Most of us probably picture bears out roaming the land, foraging for food and fishing for salmon. But right now bears are fully engaged in the activity that takes up most of their yearly cycle in the North. They are peacefully ensconced in their dens, hibernating away the winter.

Northern bears can spend up to seven months in their dens, and during this entire time they do not eat, drink, urinate or defecate. When bears hibernate, they do more than just settle down for an extra-long snooze; they recycle their urine and make other physiological adaptations that continue to impress even people who have worked with them for decades.

"I think hibernation is one of the most remarkable aspects of the bear's life history," says John Hechtel, the bear biologist with the Yukon government.

Hechtel has had more opportunities than most to get a first-hand look at hibernating bears. While studying black bears in the Fairbanks area, he would go into their dens in the spring to change the batteries in their radio collars, or make other adjustments.

He located the dens with the help of a radio receiver that picked up signals from bears wearing radio collars. But even when he knew he was close to a den, it could still take awhile to find its entrance underneath the snow.

"Even with the receiver it can be hard to pinpoint the exact location. You wonder how many times in winter you might ski by something that could be a bear den and never know it. It is something to think of all these bears sleeping beneath the snow across the landscape," he says.

He would sedate the bears once inside their dens, but they were usually awake when he first entered. Some were groggy and barely raised their heads, while others were more alert and a bit feisty.

Black and grizzly bears hibernate in almost the same way. They do not drop into a deep, deep sleep in winter like animals such as ground squirrels, whose body temperatures can plummet as low as -2 degrees Celsius in hibernation. Bears maintain a temperature of about 31 degrees Celsius in their dens, which is only 7 degrees or so below normal.

This higher temperature allows bears to wake up in the winter and respond to threats, but it has also been used to argue the case that bears are not "true hibernators."

Hechtel sees this debate as an argument over semantics. "It depends on how you define hibernation. My personal bias is that they are hibernators; there are just different forms of hibernation."

He says that early work on hibernation was done on small animals like ground squirrels whose body temperatures do drop dramatically, so this characteristic came to define hibernation.

Bears do slow down in winter, cutting their metabolic rates by 50-60 percent. Their respiratory rate slows to one breath every 45 seconds or so, and their hearts beats as little as once every ten seconds when they are most deeply asleep.

Bears head into their dens because there is just not enough for them to eat in winter, not because they want to get out of the cold. Remarkably, while living off of the layers of fat that they packed on in the summer and fall, they do not excrete any wastes.

Bears produce urea as they metabolize protein during hibernation. A buildup of urea will poison or kill most animals in a fairly short period of time, but bears recycle urea into amino acids, the building blocks of proteins.

"People studying kidney disease would love to know how bears do this," says Hechtel.

As hibernating bears metabolize their body fat, their cholesterol levels skyrocket, but -- unlike humans -- they do not suffer from hardening of the arteries or gallstones. Bears also do not lose bone mass during hibernation, or suffer from osteoporosis, even though they hibernate in a very small space where they cannot move around very much.

"It is almost like they are bedridden, but they still retain a reasonable bone mass. People studying osteoporosis would like to know how that is achieved," says Hechtel.

And to top it all, pregnant bears will give birth within the next month or so, even though they have not eaten in months.

"With most animals, if they are not getting enough food, reproduction is one of the first things that shuts down. It is hard to understand how fetuses can undergo that stage of accelerated growth while the mother is fasting," says Hechtel.

Entering a bear's den is not a recommended activity for amateurs, and Hechtel said that he would never enter a grizzly bear's den as they can be more aggressive when cornered. But the black bears in general were not difficult to work with, and he describes the chance to see them in their winter lairs as an incredible privilege.

"It is one of the most amazing things; to crawl into their dens and get some insight into their life history," he says.

John Hechtel can be reached at John.Hechtel@gov.yk.ca or by phone at 667-5652.