Why do animals hibernate?

Answered by Alana Jensen, Environmental Educator, INL ESER Program

To understand why some animals hibernate, let’s take a step back and look at the difference between endothermic and ectothermic animals.

Ectothermic animals are those whose body temperature depends on the ambient temperature (the temperature around them).  Endotherms, by contrast, can regulate their own body temperature by producing heat in the body from the food they eat. People are endotherms.

But for endotherms to be able to regulate their temperature, they need enough food to counter the effects of cold. This can be a challenge when freezing weather comes along or when food is scarce. So, in order to survive, some endothermic animals go into what’s known as hibernation.

Hibernation is an adaptation that helps some animals conserve energy by remaining inactive, greatly slowing their metabolism and reducing their body temperature for days, weeks or even months at a time.  Before the onset of winter, the animal eats a lot of food and stores it in the body in the form of fat. When it undergoes hibernation, the animal uses this fat to survive the chilling days. Though certain species of fish, amphibians, birds and reptiles are known to lie dormant during cold winter months, hibernation is generally associated with mammals.

What happens when an animal hibernates is much more dramatic than simply curling up for an long, long nap.  For some animals, hibernation doesn’t even appear to be restful. Some can emerge exhausted and have to catch up on sleep to recover!

When hibernating, an animal’s metabolism slows significantly: its heartbeat slows, it breathes more slowly (some animals even stop breathing for periods of over an hour) and its body temperature drops—in some extreme cases to below the freezing point of water (32º F).  They don’t hibernate all winter.  They do wake from hibernation from time to time to get rid of the metabolic waste that has built up. Some of them even grab a bite to eat.

True hibernators in Idaho include bats who live in caves, ground squirrels, marmots (rock chucks) and chipmunks.

While bears might be the first that come to mind, for years questions have surrounded whether bears are really true hibernators. Unlike animals that stir regularly during hibernation, bears can go for 100 days or so without needing to wake to eat, drink or go to the bathroom.  They can be aroused much more easily than true hibernators. They can also move around, with mothers able to suckle their young—quite a contrast to the stiff, ‘frozen’ bodies of smaller hibernators. For this reason, some people prefer to call it super hibernation, rather than true hibernation.

Curl Up for Warmth

Try this experiment to see why rock chucks and other mammals roll up into a tight ball inside their hibernation chamber.

Materials:  2 similar towels, a clothes dryer, 2 clothespins, a hanger

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  • Put the towels into a hot dryer for 5 minutes.
  • Fold one towel in half and then in half again.  Then roll it up like a bedroll.
  • Hang the other towel from the hanger using clothespins.
  • After five minutes, feel the hanging towel.  Unroll the other towel and feel it.  Which is warmer?
  • You should find that the rolled-up towel stayed warmer.  That’s because less of its surface area was exposed to the cooling air.  Mammals curl up into a tight ball during hibernation.  Since less of the animals’ surface area (skin) is exposed to the cold air, they stay warmer longer.

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