Feeding Frenzy

For bears, gaining body fat is the key to their winter survival. Katmai’s brown bears often eat only the fattiest parts of the salmon—the brain, eggs, and skin—and leave behind the fleshier bits humans typically prefer.

All spring, the bears munch on plants, insects, and small mammals. But when the salmon swim up the river to spawn each summer, the bears focus on consuming as many fish as possible. Each one provides 4,500 calories.

“As they prepare for hibernation, bears are actually preparing to avoid winter famine, not cold weather,” says Fitz. As temperatures drop, food becomes scarcer. Bears survive by retreating to the safety of their dens. These winter dens can range from rocky caves to hollows under rotted tree roots. Every bear spends some of its summer scouting a safe and comfortable den location.

During hibernation, brown bears do not eat, drink, urinate, poop, or leave their den. By burning body fat while they sleep, bears stay hydrated and keep warm. Their body temperature remains fairly high, while their heart rate and breathing rate drop significantly to save energy.

For bears, gaining body fat is the key to surviving the winter. The brown bears in Katmai often eat only the fattiest parts of the salmon—the brain, eggs, and skin. They leave behind the fleshy parts that humans usually eat.

All spring, the bears munch on plants, insects, and small mammals. But in the summer, the salmon start swimming up the river to reproduce. The bears try to catch and eat as many fish as possible. Each fish provides about 4,500 calories.

Bears don’t hibernate in the winter to hide from the cold, says Fitz. They actually do it to avoid going hungry. As temperatures drop, food becomes scarcer. Bears survive by making a den in a rocky cave or the hollow under a rotted tree. Every bear spends some of its summer finding a safe and comfortable den location. Then it retreats there to hibernate as winter sets in.

During hibernation, brown bears do not eat, drink, urinate, poop, or leave their den. They burn body fat while they sleep to keep warm. Their heartbeat and breathing slow down significantly. This saves energy, helping their fat supply last. 

Bears accumulate enough fat to survive hibernation by gorging on food in late summer and fall, a process called hyperphagia. During this period of extreme eating, the bears feel constant hunger.

“You know that feeling you get after a big meal?” asks Fitz. “Hyperphagic bears probably don’t feel that.” This allows the bears to eat tens of thousands of calories per day and accumulate the body fat necessary to survive hibernation—gaining up to 4 pounds per day! Brown bears never truly satisfy their hyperphagic hunger. However, they stop eating and start retreating to their dens when their food supply dwindles, usually in late November.

Bears have to store enough fat to survive hibernation. They do this by gorging on food in late summer and fall. This process is called hyperphagia (hye-puhr-FAY-juh). During this period of extreme eating, the bears feel hungry all the time.

“You know that feeling you get after a big meal?” asks Fitz. “Hyperphagic bears probably don’t feel that.” This allows the bears to eat tens of thousands of calories per day. They can gain up to 4 pounds every 24 hours! Brown bears never truly satisfy their intense hunger. But they stop eating and start retreating to their dens around late November, when their food supply starts running out. 

Katmai’s bears have become famous in recent years, thanks to an event called Fat Bear Week. During this annual competition, fans track the weight gain of the bears that congregate at Brooks River.

Each fall, more than 2,000 bears living in the park come to the river to gorge on salmon. Park rangers identify the bears by their physical characteristics, such as size, shape, face, scars and wounds, fur color, and claw color. “The bears are not tagged or handled in any way,” says Fitz.

Katmai’s bears have become famous in recent years. That’s thanks to an event called Fat Bear Week. Fat Bear Week is an annual competition. Fans around the world track the weight gain of the bears that gather at Brooks River to eat.

More than 2,000 bears come to the river every year. Park rangers tell them apart by their physical characteristics, such as size, shape, fur color, and any unique scars. This way the rangers can identify the bears without disturbing them. “The bears are not tagged or handled in any way,” says Fitz.

Then the rangers estimate how much weight the bears gain. They base it on a combination of their observations and data from studies done on bears in North America. A large male brown bear in Katmai can weigh more than 1,000 pounds by hibernation time!

“I thought it would be fun if we gave the public the opportunity to compare the changes and decide who they think is the fattest bear,” says Fitz. So in 2014, he set up a contest on Facebook where people voted for the bear they thought “won” autumn hyperphagia.

The event was so popular that Fitz and his fellow rangers expanded it to a whole week in 2015. Fat Bear Week continues to celebrate Katmai’s brown bears’ important preparation to survive the winter. Who won this year? Click here to find out!

Rangers estimate how much weight the bears gain. They do this by comparing their observations to previous studies of North American bears. The brown bears in Katmai get huge by hibernation time. A large male can weigh more than 1,000 pounds!

In 2014, Fitz set up a contest on Facebook. “I thought it would be fun if we gave the public the opportunity to compare the changes and decide who they think is the fattest bear,” he says. Many people logged onto the website to weigh in. They voted for the bear they thought “won” the autumn eating spree.

The online event was extremely popular. So Fitz and his fellow rangers expanded it to a whole week in 2015. Fat Bear Week continues to celebrate how Katmai’s brown bears prepare for winter. Who won this year? Click here to find out!