PUBLISHED April 4, 2018

Japanese macaques, also called snow monkeys, soak in a hot spring in Jigokudani, Japan. Photograph by Jasper Doest, Minden Pictures/Nat Geo Image Collection

There's a reason macaques look so serene in their photos. Submerged up to their cheeks in steamy water with snow drifting onto their furry heads—they look almost meditative.

Researchers always suspected the monkeys used the hot springs to stay warm in the winter, and a new study published in the journal Primates confirms this. While the researchers suspected this was the case, the finding could still illuminate how the monkeys cope with stress.

A Cold North

Of all the primates in the world, Japanese macaques live the farthest north. For decades, they've been observed taking baths in Jigokudani Monkey Park in Japan. Their behavior has become so endearing to people that thousands of tourists often trek north to take a peak at the snow monkeys' bath time.

In the mountainous regions of Japan, winters can be harsh. While the monkeys are lounging in a calm pool of water, they're often subjected to intense snowfall and freezing temperatures. (Read more about the world's snowiest place.)

To see exactly what kind of benefit the macaques got from their baths, a group of researchers from Japan's Kyoto University tracked 12 females. Their behavior was observed in two different seasons: their birth season, from April to June, and their winter season, from October to December. The scientists looked at which monkeys bathed the most often and for how long.

They then measured the amount of a hormone called glucocorticoid from the monkeys' fecal samples. Glucocorticoids come from a family of steroid hormones and are produced when the monkeys undergo stress—often from trying to maintain a body temperature that's not too cold and not too hot.

Soothing Baths

The team's results confirmed that the monkeys used the hot springs more frequently during winter months. Females who had higher social standing were documented taking longer baths (it's also true they were seen involved in more conflicts).

When the researchers then measured the amount of glucocorticoid in females who took longer baths, they found lower levels of the stress hormone than in females who hadn't bathed at all. This means the females experienced less stress from trying to regulate their body temperatures.

Whether the long baths influenced other stress hormones is unclear. Researchers plan to next study saliva samples to measure other hormones related to stress. They theorize that, by lowering stress, the baths could benefit the macaques' likelihood to reproduce and thus ensure their survival.