As grizzly bears prepare for hibernation they undergo a massive weight gain to accumulate fat reserves for the winter months when they do not eat, drink, or eliminate waste. The behavior is known as hyperphagia and can result in bears putting on two to four pounds per day resulting in body fat levels up to 40% by autumn. During hyperphagia grizzly bears usually consume approximately 20,000 to 30,000 calories per day and have been observed taking in 60,000 calories in a single day.
The fat stores accumulated during autumn are crucial for hibernation as bears can lose up to one-third of their body weight before spring. The fat serves not only as an energy source but is also metabolized into nutrients which allows bears to maintain muscle mass and bone density throughout the hibernation period.
The following video captured by a motion-activated trail camera in Canada’s Yukon Territory and shows a grizzly bear well into hyperphagia.
“This rotund grizzly sure knows how to prep for the big sleep that awaits. While not the biggest grizzly I’ve captured on camera, definitely the fattest, which is meant in the most admirable of ways.” -Yukon Wildlife CamsÂ
In recent years Katmai National Park has celebrated hyperphagia with Fat Bear Week. While the bear in this video didn’t qualify for voting as it lives outside the park, it deserves an honorable mention for its substantial bulk. Congratulations to 128 Grazer who took home the top spot and was crowned the winner of 2024 Fat Bear Week.
