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Wait, do bears really hibernate?! Waking up to the truth about winter's sleep

Imagine yourself going for a hike through the vast Emigrant Wilderness in the Sierra Nevada Mountains. You stop to look out over a field covered in the first light snowfall of the year. You’re captured by its beauty. One snowflake falls on your nose, then two, then more cover your hair. It’s mid-November. A crisp breeze rolls over the field and makes you shiver. What would you do next? Being a smart hiker, you probably brought a warm cozy jacket. Maybe it’s fleece, and if you’re experienced in the wilderness, it’s probably waterproof.
Now imagine yourself on this same field of freshly fallen snow, but instead of two feet in wool socks and hiking boots, you now have four large paws covered in dark brown fur. Your five claws are sharp from climbing trees to reach nuts, berries, and seeds. Your fur has thickened in the recent months. The snow is falling gently, covering your snout and ears. That same crisp breeze rolls across the field… but you hardly feel it. That 350 pounds you put on in the recent months is really paying off now. And it will continue to pay off in the months to come. You have been eating up to 20,000 calories per day, which is like eating 40 Big Mac burgers per day! Foraging takes up 20-23 hours per day, leaving you only 1-4 hours to sleep per night. But don’t worry, you’ll have plenty of time to sleep later.[1,6] Some wildlife biologists call this foraging the “fall shuffle” as bears will search up to 60 miles away from their home for food to prepare for winter.[6] Now, you turn away from the field, and journey back to your winter den. You find your warm waterproof cave nestled under three large boulders. You enter and curl up on the ground. Every part of your body begins to slow down. Your heart beat, your breathing, and your body temperature drop lower and lower. You fall into a deep slumber that may last for days or weeks at a time. You won’t eat, drink, urinate, or make scat. Instead, your body is adapted to keep your bones and muscles strong by cycling calcium, and using nitrogen from fat metabolism to build protein for muscle mass.[2,7]

But here’s the question: Are you really “hibernating” or are you only sleeping through something we call “torpor”? The answer to this question will show you that the world of winter sleeping is a far more complex one than you may have thought.

Hibernation is a process where an animal falls into a deep sleep that lasts several days to several months. They may only take one or two breaths per minute. In small mammals like the ground squirrel and hedgehog, body temperature will drop to almost freezing – temperatures around 37 degrees Fahrenheit (F). Heart rate slows in bats from 600 beats per minute (bpm) to 20 bpm.

Left: Two chipmunks snuggle for warmth during a fall nap.
Right: A bat sleeps in a torpid state in the basement of an abandoned house.
Chipmunk photo credit: Pinterest. Accessed 2/1/17.

Bat photo credit: Five Valleys Ecology. “Bats: Bat Biology.” Accessed 2/1/17.

Torpor is a state that many animals, like the chipmunk, will enter on cold winter days. It’s similar to hibernation in some ways. Breathing slows, body temperature lowers, and heart rate goes down. Unlike hibernation, however, animals in torpor will usually sleep for only a day or two, then wake up to forage for food. In torpor, body temperature only lowers a few degrees.[1] Animals that enter torpor may do so simply to take a break from the cold winter day, saving energy for hunting later.

An Alaskan bear showing off some serious snoring during hibernation.

*If you’re using CUSD internet, here’s the sciencemag article where you can watch the video: - scroll halfway down the page.

Bears land right on the line between hibernation and torpor. Their body temperature lowers at most 12 degrees, staying above 88 degrees F. [2] This is not nearly as low as smaller mammalian hibernators. Bears often wake up several times during hibernation, too, which looks like torpor. Thus, if we only look at temperature drop and length of sleep cycles, bears may not be considered “true hibernators.” In fact, before the 1980s, some people shunned the term “hibernator” to describe bears, preferring the term torpor to describe their winter sleep.[4] It wasn’t until 1981 that biologists redefined hibernation in mammals as “a specialized, seasonal reduction in metabolism concurrent with scarce food and cold weather.”[4,5] Once they realized that there was a whole lot more to hibernating than just body temperature, the definition of hibernation extended to many animals, not just those whose temperature dropped to near freezing. Today, many physiologists say bears hibernate, while some prefer calling it extended torpor.[4] In the end, it’s all a matter of how you define hibernation. Torpor usually refers to shorter sleeps, while hibernation lasts longer.

By the end of this we hope you have gained at least two things:
1)   An appreciation for the complex world of winter sleep.
2)   At least one new fancy word to show off to your friends.

Lastly, here’s a cool table that shows you the winter survival strategies of many different animal species from the Sierra Nevada Mountains. Bugs, amphibians, mammals, and more:

For more on black bears, hibernation, and what they do to prepare for winter, visit these sites and any of the references listed below:

5.    Watts, P. D., N. A. Oritsland, C. Jonkel, and K. Ronald. 1981. Mammalian hibernation and the oxygen consumption of a denning black bear. Comparative Biochemistry and Physiology, A. Comparative Physiology, 69:121-123.


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