernationlike state during the summer, it's known as estivation. It's much less common than hibernation. Hibernation in reptiles is sometimes called brumation. It differs from mammalian hibernation because reptiles are cold-blooded -- they can't control their own body temperature, so they need to spend the winter in a place that will stay warm enough.

Torpor is another word that causes some confusion. It's sometimes used as an umbrella term to describe all the various types of temperature- and metabolism-reducing functions. More commonly, it's used to describe short-term periods of reduced temperature that occur as often as every day and only for a few hours at a time. This is the usage we will use to avoid confusion.

The poorwill is the only species of bird that truly hibernates. It drops its body temperature up to 60 degrees Fahrenheit and can live up to 100 days on 10 grams of stored body fat.

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So is hibernation basically a really long nap? No. These animals aren't just sleeping, they're undergoing physiological changes that can be very drastic. The most significant element of hibernation is a drop in body temperature, sometimes as much as 63 degrees F. We'll get into the details shortly, but for now it's sufficient to say that a hibernating animal's vital signs are very different from the vital signs of an awake animal.

Sleep, by contrast, is a mostly mental change. There are physiological aspects of sleep that are similar to hibernation, such as a reduced heart and breathing rate and lowered body temperature, but these changes are very slight compared to hibernation. Sleep is also pretty easy to break out of -- if you're awakened from even your deepest sleep, you can be fully awake within several minutes. Sleep is primarily characterized by changes in brain activity. In fact, the brain waves of hibernating animals closely resemble their wakeful brain wave patterns, though they're somewhat suppressed. When an animal awakes from hibernation, it exhibits many signs of sleep deprivation and needs to sleep a lot over the next few days to recover.

Chipmunks use their expandable cheek pouches to carry large amounts of food back to their nests, where they store it for winter.

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Knowing When To Hibernate (and When To Wake Up)

Different animal species hibernate at different times, and each species has a different way of knowing when the time is right. Hibernation is most closely regulated by temperature. When it gets cold outside, animals get ready to hibernate. When it warms up, they wake up. Therefore, hibernation periods can vary depending on the weather that year. An Indian summer and an early thaw could result in a very short hibernation.

Some species keep a close eye on their food supplies. When they dwindle, the animal knows it's time to gather up whatever is left and turn in for the winter. Photoperiod (the length of the day) triggers hibernation for others.

Even if an animal has no idea what the outside temperature is, how early the sun is setting or the current state of food supplies, many would still enter a hibernation state around the same time each year. Experiments under these conditions have proven that some species will automatically enter hibernation at the appropriate time, guided by an internal biological "calendar" [source: Roots]. These circannual rhythms aren't fully understood, but all animals are affected by them, even humans. Animals that go into daily torpor depend instead on circadian rhythms, the daily version.

Preparation is required to hibernate successfully. Some animals prepare a den (also known as a hibernacula) and line it with insulating material, just as leaves or mud. Ground squirrels and lemurs do this. Polar bears dig tunnels in the snow. Other bears might spend the winter in a hollow beside a tree or a shallow cave, leaving them partly exposed to the weather. Bats are well-known for wintering in caves or attics.

Next comes food storage. Food can be kept in the den if it's nonperishable, but this requires the animal to wake up briefly during the winter to eat. Another option is to eat a large amount of food starting in late summer, building up a reserve of internal fat. Some animals even do both. If enough food can't be found to prepare for hibernation, it can be delayed.

The cape sand frog uses its hind legs to burrow into the sand, where it hibernates during the South African winter.

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The Nuts and Bolts of Hibernation

Hibernation is mainly controlled by the endocrine system. Glands in the body alter the amounts of hormones being released and can control just about every physiological aspect of hibernation.

  • Thyroid - gland that controls metabolism and activity levels
  • Melatonin - hormone that controls the growth of winter coats
  • Pituitary - gland that controls fat buildup, heart rate and breathing rate, as well as metabolic functions
  • Insulin - hormone that regulates the amount of glucose (sugar) needed by the animal

When a mammal enters hibernation, it becomes somewhat like a cold-blooded animal. Its body temperature will vary depending on the temperature around it. However, there is a minimum temperature, known as a set point. It's just like setting the temperature on your thermostat at home. When the mammal's body temperature reaches the set point, the metabolism kicks in and burns some fat reserves. This generates some energy, which is in turn used to heat things back up above the set point. Larger animals have a higher set point. If they let their temperature drop too low, it would require an enormous amount of energy to heat back up again.

Several other things occur when an animal is hibernating:

  • Heart rate drops to as little as 2.5 percent of its usual level. A chipmunk's heart rate slows to five beats per minute from the usual 200.
  • Breathing rate drops by 50 percent to 100 percent. Yes, 100 percent. Some animals stop breathing entirely. A few reptiles go their entire hibernation period without breathing, and even mammals have shown the ability to survive with drastically reduced oxygen supplies.
  • Consciousness is greatly diminished. This varies by species, but many hibernating animals are completely oblivious to their surroundings and are nearly impossible to wake up. If you were to wake up a hibernating animal midwinter, you would be effectively killing it. It would use up so much energy warming itself up in order to awaken that it would have no chance of making it to spring even if it could re-enter hibernation.

Body fat, which is packed with energy, is burned off to provide the energy necessary to maintain these minimal levels of body functions. This can be very efficient -- Arctic ground squirrels live entirely off of stored body fat for as long as nine months. Some species are unable to store enough body fat, so those animals have a lighter hibernation, allowing them to awaken periodically for a snack.

If an animal is burning fat or snacking on stored nuts all winter, what happens to all the waste? No fecal matter is produced because nothing is passing through the digestive tract and intestines. But the body is always producing urea, the waste product that is the main component of urine. Hibernating animals' bodies are able to recycle the urea. Bears don't urinate all winter, but they break the urea down into amino acids. Even though they don't drink, they don't get dehydrated either. They're able to extract enough water from their own body fat to stay hydrated.

Close-up of a lungfish in a tank of water

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Estivation and Daily Torpor

Estivation is like hibernation in hot weather. Animals that live in deserts or tropical climates practice estivation. It may not occur solely because of food supply issues, as with hibernation, but because the conditions become too hot and dry for the animal to survive. The process typically involves burrowing into the ground, where the temperature stays cool, and reducing metabolic activity in a similar manner to hibernation.

Lungfish are capable of an amazing form of estivation that allows them to live without water for as long as three years. Lungfish are primitive fish that still have lungs, allowing them to breathe air. When a lungfish's lake dries up, the fish burrows into the mud, then secretes mucus until its entire body is covered. The mucus dries into a sack that holds moisture in. Even when the mud dries completely, the lungfish stays moist and breaths through a mucus tube.

Many species of birds use daily torpor to get through colder months. The black-capped chickadee is a good example. Daily torpor (which really ought to be called nightly torpor) is like low-grade short-term hibernation. It only lasts for a few hours, and the reduction in body temperature's just a few degrees. However, studies have shown that those few degrees save a significant number of calories from being burned off overnight.

Hummingbirds have an incredibly high metabolic rate, with a heart rate that can exceed 1,200 beats per minute. Their energy consumption is so great that hummingbirds use daily torpor to conserve energy even in the tropics. Hummingbird torpor is more profound than that of other birds, with a temperature exceeding 50 percent.

For more information on hibernation and animal topics like migration and symbiosis, try the next page.

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Sources

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  • Heinrich, Bernd. Winter World : The Ingenuity of Animal Survival. Harper Collins, 2003.
  • Open University. "Animals at the extremes: hibernation and torpor." http://openlearn.open.ac.uk/mod/resource/view.php?id=192762
  • Roots, Clive. Hibernation. Greenwood Press (September 30, 2006).
  • Yan, Jun. "The Detection of Differential Gene Expression in Brown Adipose Tissue of Hibernating Arctic Ground Squirrels Using Mouse Microarrays." Physiol Genomics (February 7, 2006). http://physiolgenomics.physiology.org/cgi/reprint/00260.2005v1.pdf