Before I tell my story, I’d like to share some background on the research of the physiology of black bears during winter hibernation.
Indeed, bears do hibernate although it is different from ground squirrels and prairie dogs. Due to their large size, bears only drop their body temperature 5 to 10 degrees Fahrenheit below normal while small mammals let their body temperature drop to almost freezing. All small hibernators must arouse every 15-20 days to stretch and pee (urinate) to get rid of waste products such as urea. Bears, however, do not arouse, defecate or urinate for 5 months while in their dens. Even though their body temperature is only mildly hypothermic, their metabolism is depressed by 80%, which allows them to conserve a lot of energy over the winter.
By not arousing, bears do not get any exercise for five months. What they do is a medical marvel. First, they put on over 50% of their body weight in fat. They are adaptively obese with no heart problems or high cholesterol. They go into acute renal failure (their kidneys almost cease functioning) but rapidly recover. They recycle their urea waste and put it back into muscle protein. They have no calcium bone loss and don’t seem to show muscle disuse atrophy. Bears loose only 20% of muscle strength compared to 80% loss by a human confined to hospital bed for 3-5 months.
Our studies were designed to verify these claims and to quantify the responses. To do so, we captured bears in the summer, fitted them with radio tracking devices and located them in their dens during the early and late winter, 5 months apart. We measured their body fat, analyzed their blood and urine, measured their metabolism by implanting body temperature and heart monitor data loggers, took muscle biopsies for analysis of muscle fiber composition, and measured muscle strength with an apparatus used to measure the strength of children with muscular dystrophy in a hospital setting.
Bear dens can take different shapes, sizes, and locations. Sometimes dens are dug 5-10 feet into dirt on a hillside, sometimes they are 40-50 feet through tunnels between large boulders or in a maze of logs in a slash pile within a forest clear-cut. Most often the entrance is covered with snow 2 to 20 feet deep, which means we had to dig for 1-4 hours just to get to the den entrance.
Our digging was often noisy enough to wake the bears up which made them very grouchy. We entered the den with a syringe of anesthesia on the end of a 3 foot long jab pole and a mag light. Once the bear was located, we delivered the drug into its hip or shoulder and waited for the bear to go under the anesthesia, usually 10-15 minutes. The bears were all different. Some bears fought to get over the top of you and out of the den, others stood their ground to fight, and some just lay there bewildered about the intruder into their dream state. In order to get out of the den rapidly if an encounter got physical, we always had a rope tied to our foot as we crawled through the den. Two hard jerks of your foot meant, ‘pull me out of here now!’ Out of 50 to 80 dens, we never had a bear or a person injured, except when being rapidly extracted from a tight den when rib cages and faces were scraped across sharp rocks and roots.
Hank Harlow inspecting a bear outside of the den.
So, given all of this, I guess I can tell a story.
It was the first bear den I had ever entered; and the first time I had been out with bear biologist, Tom Beck of the Colorado Division of Wildlife. He and his group were doing a mark-capture-recapture study to determine the population structure of bears in the Uncomphagre Plateau in western Colorado.
It was great snowshoeing into the backcountry and I watched the whole operation as described above. I was totally impressed by the procedure, however, this bear was a particularly large male and the team of biologists had to give it an additional dose of anesthesia in order to finish weighing it and putting on a VHS tracking transmitter. At the end, Tom said, “Well Hank, you crawl into the den and pull him in while we push him from the outside.”
Wow, it was like getting a cork into a wine bottle. I’m at the back of the den and this 380-pound bear is blocking my way out because it is taking up all the space. The only way out is to squeeze over the top of it, one leg, one arm at a time. As I’m doing this I don’t feel or hear the bear breathing. I yelled out to Tom that I thought the bear wasn’t breathing and I’ll never forget his response. “Well, give it mouth to mouth resuscitation, right now!”
Darn, I thought, how in the world do you I do that? I had to roll this huge bear over on its back in that small space and lay on its stomach. I put my mouth over its nose and muzzle while holding its lips tightly shut. Then I took a big breath and exhaled in through its nose and mouth. I could feel its chest expand as its lungs partially filled with my breath and then I felt it deflate. In doing so, a winter’s accumulation of mucus came out of its nose; man, that was gross! All I could do was wipe it away with my glove and keep putting in breaths. I was thinking this just wasn’t doing anything, when after about five minutes, I’m looking into its eyes and they moved rapidly to the left and right and then a little puff of warm, moist air came out of its nose and condensed in the cold den air. Man, this bear was not only breathing on its own, it was looking for escape terrain and I was lying on top of it. Rapid rollover and exit out of the den was my immediate strategy. The bear was really not happy about being molested by some human invader.
We quickly put pine branches over the den entrance and shoveled it high with snow. While we hoped the bear was thinking this was just a bad dream, we were jubilant that it was alive, healthy and we’d accomplished our mission. Wow, I was hooked, I had been in my first den and just loved being in there with a bear. It was quiet, warm (above freezing), and fresh smelling (remember, they don’t defecate or urinate). That night, back at camp, Tom and I talked about hibernation physiology, disuse atrophy, and adaptive obesity and came up with the outline for a proposal to the National Science Foundation for winter survival strategies of hibernating black bears. That led to almost ten years of research on black bears and later work on sun bears in Indonesia and polar bears in the Arctic.
Hank Harlow with a black bear.
If you’d like to learn more about hibernating bears or Hank Harlow, check out these links:
Black Bear Biology and Behavior, Western Wildlife Outreach
Harlow Receives Top UW Faculty Award, University of Wyoming News, 4/20/2012
Inside the Bear’s Den with Zoologist Hank Harlow, Conversations with Richard Fidler, ABC News Australia, 3/30/2016
Lockwood, Devi, Hibernation Works for Bears, Could It Work for Us Too?, The New York Times, 11/15/2019
Peterson, Christine, University of Wyoming Professor Crawls in Bear Dens, Wrestles Komodo Dragons, Casper Star Tribune, 9/25/2013
Reception Honors UW Research Center Director Harlow, University of Wyoming News, 9/12/2013
Rogers, Lynn, Bear Center Conducts Hibernation Study, North American Bear Center Newsletter, 1999
Ryan, Kathryn, interview, Animal Lessons to Bear in Mind, From Nine to Noon, Radio New Zealand, 4/21/2015
UW’s Hank Harlow AMK Ranch Speaker, University of Wyoming News, 7/10/2015
Illustrations by Jill Bergman