KYOTO - Japanese monkeys in Nagano Prefecture’s Jigokudani valley appear to have lower levels of a stress hormone than usual when they bath in natural hot springs during winter to warm up, a study has said.
A team including Rafaela Sayuri Takeshita, a researcher at Kyoto University’s Primate Research Institute, made the finding through analyzing the excrement of the monkeys, which are famous for bathing in the hot springs at Jigokudani Yaen-Koen, a monkey park in the Nagano town of Yamanouchi.
A paper on the team’s finding was published in an edition of the international journal Primates on Tuesday by the Japan Monkey Centre in Inuyama, Aichi Prefecture.
Japanese macaques are the world’s northernmost-living monkeys and are also known as snow monkeys. As they regularly soak in hot springs in the winter, it had been assumed they do so to warm themselves up.
In their analysis, the researchers said the results indicate that the snow monkeys bathe in the hot springs to reduce stress from the winter cold and improve their chances at reproducing and surviving.
In 2014, the team observed behaviors such as bathing times of 12 female macaques, aged between 5 and 24, in the birth season in spring between April and June, and during the winter mating season between October and December.
They measured cortisol, a stress hormone, in the feces of monkeys that took baths in the hot springs at least once a week and those that did not.
The cortisol concentration for monkeys that soaked in the hot springs in winter was some 20 percent lower on weekly average than for those that did not bathe, leading the researchers to believe that the bathing caused their stress hormone levels to be reduced.
“Japanese monkeys tend to become more aggressive in the winter, and the cold temperature should also contribute to higher stress levels. But, like human beings, hot spring bathing may have reduced them,” Takeshita said.
Of the 12 monkeys that bathed in the hot springs in winter, eight gave birth in spring.
The sight of the Japanese macaques bathing at the hot spring is popular among tourists from Japan and overseas alike.