How Fat Bears Bulk Up To Hibernate (And Why We Love To See It)

[Narrator] Every year something switches in the brains

of these coastal brown bears.

Before they hibernate, they lose their ability to feel full

and pack on hundreds of pounds.

In fact, it's become an annual competition

to celebrate these fat bears.

This is 747 in summer, 2022,

and this is 747 in October, 2022.

In the span of a few months,

747 and other bears like him

at remote Katmai National Park

are gaining 20 to 40% of their body weight.

And for bears, that's a good thing.

[gentle music]

[bear vocalizing]

Fat Bear Week is a celebration of success and resilience

in our healthy population of brown bears.

It is a week-long March style madness bracket

where the public gets to vote on their favorite fat bear.

[Narrator] Katmai has some

of the fattest bears in the world

because of a strong sockeye salmon population.

The salmon run lasts from late June through September

allowing the bears to feast.

Sockeye salmon are very nutritious food.

They're actually pretty high calorie.

One sockeye salmon will contain anywhere between

two to 5,000 calorie.

Bears in average, they might catch 10 fish per day.

If they are consuming the entire fish,

they're getting anywhere between

20,000 and 50,000 calories per day.

[Narrator] And salmon is a good source of fat for bears

looking to pack on the pounds.

They're actually going to focus on

just the fattiest parts of the fish.

So the skin, the brains, the eggs in females.

They eat that, they strip the skin, they leave it,

and then some other bears or other animals

will pick off those carcasses.

There is one particular bear, bear 480 Otis,

who was observed on the webcams to consume

45 salmon within a five-hour period.

[bear vocalizing]

[Narrator] And bears are catching these fish

at Brooks Falls.

But why are bears actually fishing?

It has something to do

with a metabolic process called hyperphagia.

Hyperphagia is a general term

that basically denotes overeating.

The term can be applied to humans.

With bears, it means that there is a period of the year

right before they go into hibernation in the fall

where they're eating just as much as they possibly can

in order to try and prepare for hibernation.

[Narrator] But bears are mysterious creatures.

Researchers still don't really understand

why bears enter this state.

There's a few hormones that act as what we call

satiety factors that tell the body that we're full.

A couple of the big ones are a hormone called leptin.

And leptin is released by adipose tissue,

and that is something that helps tell our body like,

Hey, we've got enough stored energy,

so you don't need to eat anymore, you're full right now.

Another one is called ghrelin

that's released by the stomach.

And it says, Hey, I'm full right now.

You should stop eating.

So their body's not telling them I'm full.

So they'll just keep on eating and eating

without that feedback telling them to stop.

If they are not moving a lot, not working a lot,

not using those muscles, then that energy is gonna be stored

because they don't need to use it right then.

And the way that it's generally gonna be stored

for bears is in adipose tissue.

So we're getting a lot of energy

that is then being stored as fat

as they prepare to go into hibernation

so that they can use that through the winter.

[Narrator] Other animals such as hummingbirds

enter hyperphagia before long migration periods.

But for most animals like humans or cats,

hyperphagia is not a good thing.

Overeating can have a lot of negative health impacts.

You know, heart disease, diabetes, very well known.

All of these sorts of things can happen

when you've got uncontrolled eating

and that energy is not being used.

[Narrator] Bears then enter a hibernation period

and go into dens at Katmai.

What hibernation really is,

it's a reduction in the metabolic rate.

So the bear's not necessarily asleep.

In fact, they still have circadian rhythms

where they will be sort of more active during the day,

less active at night.

So they're still awake throughout this entire period.

Heart rate during hibernation

I would think of as being in the 10 to 12 beats per minute.

They're just, their metabolism has been slowed down.

Their heart rate slows down,

their respiration rate slows down.

Generally, they're not eating or drinking,

urinating or defecating,

though there are certainly exceptions to that.

[Narrator] Unlike some other hibernating animals,

bears don't actually see

too much of a drop in body temperature.

So a lot of the animals

where hibernation was originally defined

are some of the smaller mammals.

And they will drop their body temperature down

closer to ambient temperature

and hold very close to freezing.

But brown bears will have a body temperature

of around 36, 37 degrees Celsius.

And then during hibernation,

they're only dropping down to maybe 31 degrees Celsius,

about a six degree drop in body temperature.

[Narrator] Bears can also give birth while in hibernation.

Usually they give birth from late December

up until early February.

Their metabolic rate will increase,

not all the way back up to active state,

but it will increase quite a bit.

And then they give birth,

and immediately after giving birth essentially,

the metabolic rate will drop straight back down

to a full hibernation period.

And the cubs will just kind of climb underneath mom,

and they'll start nursing while mom continues to hibernate.

[Narrator] And all that fat that's been stored up

helps them to survive a long winter.

Bears store all this fat going in hibernation

so that they have energy

so that they can survive through the winter.

They'll lose between 25 and 40% of their body weight

over the winter.

That's gonna be higher for, like, females that give birth.

They're gonna be closer to the 40% range.

Some of the really large males

tend to be closer to 25% range.

One interesting thing,

that fat is not only giving them energy,

it's also providing, so that the catabolism of fat

also provides water.

So we mentioned that they're not drinking all winter long.

This is one way that their body is able to get

a little bit of water since they're losing water constantly

through respiration and stuff like that,

they do need to offset that somehow.

And the catabolism of fat

actually creates water that they're using as well.

Doing any of this type of work on bears

can be very challenging.

Hibernation is fairly well studied physiological state.

Despite that, we still don't really know

what triggers hibernation.

It's really tough to nail down precisely individual factors,

particularly when a lot of times

it's not one factor that's triggering it.

It's multiple things that are working in concert

that might lead to bears

changing physiological states like this.

[Narrator] So who will win

this year's Fat Bear Competition?

One of those big contenders

is the reigning champion, 747.

He still looks really, really good.

When he came out of hibernation,

I was fortunate enough to see him the first week of June.

And I was like, Oh my goodness.

This is a bear that lost one third of his body weight.

There's some other large males

that are also looking really good.

Number 32 Chunk.

There are also some females that are in the running for it.

Bear 128 Grazer.

She is looking really, really good.

[bear vocalizing]