The Bear Necessities of Hibernation

A warm, musty smell hovered around the den’s entrance. By shining a flashlight into the hole, we could just make out a thatch of thick, black fur behind a screen of tree roots. Kneeling awkwardly on the snow, we each took turns trying to reach our cameras into the opening for a better angle. It wasn’t until I uploaded the photos at home that I noticed the single brown eye staring back at me through a gap in the roots. Peekaboo!

For many years, scientists have wrestled with the question of hibernation, and if bears are “true hibernators.” One of the sticking points, it seems, was bears’ ability to wake up quickly during the winter, and be immediately active. The unquestioned hibernators – like ground squirrels – get so cold that they must warm up a bit before they can move quickly. A ground squirrel would probably not have opened its eyes in response to my camera flash. The bear did – and then promptly rolled over to go back to sleep. Subsequent photos just show an expanse of back fur.

Hunter, the 12-year-old German shorthaired pointer who discovered this den, experienced the bear’s relatively light sleeping, too. By the time his human arrived at the scene of a commotion, the bear’s growling snout was partway out of the hole. Luckily for old Hunter, the bear didn’t deem him enough of a threat to exit any farther. Still, having the option to defend yourself in an otherwise vulnerable time is an incredible advantage for bears. Chipmunks hibernating under the snow may never wake up if a weasel comes knocking. They become lunch before breakfast. 

As scientists learn more about the winter physiology of bears and other hibernators, they’ve had to refine the definition of hibernation. While it used to focus on animals that show a significant drop in body temperature, the emphasis is now on a specialized, seasonal reduction in metabolism concurrent with scarce food and cold weather. What’s more, scientists recognize that hibernation is on a continuum with the short-term bouts of decreased activity known as torpor. Not only have bears been restored to their place of esteem as hibernators, but many scientists consider them super hibernators.

Amazingly, black bears generally do not eat, drink, defecate, or urinate during hibernation. They subsist off the 30 percent extra body fat they acquired during the fall feast. Even though this high-fat diet causes a doubling of their cholesterol levels (a serious problem for humans), bears do not experience gallstones or a hardening of their arteries like we would. In fact, we can use a substance secreted from a bear’s liver to dissolve our own gallstones without surgery. Urea, a potentially toxic waste product created during the metabolism of fat that would typically be flushed out with water, is broken down into protein and used to build the bear’s lean muscle. Additionally, despite the long period with no weight-bearing activity, bears do not lose bone mass. Once scientists figure out the mechanism that keeps bears’ bones strong, we may have a new miracle cure for osteoporosis.

A hibernating bear’s breathing slows significantly, from 40 breaths per minute down to eight. This is matched by a 50-60 percent reduction their metabolic rate. Nevertheless, bears’ huge bulk and thick fur enable them to stay within 12 degrees Fahrenheit of their normal 100-degree body temperature. The den’s small opening, snug fit, and a layer of duff on the floor also help them retain heat, although bears are commonly found hibernating in relatively unprotected places as well.

In contrast, ground squirrels and chipmunks let their body temperatures drop to just above freezing. Virtually no normal functions can continue, and so they must wake up every week or so to warm up, move around, urinate, eat, and experience deep, refreshing sleep. Bears can wake up, but they don’t have to – unless, of course, a nosey dog and his people come sniffing around.

We don’t know for sure when this bear entered its den, but hibernation is usually triggered by a combination of weather and lack of food. With this year’s late-coming snow cover, bird feeders were getting raided much later than usual, since bears were in no hurry to den up. One researcher observed that the final den entry often occurs during a snowstorm so that fresh snow will hide any signs that could lead unwanted guests to the sleeping bear.

Indeed, no bear tracks marked the snow near the den’s entrance – just the prints of one snoopy old dog and a few curious naturalists interested in seeing a super hibernator in action.

Bears are able to maintain a high enough body temperature during hibernation that waking up to check on unwanted visitors is a relatively quick process. Photo by Emily Stone.

Without the aid of Hunter (not pictured) we probably never would have found the den. Photo by Emily Stone.