Winter Wildlife Chronicles: Bear Underground

Over the past four years, I've run two marathons and nine half marathons. That involves a pretty significant amount of training time. But even with all this running, if I take a few weeks off, my stamina decreases. My legs aren't as strong. In some ways, I have to start building up my endurance all over again.

Most of us know this and understand this. If someone is confined to a bed, their muscles atrophy and bones weaken. When it comes to the human body, it really is "use it or lose it."

Not so for black bears.

As Bernd Heinrich describes in his excellent book Winter World: The Ingenuity of Animal Survival, black bears can spend five months in a den, without eating and almost entirely immobile--and not lose muscle mass or bone strength.

Heinrich is fascinated by the bear's adaptations. He's an accomplished scientist and writer, who is also that rarity among biologists today: a skilled field naturalist. He also happens to be a record-setting ultra-marathoner, so he understands the physiology of exercise.

Bear hibernation is not easy to categorize. Bears are immobile, but they can awaken easily--a fact that makes studying bear hibernation quite difficult.

Heinrich writes that bears, unlike most hibernating mammals, don't lower their body temperature. They maintain a high metabolic rate. And despite this, they do not need to drink or urinate all winter.

Biologists have found that black bears metabolize their urea into nontoxic creatine, and nitrogen wastes are recycled back into protein.

But that still doesn't explain how bears remain, as Heinrich calls them, the "ultimate couch potatoes." How can bears lie inactive all winter long, and spring out of their dens in fine physical shape? It does not seem like it should be possible. The fact is, much about their physiology remains unknown.

Biologists are refining ways to work with black bears in winter (above, Conservancy staffer Justin Petty participates with an Idaho Department of Fish and Game winter bear survey). But even with these high-profile mammals, there is still much to be learned.

What we do know is that in dens around Idaho, right now, bears are lying--not quite asleep, not quite awake, not eating or drinking but not suffering from starvation or thirst, not moving but able to move quite well. --Matt Miller