Fall & Fat Bears

September 22nd marked the beginning of fall. For us humans, this might mean raking autumn leaves, wearing cozy sweaters, or drinking pumpkin spice lattes. For American black bears and brown bears, however, the beginning of fall is about one thing: consuming as much food as possible, in preparation for a long winter nap.

This phase during which bears put on weight to prepare for winter is called hyperphagia. During hyperphagia, bears are constantly foraging for food. Adult male brown bears can put on as many as 500 pounds of weight in the months before winter.¹ This seasonal weight gain has led Alaska’s Katmai National Park to establish Fat Bear Week in early October, during which fat bear fans from around the world vote for their favorite fat bear. This year, 25-year-old Otis AKA #480 claimed his fourth title, defying the odds by becoming winner of Fat Bear Week despite having lost multiple canine teeth. While celebrating overweight animals may not sound like it’s in the bear’s best interest, Fat Bear Week is really a celebration of Katmai’s healthy ecosystem.² Thanks to the plentiful salmon, Katmai’s brown bears have resources to build up fat reserves. For the bears that brave Alaska’s freezing winters, fat is survival.

While the period of dormancy seen in bears during winter is generally referred to as hibernation, some debate exists as to whether bears truly hibernate. During winter, bears enter dens and can go as long as 6 months without eating, drinking, urinating, or defecating. They survive by breaking down the layers of fat they built up during hyperphagia. Their heart rate and metabolic rate drop and they breathe much more slowly. However, their body temperature only drops about 12 degrees, which is significantly less than true hibernating mammals like ground squirrels, whose temperatures drop below freezing during hibernation.³ The winter dormancy in bears is sometimes referred to as torpor, because of the comparatively small dip in internal body temperature. During torpor, bears are able to wake up if disturbed, further distinguishing them from true hibernators.

Brown and black bears are intelligent and adaptable animals who are able to survive in a wide variety of climates and habitats throughout North America. Bears torpor for differing amounts of time depending on the climate in which they live. While bears in extremely cold places can torpor for half the year, black bears in more mild climates may go into a period of dormancy for just a few weeks, if at all.⁴ Bears awake in the spring having shed up to 30% of their body weight, and ready to chow down once again on spring’s new growth.5,6

Understanding the seasonal patterns of bears is important for humans who live or travel in bear country. People who share habitat with bears have a responsibility to ensure that wild bears don’t have access to human foods, especially during hyperphagia. When bears learn to search for food in human areas, it can put them in danger from car strikes, and if a bear repeatedly gets too close to human settlements, wildlife officials are forced to euthanize the bear.

Creating a world where people and wildlife coexist starts with understanding the needs and patterns of our non-human neighbors in order to accommodate them. This fall, let’s hope the bears are finding lots of natural food, and wish them a peaceful winter’s rest.

Learn more about how you can stay bear safe here.


  1. Cooper, K. (2018, October 5). Why a US national park is holding a ‘Fat Bear Week’ contest. BBC.
  2. Rosen, Y. (2021, September 29). Ahead of winter hibernation, Alaska celebrates Fat Bear Week. amNY.
  3. National Park Service. (2020). Arctic Ground Squirrel.
  4. Natonal Park Service. (2020). Black Bears in Big Bend.
  5. National Park Service. (2017). When Bears Wake Up.
  6. Fitz, M. (2013, November 13). Bear Hibernation. National Park Service.