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Bear Update: Visiting a bear den

By Michael Allen, Whistler Black Bear Project I crouch on a worn pair of Tubbs snowshoes. Pale grey and cracking, these extra-large bundles of aluminum and plastic have provided me with winter mobility to over 1,000 bear dens during the last decade.

By Michael Allen,

Whistler Black Bear Project

I crouch on a worn pair of Tubbs snowshoes. Pale grey and cracking, these extra-large bundles of aluminum and plastic have provided me with winter mobility to over 1,000 bear dens during the last decade.

At nearly 1700-metres elevation on Whistler Mountain’s North Slope, I shift my weight as I hunch back on creaking bindings and glance through a small opening to a clump of trees – fir and hemlocks huddled against sub-alpine fury. Snow crystals launch through -17 C winds from airstrips off stunted, frozen conifer boughs. I breathe onto my bare finger tips and remove a pair of binoculars. Twenty metres side-slope a shadow stares at me from the clump of trees. I stare back.

Within that shadow is a lethargic blackness I have marvelled at for the last 18 winters. The search for hibernating bears in secret winter dens is the jewel of my research, but not without its physical and mental demands. Standing to force the ache and numbness from my legs, I am nearly thigh-deep in powder.

I snowshoe down-slope of the tree clump away from the dead tree then back up, stopping just three meters side-slope and in full view of the black hole. I stop, listen, eyes fixed into darkness. I glide my large snowshoes quietly through dry powder up to the black hole in the dead tree.

Tree cavity dens are the norm for denning bears in Whistler. Basal cavities within dead (or dying) standing snags provide dry, secure shelter from winter weather and outside intrusion of animal or human kind.

I switch on a LED lamp and quietly peer in through an 18-cm wide crack. This is not the entrance but an old knot given way to falling out and cracking. Tips of black hair coat the jagged edge of the opening. I carefully and quietly collect about 10 shafts and place them into an empty film canister.

Bears usually stand or climb inside tree cavities snagging hair at knots or cracks besides the den entrance and denning bed substrate.

Doubtful that follicles are present, good samples of shafts still may hold DNA. I don’t know the identity of this particular bear but now with the tool of genetic profiling from hair DNA I can match this sample’s profile with known bear profiles from my population database.

Most of the time I know the ID of bears that I snow-trail to dens in November and December. Although not knowing, is interesting too – to the sound of three new-born cubs on a few den visits in March. New-born cubs can be detected by vocalizations during March and April.

I first direct the beam above and below the hole in the tree just to be sure I haven’t woken the bear. Disturbed bears are usually docile, but I really haven’t disturbed enough to warrant a sample. That might be a good thing.

I don’t want to encourage activity for obvious physiological constraints on the fasting bear. In short, a hibernating black bear’s metabolic state allows accumulated fat stores (from summer-fall berries) to combust energy while no food/water consumption, defecation, nor urination takes place for up to seven months. The dangers of waste products (urea) are filtered through liver and kidneys and reabsorbed back into the bloodstream. Nitrogen from urea is used to reconstruct protein. Protein supply allows fasting hibernating bears to maintain muscle and organ tissue.

I adjust my light downward beneath snow surface to illuminate the snag’s basal cavity. Light catches a dark, furry curve outlining a portion of the bear’s body. Following the curve I can see that the bear is lying with its back to the entrance of the den and head tucked sideways into the forepaws. I quietly adjust my footing and body position so that I can view the head and chest cavity. I pull out a stopwatch and look back into the cavity. The bear’s chest falls. I wait. It seems like minutes, then without warning the chest expands. I watch closely and activate the stop watch as the chest becomes still after exhalation. Forty-eight seconds later the chest expands without warning. During the expansion a slight shaking is detected. The shaking may be attributed to muscle contractions during periods of inactivity (exercise). Sometimes noticeable, sometimes not, depending on the size of the bear.

Brian Barnes a professor of zoophysiology at the University of Alaska found a very efficient heartbeat pattern in a limited oxygen environment. While monitoring brainwave, heart rate, and respiration rate he found that the heart beats once every 10 seconds during deepest hibernation. During inhalation, which I observed to last 5-7 seconds in Whistler bears, heart rate speeds up to a few quick beats enabling blood to gain oxygen and the bear’s breath to pick up carbon dioxide and water. After exhalation, heart rates drop down to lows of 8-10 beats per minute.

Respiration rates determined for Whistler bears range from 35 to 55 seconds but average around 40-45 seconds.

In Alaska they found that bears sleep most of the time during hibernation. Sleep is necessary to repair brain cells. Bears do periodically shift body position, depending on individual and environmental conditions.

I withdraw the LED and insert a one metre long temperature probe. The probe is hung approximately 30-cm above the bear’s body. It reads - 9 C. Ambient temperature is -13 C. Tree cavities keep bears dry not warm – bears are already warm.

Snow depth above and below the tree den and distance to the nearest ski or snowboard track – 1 metre away – is also recorded. With 275 cm of snow over the den’s entrance this bear should be insulated from ambient activities. I peer in one last time – closed eyes and sound asleep.

Anyone with questions about black bears or want to report winter bear sightings may contact or call 604-902-1660. Thanks to Pique Newsmagazine for sponsorship of these columns.