How Do Bears Prepare for Hibernation?

As temperatures begin to cool, many of us look forward to staying inside away from the wintry weather. We may choose not to spend much time outdoors until things look much greener. Of course, we’ll still remain active during this stretch, even if we’re inside. Bears, on the other hand, will be especially inactive during this same period as they hibernate. However, they do have a lot of activity to get done before they hole up for the winter. Whether it’s packing on weight, securing a den, or preparing to keep their upcoming cubs safe during the long cold months, they sure are a busy species in the fall.

Why Do Bears Hibernate?

In the winter, temperatures plummet, snow covers the ground and it can be very difficult for animals to find food. As a result, most bears throughout the world have developed an adaptation to get themselves throughout this season of scarce resources and frigid temperatures: A months-long hibernation.

Do All Bears Hibernate?

If there is enough food on hand, bears don’t necessarily need to hibernate, unless they’re pregnant or the winter conditions are especially harsh. There are some bears that aren’t as apt to hibernate, too.

Sean Farley, research biologist and bear specialist at the Alaska Department of Fish and Game, explains, “Not all bears hibernate, but in Alaska, most bears do, including the males. Our Alaskan brown/grizzly bears and black bears den more consistently than polar bears. It is not uncommon for male polar bears and polar bears that are not pregnant or with young to remain active through periods of low food availability.”

He adds that bears in captivity usually don’t hibernate, but they will slow down and sleep more. However, some zoos are now allowing their bears to hibernate, which is healthier for them.

How Much Do Bears Eat to Prepare?

Bears begin their excessive eating period – or hyperphagia – in the fall. During this stretch, they can eat up to 20,000 calories per day, often putting on up to three pounds on a daily basis. I guess our Thanksgiving chow downs aren’t impressive as we thought, huh? With all this feeding, bears add on some extra inches. Black bears in Denali National Park, for example, will have another 4-5 inches of body fat by autumn’s end.

Because bears are omnivorous, they can eat a variety of foods to get that weight up. It can come from berries, ground squirrels, carrion, and whatever else they find.

Interestingly, during mating season, fertilized eggs don’t immediately implant in the female’s womb. This allows her more time to conserve her energy for hibernation. If she hasn’t put on enough weight by the time she enters her den, the fertilized eggs will abort.

How Do They Choose Their Dens?

Where a bear chooses to locate their den varies by the species or location. In Yellowstone, grizzlies will pick a spot on higher elevation slopes, generally at the base of a tree. Black bears pick areas that are slightly less elevated and look for spots under windfalls, in hollow trees or caves, and in previously occupied dens.

Sometimes their choices can be a bit odd, though.

Farley explains, “In one famous Pennsylvania bear study the researcher found bears denning in road culverts, underneath home porches or simply curled up on a nest of leaves.”

How Long Do They Hibernate?

The length of hibernation can be broad, with changes based on location and species. Just in Alaska, for example, bears further north can remain in their dens for up to seven months, while coastal bears may only hibernate for two months.

Things are variable in the Yellowstone ecosystem, as well. Bears will typically move into their dens in late October through mid-November, with final entry corresponding with severe snowstorms. How long they remain in their dens depends on the type of bear.

The Yellowstone website explains, “Male bears emerge first, usually from early to mid-March, followed by solitary females and females with yearlings or two-year-olds in late March through mid-April. The last to emerge are females with new-born cubs, from mid-April through early May. Males, sub-adults, solitary females, and females with yearlings or two-year-olds usually leave the vicinity of their den within a week of emergence while females with new-born cubs remain in the general vicinity of the den for several more weeks.”

How Do They Emerge From Their Long Rest?

As noted before, one of the main changes is that they may emerge with cubs! When black bears come out of their dens around three months of age, cubs can range in size from 4-8 pounds, while grizzly cubs will weigh in at 10-20 pounds at around ten-weeks-old.

As for changes in adults, the Yellowstone website explains, “Bears live off of a layer of fat built up during the summer and fall months prior to hibernation. Waste products are produced, however, instead of disposing of their metabolic waste, bears recycle it. The urea produced from fat metabolism is broken down and the resulting nitrogen is used by the bear to build protein, which allows them to maintain muscle mass and organ tissues.”

They will largely lose fat during this stretch as a result, generally shedding 15-30% of their body weight.

Once they emerge, they’ll be on the prowl for more food, ready to start their yearly cycles anew.