Who doesn’t love a spa day? A bit of pampering and relaxation in hot pools to reduce the stress from one's daily routine. Well, it turns out that this enjoyment extends to some of our primate cousins. Japanese macaques have been witnessed taking hot baths for the last 55 years, a behavior that researchers think helps the animals stave off the stress of cold conditions.

The research was published in Primates, which is the official journal of the Japan Monkey Centre. Rafaela Takeshita of Kyoto University and her colleagues studied 12 female macaques during the spring birth season (April to June) and the winter mating season (October to December). They made a record of who bathed the most and for how long they were in the water, and then collected fecal samples during extreme cold spells.

The fecal samples were used to measure the concentration of fecal glucocorticoid metabolites, a steroid hormone that can be used to monitor stress levels. The team discovered that dominant females who spent the winter in more aggressive conflicts used their status to spend more time in the bath, leading to reduced stress levels.   

"This indicates that, as in humans, the hot spring has a stress-reducing effect in snow monkeys," Takeshita said in a statement. "This unique habit of hot spring bathing by snow monkeys illustrates how behavioral flexibility can help counter cold-climate stress, with likely implications for reproduction and survival."

The researchers also showed that the 500 or so daily visitors that come to watch the macaques bathe in the hot spring don’t affect their stress levels. Takeshita’s team is now interested in expanding their research. They are interested in short-term changes in stress hormones and if the hot baths affect those as well. The team thinks that by using serum or saliva samples, they may gain a more detailed picture of macaque stress levels.

Snow macaques began this habit of spending time in hot pools in 1963, when a young female was seen in an outdoor hot spring from a nearby hotel. The behavior quickly spread to other females, and by 2003, one-third regularly bathe in the winter. The monkeys reside inside the Jigokudani Monkey Park in Nagano where, for hygienic purposes, a special hot spring has been built for them.