Why Do Bears Hibernate in Yellowstone?

The why and how of bear hibernation is fascinating, and a scientifically advantageous subject. The “why” has been very obvious to observers of bear behavior from the beginning. Bears hibernate to better survive winters where there is limited access to food.

The “how” really relied on the studies of two twin brothers in Yellowstone National Park back in the 1960’s. These men, Frank and John Craighead, were able to successfully track bears to their dens where they could monitor breathing, heart rate, and general behavior. 

Being the first study to use radio telemetry to track wild bears; this study was groundbreaking and the scientific community has continued the brothers work, giving us a great deal of information about the mysterious process of hibernation. 

Why Do Bears Hibernate?

The first big question that most people ask is, are bears true hibernators?

Many people have argued that bears do not truly hibernate because the bear’s body temperature stays above 88 degrees which is quite warm. Additionally, not all bears hibernate, and those that do don’t hibernate equally.

Black bears in Mexico might hibernate for only a few days, while grizzlies in Alaska might stay in the den for up to six months. Male polar bears often don’t den at all and the grizzlies of Yellowstone have been known to wake up randomly during the winter and leave their den. On average though, the bears of Yellowstone hibernate for five months, with black bears going to den first, normally in October, followed by pregnant female grizzlies, and large male grizzlies going to den last. 

While in the den, they lower their body temperature 8-12 degrees, and break down fat stores for energy. Some protein is used as well, but bears largely conserve their muscle mass and do not become noticeably weaker during hibernation. They do not eat, drink, urinate, or defecate. They breathe once every 45 seconds, and have a heartbeat of 8-21 beats per minute.

All of this sounds incredibly stressful for the bear’s body, but the bear’s ability to recycle nitrogen as they metabolize their fat and turn that nitrogen into protein prevents a true state of starvation. Some bears might even gain lean muscle in the den; however, body fat content is lost dramatically and the bears often emerge with fifteen to thirty percent less body weight in the spring.

Preparing for Denning

To prepare for denning, the bears must first go through a period we call hyperphagia. During this period, grizzlies go from consuming approximately 6,000 calories a day to over 20,000 calories. They feed almost constantly on berries, white bark pine nuts, and any animal protein they can find. In late fall and after hyperphagia they transition to voluntarily eating less, but they continue to drink to free their body of wastes. During this period, they may rest 22 or more hours per day, and their heart rates fall from 80-100 per minute to 50-60 per minute. Now is the time to build a den.

Grizzly Hibernation Behavior

There is a lot of careful planning that goes into grizzly den placement. The grizzlies of Yellowstone need to get through one of the harshest and coldest winters in the lower 48, so where they spend it is incredibly important.

Scientists have observed that grizzlies tend to locate their dens on the mid to upper third of thirty to six degree slopes. They look for a northern exposure, and they prefer elevations averaging 8,000 feet, but as high as 10,000 feet in some cases. Pregnant females tend to den higher than other bears, because they will need to spend more time in the den, and come spring the melting snow will destroy dens at lower elevations first.

Once a suitable spot is selected, grizzlies will excavate a den only slightly larger than their bodies. The smaller the space, the warmer they will stay as outside temperatures dip as low as -66, which was seen in 1933 and is still the record for the ecosystem. 

Black Bear Hibernation Behavior

Black bears will sometimes use hollow trees or caves, and in a Pennsylvania study they were observed curled up on a nest of leaves and utilizing road culverts, and the underside of porches. This behavior would be extremely dangerous in grizzly country, because grizzlies will occasionally take advantage of black bears denning earlier to prey on black bears while they sleep. Grizzlies prefer to dig a den, often at the base of a large tree or on a densely vegetated north-facing slope.

North facing is always best, being the coldest part of the mountain, and that placement allows the southwesterly winds to accumulate snow on the north slopes and better insulate the dens. Construction of the den is fairly quick, taking an average of three to seven days. During this time a bear could move as much as a ton, or 2,000 pounds of material. 

Hibernation and Cubs

Grizzly cubs are born about two months into hibernation, in late January or early February. Both grizzly and black bears breed from May to July, but gestation does not begin until late November. The grizzly cubs are 8 inches long, and weigh 8-12 ounces. These babies won’t hibernate, but will nurse and grow quickly, leaving the den with their mother early to mid-May. At this point they should be nearing ten pounds and will be able to follow their mother in search of food, which she desperately needs.

Yellowstone bears will often seek out hydrothermal areas in the weeks after leaving the den, where the underground, magma-heated, water can rapidly accelerate the emergence of green plants. The grizzlies also focus on animals that might have died and remained frozen during the winter. A bison that has fallen through the ice and drowned, or a deer that became trapped in deep snow is an early season boon for a hungry bear. 

All in all, grizzlies as well as black bears, handle the deep freeze of winter remarkably well thanks to their hibernation abilities, and this has led scientists to wonder if there could be lessons learned from these animals that could advance our own medicine, travel, and overall health.

A study in Australia on hibernating squirrels has shown potential in treating Alzheimer’s disease, and the ability of a bear’s cells and tissues to survive very cold temperatures could help us better preserve and transport human tissues. The lack of muscle deterioration or bone loss in hibernators is still not totally understood, but could lead to treatment for muscle disorders and degenerative bone diseases.

There is a great deal that could come from the study of bear hibernation, and scientists will continue to observe, measure, and record data about this incredible adaptation.

See the Bears of Yellowstone with BrushBuck Wildlife Tours