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What is Hibernation?

S. Mithra
S. Mithra

Animals enter hibernation during winter to conserve energy by going into a deep sleep-like state. Mammals, such as gophers, bears, skunks, raccoons, hamsters, and bats, lower their metabolism and enter a state of torpor, but they are not asleep. With a slowed heart rate and lowered body temperature, these animals have adapted to survive cold winters with little or no sustenance.

The dormant state means the hibernating animals minimally eat, drink, move, think, or defecate. Some "deep" hibernators, like bears, almost never rouse themselves once they are secure in their den. Other animals, especially rodents, frequently come out of this state to snack on food harvested during the summer and fall. Cold-blooded creatures, such as reptiles and amphibians, also can be said to hibernate. They are always the same temperature as their surroundings, however, so this "sleep" means something different. For instance, wood frogs actually freeze solid over winter, while a natural antifreeze, glucose sugar, protects their organs.

Bears are considered "deep" hibernators, which rarely move out of their dens in winter.
Bears are considered "deep" hibernators, which rarely move out of their dens in winter.

Once in wintering mode, in a snug den, most animals do not need significant external energy sources. They survive plummeting temperatures by lowering their own body temperature, sometimes to within degrees of the freezing point of water. Physiologically, their bodies reduce their need for energy by almost stopping their heartbeat.

In the months leading up to the cold season, the animal has stored fat by eating more than usual. It receives the little energy it does need by breaking down stored carbohydrates and fats. Organs and muscles even donate some sustenance. A bear actually borrows protein and water from its organs, because it can regenerate them to healthy levels once spring comes.

Bats are one of the creatures that lower their metabolism and enter a state of torpor during winter.
Bats are one of the creatures that lower their metabolism and enter a state of torpor during winter.

No one knows exactly what triggers hibernation in various animals. It might be a change in light exposure, measured by melatonin levels, which alerts animals to the coming winter and nudges them to seek out a burrow. A lessening food supply might be what makes them drowsy and lethargic. Biologists have been successful at triggering this state in certain species, like rodents, in the laboratory.

Many frogs hibernate.
Many frogs hibernate.

Of course, biologists also use their research to solve human problems. Some scientists think that humans might hibernate one day, such as in order to travel to Mars or lose weight. Researchers are looking for clues to healing people's liver disorders, kidney disease, starvation, or obesity, by studying mammals in this state as well. Humans might carry dormant genes that, when triggered, could regenerate damaged muscles and organs.

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Discussion Comments


Crocodiles hibernate too, which is pretty cool.

I wonder what would happen if someone disturbed a hibernating animal. Would the animal wake up or keep sleeping?

@healthy4life-- Me too. Unfortunately, our biology just isn't set up that way. We can't even bear hunger for a day. How could we hibernate?

@feasting-- Yea, squirrels don't hibernate, they're just less active in winter. I looked it up because I keep seeing baby squirrels running about in December. They don't seem upset from the cold at all. They're having a good time.


I've often thought it would be nice to be able to hibernate through the winter. I hate cold weather, and I'm extremely sensitive to it. I have to wear more clothing than anyone I know in order to stay warm during the cold months, and I would love to just snuggle down beneath some warm blankets and sleep the months away.


I've read that a turtle's hibernation is different than that of warm-blooded animals. Since they are cold-blooded, they can't digest food well when it's cold outside, so they have to stop eating awhile before they will begin hibernation.

This lets their digestive systems get totally cleared out. Then, they can burrow down somewhere safe for the winter.

Other animals do the opposite, eating extra food to store up fat for the winter. Reptiles actually fast instead.


@feasting – You see them in winter because they don't hibernate in the way that bears do. They go into torpor, so they still need to eat now and then.

I have a lot of trees in my yard, and I've noticed that they go into holes in the trunks to build their nests. They do tend to hide out here for longer periods when it's cold than when it's warm.


Do squirrels hibernate in climates with moderate winters? I live in the South, and I see them running about all year round. I'm thinking that maybe this is because the temperature rarely falls below the forties here.


I did research on bears, how they hibernate, and it says they hibernate three to four months, which is a long time. Imagine sleeping for thre months straight without waking up at all. It's crazy!

Anyway, bears also have their babies while hibernating. The male bear and the female bear hibernate, not together, but away from each other in their own cave.

Lots of animals hibernate, including some bats, ground squirrels and some other rodents, mouse lemurs, many insectivores, monotremes, some snakes and marsupials.


I thought only some mammals hibernate. I never knew that reptiles do too. I guess I was wrong.


@burcinc-- Actually, it doesn't. Most animals which do hibernate do so during winter because they are not able to find enough food during those months and also because the temperatures are too low for what their body requires. But a type of hibernation can take place in other climates too. When climate and food conditions become too difficult due to too much cold, heat or even too dry weather, hibernation allows animals to get through those difficult periods.

For desert ground squirrels, hibernation is necessary not only in winter but also during the summer when it becomes too hot and dry. Lemurs also hibernate during Madagascar's dry season as do some frogs.


Why does hibernation happen only during winter?


I thought that all animals that hibernate do so by themselves, but its not true. Raccoons, snakes, squirrels and mice cuddle together in sometimes hundreds to stay warm during the cold hibernation season. For some of these animals, hibernation and mating is the only time they actually get together.


Of course, now the term hibernate has taken on a new meaning, like many other biology terms. Computer hibernation is when a computer or laptop "rests" while not in use. It is still on, and the information is still accessible, because you can often "wake" it just by touching the mouse.

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    • Bears are considered "deep" hibernators, which rarely move out of their dens in winter.
      By: byrdyak
      Bears are considered "deep" hibernators, which rarely move out of their dens in winter.
    • Bats are one of the creatures that lower their metabolism and enter a state of torpor during winter.
      By: cheri131
      Bats are one of the creatures that lower their metabolism and enter a state of torpor during winter.
    • Many frogs hibernate.
      By: andrewburgess
      Many frogs hibernate.
    • Skunks hibernate.
      By: Eric Isselée
      Skunks hibernate.
    • Raccoons are hibernating mammals.
      By: chas53
      Raccoons are hibernating mammals.