The Japanese snow monkeys that live in Jigokudani Monkey Park famously enjoy bathing in hot springs—an adorable habit that hasn’t been observed among other groups of macaques.
Scientists have long believed that this behavior can be chalked up to a logical explanation: the monkeys are cold, and dipping in the hot springs helps them keep warm during frigid winters in northern Japan. But researchers did not have the data to back up this hypothesis. So, as James Gorman reports for the New York Times, scientists at Kyoto University recently set out to study the snow monkeys of Jigokudani, to shed some light on why the critters enjoy chilling in hot springs.
Japanese macaques, also known as snow monkeys, are the most northerly species of non-human primates. Researchers believe that snow monkeys in Jigokudani maintain their body temperature by growing longer and thicker fur during winter. The group’s predilection for warm baths was first reported in the winter of 1963, when a juvenile female snow monkey was seen soaking in an outdoor hot spring that belonged to a hotel near Jigokudani. Pretty soon, other macaques were joining in on the fun. They disappeared when the weather started to warm up, only to return again during the following winter.
Due to hygiene concerns, the Jigokudani park decided to build the monkeys their very own hot spring, where the macaques now bathe regularly during the winter months. Working on the assumption that the monkeys use hot springs to keep warm, the Kyoto University team tested the animals’ feces for levels of glucocorticoids, a metabolite that is linked to biological stress, explains Rachael Rettner of Live Science. Previous studies have shown that cold temperatures cause glucocorticoids to increase in a number of primates—among them Japanese macaques.
Researchers tracked 12 adult females during the spring birth season (from April to June) and the winter mating season (from October to December). The results of their study, published recently in the journal Primate, showed that the snow monkeys used the hot springs more frequently during the winter. The team also found that the in the colder months, the macaques had lower levels of fecal glucocorticoids during weeks that they bathed, compared to weeks they did not.
Dominant females had higher stress levels than their subordinates, likely because they were involved in more aggressive conflicts. But the dominant snow monkeys also spent more time in the hot springs—a benefit of their high social rank—which was associated with lower fecal glucocorticoid concentrations.
“This indicates that, as in humans, the hot spring has a stress-reducing effect in snow monkeys,” Rafaela Takeshita, a primate researcher at Kyoto University and lead author of the new study, says in a statement. “This unique habit of hot spring bathing by snow monkeys illustrates how behavioral flexibility can help counter cold-climate stress.”
Unlike many humans, however, the monkeys did not seem to mind strangers getting up close and personal during their bath time. Some 500 visitors flock to Jigokudani every day to watch the monkeys kick back in the hot springs, but the researchers found that their presence seemed to have no effect on the animals’ stress hormones.