Tuesday, October 13, 2009

Bear Dreams

The Big Dipper is often portrayed as the tail of Ursa Major, the "Great Bear" constellation. And in October, when the Great Bear's tail dips down to its lowest point in the midnight sky, it is time for grizzly bears to head below ground.

Grizzly bear sow and cub (c) John AshleyMost bears stop eating in winter because it is more efficient to sleep than it is to plow through snow, digging for dwindling supplies of plants and berries. The calories spent searching would outnumber the calories gained by eating.

Some interesting exceptions occur in Glacier and Yellowstone parks, where wolves have returned. Some bears have learned to spend the winter awake, actively usurping and scavenging deer and elk meat from wolf and lion kills. Nowadays, this is exceptional behavior, but it might have been the norm back when bears and wolves lived with few if any human neighbors.

In early fall, adult bears prepare for winter by digging one or more dens into steep, north-facing slopes above 6,000 feet elevation. A short, narrow tunnel slopes downward to a larger resting chamber that is often lined with grasses and leaves, and the excavation may measure up to 12 feet in length.

Two paw prints, two very different animals (c) John AshleyGrizzly sows are the first to den up, followed by the bigger boars about a month later. The bears often wait for a major snowstorm before heading up the mountain. This timing not only hides their tracks, it also covers the den opening and buries it in an insulating blanket of snow. Down at the bottom of their lair, the ground temperature stays above freezing.

For the rest of winter, the bears survive on energy metabolized from the summer's fat layer under their thick fur. While the great bear sleeps, its body temperature falls a few degrees, the heart rate slows to 8-12 beats per minute, and oxygen consumption drops by half. It does not eat, drink, urinate or defecate. Still, this is not true hibernation, and the bear remains alert and can awaken easily. They might even move to another one of their dens mid-winter, but they typically don't re-use dens that were dug in previous years.

During the winter, pregnant sows give birth and begin nursing their one-pound cubs with milk; this caloric balancing act is simultaneously complex and graceful. Young bears that survive the summers will den again with their mothers during the next two winters. In the third year, the sow will be ready to mate again.

Big boars are the first to emerge, usually by March or April, weighing 30% less than they did when entering their dens. Sows with cubs will rise about a month later, giving the young bears more time to grow a little bigger before climbing out into a new world.

By late spring, the Great Bear constellation has climbed back up to its highest point in the evening sky, and Montana's great bears are back on top of the food chain -- right where they belong.

Behind the lens: While photographing these bears in Glacier Park, I noticed that the front of my lens was dotted with water droplets where snowflakes had skipped across. Subsequently, each image was blurrier than the one before. Lens hoods are helpful, but now days I also keep a dry towel around my neck when working in rain or snow.

Also please note, this photograph was taken from the safety of my truck. I never approach bears, and neither should you.  If one of us gets mauled, the bear would most likely get shot. I've never seen a photograph that was worth a bear's life.