Over the past few months (August through November), bears across Montana have been preparing for hibernation. They get ready to hibernate by overeating and gaining as much weight as they can in a period called hyperphagia. At the peak of hyperphagia, grizzly bears have been known to consume up to 20,000 calories per day! This extra weight helps them survive for months without eating, drinking or passing waste.

Denning season

By December, most of our bears have begun to hibernate. In Montana, many grizzly bears will select a den site on a steeper slope at higher elevations. Bears seem to prefer to dig dens below the roots of a tree. Black bears and grizzlies often dig dens, but sometimes natural rock dens or dens sites nestled below upended tree root balls are found and used. We’ve also seen grizzlies den in hillsides on the prairie and even see bears hibernate without using a den at all. Bears dig dens in about three to seven days, during which a bear may move up to a ton of material. Dens usually consist of an entrance, short tunnel and a chamber with bedding material that has air pockets and traps body heat. Bears don’t usually reuse the same den, though they may den in the same area for years. Bears regularly prepare their den by gathering soft vegetation like pine needles or grasses as soft, insulating bedding. But why do bears hibernate in the first place?

It’s not sleep; it’s survival

Grizzly bears and black bears hibernate to survive a shortage of food – which is brought about by winter in our part of the world. In contrast with sleep, hibernating animals slow way down to survive periods with less food or to survive extreme conditions. A lowered metabolism means they can survive while using less energy. For bears, this means their bodies cool down by about 10 degrees Fahrenheit, to around 90 degrees. Their heart beats at about 8 to 12 times per minute, and they take only one or two breaths each minute.

You may have heard people talking about whether bears are “true hibernators.” Let’s settle this discussion. There are several ways that animals experience a lowered metabolism for survival. When animals slow down metabolically in the summer to survive extreme heat or drought it’s called estivation. Some animals, such as chickadees, even go through a daily torpor during very cold nights in winter. For reptiles and amphibians, animals that can’t produce their own body heat, winter dormancy is called brumation. Grizzly bears, ground squirrels and yellow-bellied marmots are examples of animals that hibernate during winter. Debate about whether bears are “true hibernators” arose from the fact that bears body temperature doesn’t drop as low as small mammal hibernators, and bears are able to respond to disturbance more easily than hibernating rodents. Interestingly, bears are much more efficient at using stored energy, even though their bodies don’t get as cold during hibernation. However, these are all just variations on a continuum of ways that species hibernate. Some bear biologists have even described bears as super-hibernators.