Ready for 'Fat Bear Week?' Park ranger Felicia Jimenez and wildlife biologist Dr. Michael Saxon join WIRED to talk to us about all things brown bear bulk, from how they fish for tens of thousands of calories a day to the evolutionary reasons behind their salmon snacking prowess.
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00:00 Every year, something switches in the brains of these coastal brown bears. Before they hibernate,
00:04 they lose their ability to feel full and pack on hundreds of pounds. In fact, it's become an
00:09 annual competition to celebrate these fat bears. This is 747 in summer 2022, and this is 747 in
00:16 October 2022. In the span of a few months, 747 and other bears like him at remote Katmai National
00:21 Park are gaining 20 to 40 percent of their body weight. And for bears, that's a good thing.
00:30 Fat Bear Week is a celebration of success and resilience in our healthy population of brown
00:38 bears. It is a week-long march-style madness bracket where the public gets to vote on their
00:46 favorite fat bear. Katmai has some of the fattest bears in the world because of a strong sockeye
00:52 salmon population. The salmon run lasts from late June through September, allowing the bears to
00:56 feast. Sockeye salmon are very nutritious food. They're actually pretty high calorie. One sockeye
01:01 salmon will contain anywhere between two to five thousand calories. Bears on average, they might
01:07 catch ten fish per day. If they are consuming the entire fish, they're getting anywhere between
01:12 20,000 and 50,000 calories per day. And salmon is a good source of fat for bears looking to
01:19 pack on the pounds. They're actually going to focus on just the fattiest parts of the fish. So
01:24 the skin, the brains, the eggs and females, they eat that, they strip the skin, they leave it,
01:30 and then some other bears or other animals will pick off those carcasses. There is one particular
01:35 bear, Bear 480 Otis, who was observed on the webcam to consume 45 salmon within a five hour period.
01:44 And bears are catching these fish at Brooks Falls. But why are bears actually fishing?
01:51 It has something to do with a metabolic process called hyperphagia. Hyperphagia is a general term
01:56 that basically denotes overeating. The term can be applied to humans. With bears, it means that
02:00 there is a period of the year right before they go into hibernation, the fall, where they are
02:04 eating just as much as they possibly can in order to try and prepare for hibernation.
02:10 But bears are mysterious creatures. Researchers still don't really understand why bears enter
02:15 this state. There's a few hormones that act as what we call satiety factors that tell the body
02:19 that we're full. A couple of the big ones are a hormone called leptin. And leptin is released by
02:24 adipose tissue. And that is something that helps tell our body like, hey, we've got enough stored
02:29 energy, so you don't need to eat anymore, you're full right now. Another one is called ghrelin,
02:33 that's released by the stomach. And it says, hey, I'm full right now, you should stop eating. So
02:38 their body's not telling them, I'm full. So they'll just keep on eating and eating without
02:43 that feedback telling them to stop. If they are not moving a lot, not working a lot, not using
02:47 those muscles, then that energy is going to be stored because they don't need to use it right
02:51 then. And the way that it's generally going to be stored for bears is in adipose tissue. So we're
02:55 getting a lot of energy that is then being stored as fat as they prepare to go into hibernation so
03:00 they can use that through the winter. Other animals such as hummingbirds enter hyperphagia
03:04 before long migration periods. But for most animals like humans or cats, hyperphagia is
03:10 not a good thing. Overeating can have a lot of negative health impacts, heart disease, diabetes,
03:15 very well known. All of these sorts of things can happen when you've got uncontrolled eating and
03:19 that energy is not being used. Bears then enter a hibernation period and go into dens at Katmai.
03:25 What hibernation really is, it's a reduction in the metabolic rate. So the bear is not necessarily
03:29 asleep. In fact, they still have circadian rhythms where they will be sort of more active during the
03:34 day, less active at night. So they're still awake throughout this entire period. Heart rate during
03:38 hibernation I would think of as being in the 10 to 12 beats per minute. They're just, their
03:42 metabolism has been slowed down, their heart rate slows down, their respiration rate slows down.
03:46 Generally, they're not eating or drinking, urinating or defecating, though there are
03:50 certainly exceptions to that. Unlike some other hibernating animals, bears don't actually see
03:55 too much of a drop in body temperature. So a lot of the animals where hibernation was originally
03:59 defined are some of the smaller mammals, and they will drop their body temperature down closer to
04:04 ambient temperature and hold very close to freezing. But brown bears will have a body
04:10 temperature of around 36, 37 degrees Celsius. And then during hibernation, they're only dropping
04:15 down to maybe 31 degrees Celsius, about a six degree drop in body temperature. Bears can also
04:22 give birth while in hibernation. Usually they give birth from late December up until early February.
04:27 Their metabolic rate will increase, not all the way back up to active state, but it will increase
04:32 quite a bit. And then they give birth and immediately after giving birth, essentially,
04:37 the metabolic rate will drop straight back down to a full hibernation period. And the cubs will
04:41 just kind of climb underneath mom and they'll start nursing while mom continues to hibernate.
04:46 And all that fat that's been stored up helps them to survive a long winter.
04:51 Bears store all this fat going in hibernation so that they have energy so that they can survive
04:55 through the winter. They'll lose between 25 and 40% of their body weight over the winter.
05:00 That's going to be higher for females that give birth. They're going to be closer to the 40%
05:05 range. Just some of the really large males tend to be closer to the 25% range. One interesting thing,
05:10 that fat is not only giving them energy, it's also providing... So the catabolism of fat also
05:15 provides water. So we mentioned that they're not drinking all winter long. This is one way that
05:19 their body is able to get a little bit of water. Since they're losing water constantly through
05:23 respiration and stuff like that, they do need to offset that somehow. And the catabolism of fat
05:26 actually creates water that they're using as well. Doing any of this type of work on bears can be
05:30 very challenging. Hibernation is a fairly well-studied physiological state. Despite that,
05:36 we still don't really know what triggers hibernation. It's really tough to nail down
05:41 precisely individual factors, particularly when a lot of times it's not one factor that's triggering
05:47 it. It's multiple things that are working in concert that might lead to bears changing
05:50 physiological states like this. So who will win this year's fat bear competition? One of those
05:55 big contenders is our reigning champion, 747. He still looks really, really good. When he came out
06:02 of hibernation, I was fortunate enough to see him the first week of June. And I was like, "Oh my
06:07 goodness, this is a bear that lost one third of his body weight." There's some other large males
06:11 that are also looking really good. Number 32, Chunk. There are also some females that are
06:18 in the running for it. Bear 128, Greaser. She is looking really, really good.