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Battle of the Chonk: Here's Why Fat Bears Get So Fat

An adult male bear can easily go from around 700 pounds at the start of the season to toppling more than 1,200 pounds

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Brown Bears gather at Brooks Falls in Katmai National Park to feed on the sockeye salmon (Photo: Rebecca Harding/Getty Images)

The battle of the chonk is back, and Alaska’s Fat Bear Week is officially in full swing.

Hundreds of thousands of fish have returned to the salmon runs of Alaska’s Katmai National Park and Preserve to spawn, and with them, the fat bears. For weeks now, the region’s brown bears have been gorging on salmon in the Brooks River, packing on the pounds as they compete for the title of fattest bear—an honor that comes with social media fame.

Fat Bear Week, a March Madness-style contest that pits a dozen of the state’s beloved brown bears against one another in single-elimination brackets, is now in its eleventh year. Over that time, it has grown from a cheeky one-day event created by a former park ranger to a global sensation, with thousands of people from around the world monitoring the live camera streams beaming out the bears from the Brooks River and Brook Falls.

More than one million people voted in last year’s contest for the bear they thought had succeeded in putting on the most weight—generally a best guess from before and after images since there is no final weigh-in. This year’s ursine champion will be crowned on September 30.

Salmon numbers along the Brooks River are especially high this year, organizers say, surpassing anything seen in recent memory. With the bears already displaying an impressive heft, the National Park Service and decided to kick things off a few weeks ahead of the contest’s usual October timing.

Although around 100 bears arrive at Brooks River every year to fatten up, only 11 are featured in the 2025 Fat Bear Week. A few fan favorites have already returned to the limelight, including 2023 and 2024 winner Grazer—the first mother bear to ever win the contest who is now raising her third litter.

Last year’s 1,200-pound runner-up, Chunk, is also back. He turned up to the river in June with a broken jaw, suggesting he had been injured in a fight with another bear. That might make him less competitive this year, but so far, park staff say he has adjusted his behavior to avoid other large males, and learned to eat salmon without the full use of his mandible.

Notably absent is Otis,��a four-time winner famous for his fishing strategy of patiently sitting on the shore of the Brooks River to scoop up fish, rather than hunting in the water. But Otis is—or was—nearing the age of 30, the end of a bear’s lifespan in the wild. He also failed to turn up to Brooks River last year, causing rangers to speculate he may have already passed.

Still, there are plenty of chubby newcomers in the mix this year, ranging from young males to single females battling for the bulge.

Vying for the honor of being named the chonkiest bear is about much more than stardom for these bruins. Fat means survival. And in this arena, Katmai’s bears are better at bulking up than almost anyone else.

Why Bears Feast Every Fall

When the bears arrive at Alaska’s salmon springs in early August, they are entering a physiological state known by scientists as —a frenzied period of gathering food and gorging before the long, cold winter arrives. Scarfing down as many calories as possible allows the bears to survive hibernation when they won’t eat for months.

Many species like to load up on calories when food is plentiful in the late summer and early fall. It’s what drives squirrels to collect nuts and urban black bears to rummage through garbage with increased vigor.

“Some animals store food as food, like squirrels and chipmunks and pikas. But others store it as fat. And that’s what bears are specialized to do,” says bear biologist , who has spent more than 40 years studying grizzlies in British Columbia. “They’ve got to make hay when the sun shines. A bear can store a season of abundant food in fat.”

In Alaska, though, everything is ramped up a notch.

“Bears on salmon streams have that enormous pulse of energy and protein that indeed allows them to max out their intake and fat storage capability,” says , director of research at Washington State University’s Bear Center, who studies the nutritional requirements of bears.

While Kodiak brown bears and polar bears are larger than Katmai’s brown bears, bear in the park��gain the most weight proportionally in a concentrated period of time. During the peak of the salmon runs, a single bear will eat between 10 and 40 salmon per day, or about 112 pounds of salmon, explains Robbins. That would be equivalent to a human consuming around 30 pounds of salmon every day.

An adult male bear can therefore easily go from around 700 pounds at the start of the fishing season to toppling more than 1,200 pounds.

Part of the appeal of Fat Bear Week is that its contestants are roughly on equal footing. There are no men’s and women’s finals to split up the bruins. Male bears are nearly twice the size of females, but both can rapidly balloon up from their individual starting weights.

Over the past decade, three winners have been female—Grazer, Beadnose,��and Holly—and two have been male—Otis and 747.

“All brown bears try to max out their intake and weight gain in the fall,” says Robbins. “This is driven evolutionarily by the fact that the largest males do most of the breeding, and the fattest females have the highest probability of producing surviving cubs.”

Still, some female bears can hit the glass ceiling of Alaska’s salmon streams, missing out on the best fishing positions. Past research has found that females with cubs due to the risk of an aggressive male bear killing her young. In turn, those females tend to be significantly lighter and have a harder time in winter.

“The big dominant males hog the best sites,” says McLellan. “Sometimes you’re watching them and you get mad at these guys because they are so fat, and catching so many fish, and the moms with cubs are having a hard time getting enough.”

During Hibernation, Bears Lose Up To 40 Percent of Their Body Weight

Following the blubber blowout, Alaska’s fat bears will retreat from the spotlight to their winter dens, tucked into hillsides or under hollowed-out trees of Katmai. Here, they’ll enter a deep, sleep-like state that runs from around late October to April.

During hibernation, bears lose an enormous amount of body weight. The bear’s heart rate drops. Its metabolic rate slows. It takes just one breath every minute. The bear does not eat, drink or defecate during this time. To sustain itself, the animal burns off stored fat. Its body recycles urea and other nitrogenous wastes into proteins to stave off muscle loss. By the end of hibernation, the bruin will have lost nearly a third of its body weight, without sacrificing muscle or bone density—a feat not possible in most other species.

Expectant females will wake around January or February to give birth in the den, caring for the cubs until they emerge later in spring for the green-up. Female bears experience a unique reproductive phenomenon known as “delayed implantation.” After mating, the fertilized embryo remains dormant and will only implant on the female’s uterine wall once she has accumulated enough body fat, about 20 percent, to get her and the pregnancy through hibernation and the cub-rearing stage.

If she doesn’t gain enough weight, her body reabsorbs the embryo. This allows for natural crowd control during bad food years.

“Fatter females produce cubs earlier and they produce either more milk or better quality milk,” says Robbins. “The combination of these two things means that cubs born to fatter females have longer to grow in the den before they emerge, they grow faster, and therefore they are larger when they come out of the den and are better able to stay with their mother.”

Coming out of the den bigger and fatter than other bears also makes spring foraging easier, says McLellan, because the bears can stick closer to home.

“That’s often the time when males kill cubs,” he says. “If females can stay in a remote basin and not eat much through May and June, then her cubs will do well. But if she’s skinny and she has to go down where the food is best, then she takes the risk that others might kill her cubs.”

Can’t get enough of fat bears? An��excellent way to pick your bear for each day of the bracket is to try to figure out which one speaks to you the most—the real you, if you were a bear lucky enough to live in the salmon-rich waters of Katmai. We’ve made that process easier by building a quiz to help you determine your match.