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The bear started to switch from prehibernation to hibernation physiology. This switch is triggered by chemical signals in the blood. Once in hibernation, bears will stay in their winter dens for as many as five months at a time, conserving their hard-won energy resources (fat) that they have accumulated during their feeding frenzies of the previous fall. Running around searching for food when little is to be found is deficit energy spending. Evolutionary logic dictates that appetite would be suppressed in a bear ready to hibernate, because a hungry bear would continue to be a food-searching bear, and deficit-spending bears become dead bears.

There are two ways to try to beat an energy crunch brought on by winter. One approach, used by the kinglet and human beings alike, is to work harder and harder to try to maintain a profit margin, even as we pay ever-higher heating costs in the face of ever-dwindling resources. There comes a point, however, when it is better to drop out in an effort to save as much energy as possible. The latter is sometimes a matter of necessity and it is common in many animals in winter. It may even occur in humans, given the right circumstances. Here in midwinter at the high latitudes of Vermont and Maine, I start to feel sleepy at about 5 P.M. and I have little trouble curling up in a snug bed as soon as it gets dark. In the summer at that time there are still four more hours of daylight to come, and I would still be running around. Of course my semi-hibernation tendencies are blunted ever so slightly by social pressures. In my culture it is just plain lazy to sleep fourteen hours per day. So, as a result of social conditioning I routinely extend my winter day with artificial light in the evening and with caffeine in the morning. Most important, my natural tendencies may be suppressed because I’m not on a stringent diet that winter would normally impose. My calorie intake is undiminished and sufficient to keep up my energy level. Unlike the bears in the woods, we New Englanders don’t need to go into hibernation in October when the nuts run out.

My students and I did not succeed in tracking our bear to its den. If we had, we might have found it under a brush pile, in a hollow tree, under roots, or upon a heap of branches in a stand of dense young spruces or balsam firs. Bears may even curl up and hibernate in the open with the understanding that they eventually will be buried with snow. Grizzlies dig their dens, moving up to a ton of earth to carve a tunnel into a hillside, their snug hibernation chamber at the end of it. They cover the floor of their chamber with bedding material of branches, grass, or duff scraped from the ground nearby.

Bears may prepare their overwintering den a few days or weeks before entering them. Some wait for a big snowstorm before finally crawling into their prepared sanctuary. They are flexible and individual differences abound. However, most bears engage in a feeding frenzy in late summer and early fall, in which they down about five times their normal food intake, putting on a five-inch layer of fat. By late fall they slowly lose their appetites until they eat nothing and when they leave their dens in the spring, they are no more hungry than before entering. Instead, provided they retain some fat, it takes them a long time to regain their appetite. Appetite suppression during hibernation is probably under the control of leptin, a “satiety” hormone secreted by fat cells that circulates in the blood and affects the appetite centers in the brain (Ormseth et al. 1996). In spring, leptin levels decrease and appetite increases.

When are bears in hibernation, if ever? This common question is not a good scientific question, because the best answer is “it depends.” We often seek precision by pigeonholing through definitions, whether with respect to what is right or wrong, alive or not alive, hibernating or nonhibernating. The problem is that animals don’t stay within such simple boundaries. They don’t obey rules, so any