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 non-scientific 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.

Hello, YouTube, I'm Jim Gorman, the host of ScienceTake, the show you just saw.

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