Everyone loves watching the bears get fat in Yellowstone National Park every year. If you haven’t seen the fat bears, it’s something you should look into. Although it might be fun to watch them get bigger and bigger, they are doing it for a very important reason. They are gearing up their bodies for hibernation through the cold, desolate winter months. Bears may be the most famous hibernators, but more animals hibernate than you might think. Why do so many animals hibernate?

When life is hard

Brown bear emerging from its rocky cave with snow on the ground
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Contrary to popular belief, winter is not the only reason that animals hibernate. Hibernation is a means to survive in any difficult period. For the bears in Yellowstone, that difficult period is winter when all the food sources are gone or buried by snow. Instead of wasting their energy looking for food, they hibernate and simply wait it out. Hibernation slows down all the metabolic processes to conserve energy. It’s similar to putting your phone on energy-saving mode. The processing power goes down, but your battery lasts much longer.

Hibernating in the heat

Up close view of lemur resting on tree branch with vines and trees in background
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In hotter climates, the winters aren’t always cold — but animals hibernate all the same. Take some of the animals on the African island of Madagascar, for example.

Winter averages in Madagascar range from 70 to 80 degrees Fahrenheit, but it can fluctuate up to 36 degrees in a day. Regulating body temperature is one of the most energy-consuming activities that warm-blooded animals do. So, instead of fighting the drastic temperature swings, fat-tailed lemurs simply wait it out. They stock up on food in the summer and hibernate through the winter, even though the daytime temperatures are quite pleasant. While in hibernation, the lemurs’ body temperature can fluctuate with the temperature outside to help conserve energy.

Unlike bears who use their body fat to stay warm in the winters, the lemurs don’t need the extra insulation. In fact, extra insulation would cause them to overheat during the hot days! Instead, they store all the fat in their tails. They start to look a little silly as wintertime approaches.

In times of distress

Hedgehog resting on a pile of golden autumn leaves, preparing to hibernate in the fall
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Hibernation isn’t just a seasonal trend. Many animals use it as a defense mechanism in times of peril.

In the Australian bush, where it’s common for wildfires to tear across the landscape in the dry summers, most animals know to run at the first sight of the flames. Echidnas, however, have an unusual tactic to avoid harm. They hide in a hole or a downed tree and enter a state of hibernation. Not only does this allow their body temperature to fluctuate, it also allows them to survive through the period following the flames when food is scarce. The echidna’s main food source is ants, most of which are unable to escape the flames.

Hibernation is not sleep

Up close view of brown bear falling asleep on a rock
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When animals hibernate, their body activity drops to about 5% of what it would be if they were active. This includes brain activity, breathing, and heartbeat. When bears hibernate, their heartbeat drops to about 4 beats per minute, and they only need to take a breath once or twice per minute. After living at such a low-functioning rate for a few months, many bears wake up exhausted and need some extra sleep to make up for it.

The animals also aren’t unconscious the whole time. Bears have been known to move around the dens in the middle of the winter and mother bears can still produce milk and feed their young. Other animals even leave their dens if they get flooded or damaged, all without exiting hibernation.

Other hibernating animals

Group of alligators blending in and resting in a dark green pool
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While bears may be the most famous, they are by far not the only hibernators in the world, or even in North America. Many smaller animals, specifically herbivores, hibernate when food gets scarce. Some other hibernators you might not have known include:

  • Bumblebees
  • Hedgehogs
  • Ground squirrels (not tree squirrels)
  • Bats
  • Turtles
  • Moths
  • Groundhogs
  • Hummingbirds
  • Skunks
  • Ladybugs

Many amphibians and reptiles enter a lethargic state called brumation during cold weather. It’s essentially the same as hibernation except for cold-blooded animals. Alligators can enter brumation while underwater, leaving just their noses above the surface to breathe. Sometimes you can even see their snouts sticking out from beneath a sheet of ice! They’re not dead. They’re just waiting out the cold.