Contributor: Tom Smith

3/6/2012 4:21:03 PM

First Den Site


We took our first excursion into the Frozen Land yesterday, and it carried us to a large gravel bank next to the Beaufort Sea. The sea is actually more like the Beaufort Ice Sheet now: ice 10 feet thick or more that feathers so cleanly into land that we often can't tell the difference between tundra and ocean travel.

Den site

This gravel bank interrupts snow being blown across the tundra so that it piles into huge drifts on the leeward side. Polar bears are drawn to sites like this for digging their dens—and for the past 11 winters a bear has chosen to hibernate and give birth to her young here. 

Using an infrared camera (known as a FLIR - forward looking infrared) to scan drifts, we quickly identify a hotspot indicating a bear snuggled warmly beneath the otherwise featureless drift. Heat from within has worked its way through the snowpack, warming the surface 14 degrees above ambient. Though undetectable by either our eyes or touch, this heat signature shouts out "bear!" to our hand-held FLIR. The area of warmth is meters wide - something not associated with smaller mammals like a denning arctic fox, so we're quite sure it's a polar bear.  

Gravel pad den
The white spot in the center is the denning family. The gray area is a snow drift.

Research has shown that a denned mother polar bear (400 pounds) radiates about 200 watts of heat, much like a large incandescent light bulb. As such, den temperatures hover at, or above freezing, which may sound like a miserable environment for newborns until you realize that through the thin den walls (1 meter or less) the Arctic shivers in temperatures that may be as much as 100 degrees colder. So the den is cool, but not cold. 

Cubs born in January weighing 1-2 kg are unable to see or hear (like newborn puppies), and are covered by a very thin layer of snow-white hair. Although the cubs are furred, it's not enough to keep them warm, so their mother pulls them in close. Within her warm folds of fur, they suckle on milk that is more than 30% milk fat—something akin to drinking melted butter. As a result, cubs grow very fast and by the time they leave the den three months later are five or more times larger than they were as newborns. 

Young polar bear cub

Rather than freeze to death awaiting mother bears and cubs to break out of their dens, we have designed camera systems to do that work for us. Within heavily insulated coolers, high definition digital cameras sit in heated comfort and record den site activity. PBI's BJ Kirschhoffer has engineered these units so that they are self-heated, write to solid state hard drives, are powered by the sun, and can be left unattended for weeks. In this manner we minimize disturbance of bears yet are able to capture their activities from den break out to abandonment.  With this gravel pad den we begin the 2012 season. 

Camera set up

Being a university professor I must fly south after 1 week on the slope - my students await. But BJ and Jay remain behind to carry on. Like most Arctic visitors, I leave changed, touched by its stark beauty and simplicity and am awed by the amazing polar bear that calls this hostile-to-humans environment home. Organizations like PBI make this work possible and for that, I'll be forever grateful.  

Photos: Denning site, copyright BJ Kirschhoffer. FLIR image, courtesy Dr. Tom Smith. Cub, copyright Dr. Andrew Derocher. Camera set-up, copyright BJ Kirschhoffer.

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