than any other nonhuman primate.

And one troop has become downright famous

because they're the only monkeys in the world

to spend hours in the winter soaking in hot springs.

Only recently, scientists have started investigating

the behavior of the macaques.

But before we get to that, here's a brief history

of how a group of macaques developed the hot tub habit

in the first place.

Macaques live all over Japan.

But this group's home is near Nagano,

where the winter temperatures are often below freezing,

but the landscape is sprinkled with natural hot springs.

But it wasn't until 1963,

so the story goes, that a monkey first joined human visitors

in a hotel bathing pool.

Of course, one macaque in the pool

soon became many macaques, which upset the humans.

The solution: build a park and hot spring bathing pools

just for the monkeys.

Happy monkeys, happy humans.

The macaques soon became an attraction,

drawing more attention and more visitors

to the mountains of Nagano.

Back to the present.

Researchers are now focusing on why the monkeys bathe.

Probably to stay warm, but that's just an assumption.

Since cold causes stress, increasing levels of hormones

called glucocorticoids,

scientists tested levels of these hormones in the monkeys.

They didn't draw blood or collect saliva.

They collected and tested feces.

And since the monkeys are so used to human tourists,

they paid no attention to the researchers.

As suspected, stress levels were lower during periods

when the macaques were bathing.

Interestingly, the higher-ranking females

had more access to the pool and more time bathing.

Takeshita herself found a kind of nonscientific inspiration

in the monkey bathing.

Many times after coming back from the field

I would go to hot springs.

They also show how a small group of animals

can develop a unique behavior, their own kind of culture,

passed down from generation to generation.

I wonder what they think about while they soak.