Some of what is known about grizzly bears is settled, while some is still being worked out, and the winter sleep is a good place to begin: hibernation research has mostly been done on small mammals in laboratories, and little is known about what triggers or ends it in the wild, which is why researchers studied free-ranging brown bears.
In a study that ran over three winters, the bears' activity, heart rate and body temperature began to fall several weeks before they entered the den, and they entered when snow arrived and the air temperature reached 0 degrees Celsius, and researchers think that food in autumn likely triggers hibernation, and that conditions in winter and spring are linked to when bears leave the den.
The sleep itself is still being described. In Scandinavian brown bears, daily rhythms in body temperature and heart rate slowed during hibernation to rhythms that run longer than a day, and these rhythms were influenced by the amount of snow in the denning area.
Even the name of the winter sleep is not settled. A U.S. Department of the Interior page calls it torpor, while researchers in Scandinavia call brown bears shallow hibernators, because during the den period their body temperature drops only a little, to about 33 to 34 degrees Celsius.
In September and October, whitebark pine nuts are the most important food for Yellowstone grizzlies in years when the seeds are plentiful, yet mountain pine beetles have killed trees in about 82 percent of whitebark pine stands in the Greater Yellowstone area. When the seeds were scarce, Yellowstone grizzlies seemed to eat more animal matter, and their body condition did not change in poor seed years, and if high-calorie foods are scarce, grizzlies can usually shift to other foods that have fewer calories but are widely spread across the land and easy to find most years.
Grizzly bears have one of the slowest reproductive rates among land mammals. A female grizzly mates in spring or early summer, but the embryo does not start to grow until late fall, and a female that has not stored enough fat in the fall may not have cubs that winter.
How many kinds of grizzly are there? A grizzly bear is one kind of brown bear, and scientists agree there is only one species of brown bear. Scientists once named many grizzly subspecies from body measurements, but genetic studies suggest there are fewer, and although two are currently recognized for North America, the mainland grizzly and the Kodiak bear, the exact number of brown bear subspecies is still under debate.
Brown bears in the lower 48 states lost about 98 percent of their former range, and the Greater Yellowstone area and northwest Montana are the only places south of Canada that still have large grizzly populations.
As of November 2026, grizzly bears in the lower 48 states are protected as a threatened species under the Endangered Species Act, and, as of November 2026, Wyoming, Montana and Idaho have all petitioned the U.S. Fish and Wildlife Service to consider removing Greater Yellowstone grizzlies from Threatened Species status.
The last question is what counts as recovered, and scientists judge whether the Greater Yellowstone grizzly population has recovered using estimated population size, where females with cubs live, and how many bears die each year.
Words to know
- torpor: deep rest
- subspecies: kind within species
- hibernation: deep winter sleep
