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How do bears hibernate? Bears in Winter Part 2

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In Part One I answered the question “Why do bears hibernate?”

Read Part One

If you missed it, I recommend reading that first.

Now that we know WHY bears hibernate…let us explore HOW. How do they pull off this physiological feat, and what makes bears in winter so fascinating and formidable?

Bears undergo a specialized form of hibernation, unique to bears alone. These miraculous mammals enter a low energy state (as described in Part 1). They do not eat, drink, urinate, or defecate for a period of many months. Through it all, they manage to maintain muscle mass, even though they remain in a sedentary state from roughly November to April. How do they do it?

Amazingly, bears are able to take the "waste", or urea from the metabolic process and convert it into amino acids — the building blocks of protein. This process allows muscle cells to actually grow while they rest. In this way they are able to continuously re-build their muscles, rather than experience atrophy (as humans do, even after a short period of inactivity).

Incredible, right?

The evolutionary marvel of bear physiology gets even more impressive when you consider what female bears can accomplish during the winter months. Mother bears give birth and produce milk for their cubs without eating or drinking for the entire winter.

Say what? Read that last sentence again, please. Unbelievable, isn’t it?!

Black bear mother and cub in den tree

This is not what most people imagine when they picture a bear’s den, but hibernating black bears routinely den and give birth in the safe and sheltered environment of a hollow standing tree (note the young cub peeking up from under the mother). This image was captured as part of the long-running black bear study in the Great Smoky Mountains National Park (photo: Frank T. van Manen, my internship supervisor at the University of Tennessee in 1993). A common den tree species was Tulip Poplar (also known as Tulip Tree, Liriodendron tulipifera), a fact that has stayed with me, recommending this wonderful native tree to clients in New England 20+ years later.

“Female bears give birth and produce milk for their cubs without eating or drinking for the entire winter. ”

These physiological feats are so unparalleled, that even NASA has funded studies of hibernating bears in an effort to better understand how to induce artificial hibernation states in astronauts during deep space missions to Mars, and even how to develop cryosleep chambers, an advance that could make the science fiction scenes from Alien, a reality.

I'll be staying here on Earth with the bears, thank you very much. I'll stay here and reminisce about my former life as a roving field biologist crawling into bear dens — remembering that feeling of being in that deep, dark hole, all alone with mama bear and her cubs. I remember it like it was yesterday, the sound of her slow heartbeat, the smell of cedar bark lining her bed, and the muffled voices of the rest of the bear crew, just outside the den entrance waiting for me to hand out the cubs.

In those precious moments I was in complete awe of the quiet miracle of hibernation, of mama bear nursing her cubs through the winter on an empty stomach. Each time, I was flooded with an almost overwhelming sense of gratitude that I was able to witness — so intimately — a true miracle of nature.

cub photos courtesy of Maine Department of Inland Fisheries & Wildlife

You might be wondering….

Why was I in a bear den? Why was I taking the cubs out of the den? Why do those cubs have ear tags?

In the 1990s I worked on a number of black bear studies and visited winter dens to help collect reproductive and survival data. Read more about this work in my article about beechnuts, and the DownEast article below.

Beechnuts & Bear Biology

Want to delve even deeper into the magical, marvelous world of bear hibernation?

Ron Joseph — my dear friend and mentor — penned this exceptional article for DownEast Magazine.

Read Ron's Article

Important PSA! As soon as the weather turns warm, or even during mid-winter warm-ups, take down your birdfeeders so you don’t create a human-bear conflict. These conflicts put yourself at risk, and are very bad for bears. Learn more…

Hungry Bears on the Move

I hope you enjoyed this Phenology Note!

Join me in witnessing the seasons of our wild world.

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#ThePersonalEcologistI partner with eco-minded landowners to create thriving wildlife habitats in their backyards, gardens, fields and farms, woods or campuses - at any scale.I have 25 years of experience in my field, and a lifelong commitment to wi…

#ThePersonalEcologist

I co-create biodiverse habitats with eco-minded stewards throughout the Northeast - at any scale.

I have 30 years of experience and a lifelong commitment to wildlife conservation.

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Deborah Perkins