Japanese snow monkeys get more than just relief from hot springs
When temperatures drop and snow falls, it’s understandable to envy a snow monkey bathing in a steaming hot spring. Officially called Japanese macaques (Macaca fuscata), primates are well known for taking advantage of warm waters during snowy winters. Although the warm water helps keep their bodies warm in parts of Japan that can be covered in feet of snow for months, this unique behavior may have more to do with what is seen.
“Hot spring bathing is one of the most unusual behaviors observed in non-human primates,” said Abdullah Langgeng, a Ph.D. student at Kyoto University.
Unusual and also potentially beneficial. Such bathing may influence macaques’ intestinal parasites and microbes, according to a study co-authored by Langgeng and recently published in the journal Primates.
Japanese macaques live on three of Japan’s four main islands (Honshu, Shikoku, and Kyushu), as well as the smaller islands of Awaji, Shodo, Yakushima, Kinkazan, and Kojim. Their northern limit is at the tip of the island of Honshu. More than a meter of snow can cover parts of this area for several months of the year and temperatures can reach -4 degrees Fahrenheit. The snow monkeys that live here are considered the northernmost wild populations of non-human primates in the world.
As part of the study, the team traveled to the Jigokudani Snow Monkey Park in Nagano Prefecture, central Japan, on the island of Honshu. For two winters, scientists followed a group of female macaques, comparing those that regularly bathed in hot springs with those that did not. They also collected fecal samples to monitor the monkeys for parasites and performed genetic sequencing of organisms in their gut microbiome.
The combination of observations and tests helped the team test whether or not bathing influences the macaque holobiont, a biological system consisting of a host and all of the parasites and microbes associated with it.
They found that bathing in hot springs subtly reshapes snow monkeys’ relationships with their parasites and gut microbes. Bathing macaques had different distributions of lice and gut bacteria. Soaking in warm water can disrupt lice activity or egg placement in their fur.
As for gut microbes, the team observed similar subtle changes. Overall microbiome diversity was similar between bathers and non-bathers. However, several types of bacteria were more abundant in monkeys that didn’t bathe. Despite concerns that shared hot springs could increase exposure to intestinal parasites, bathing macaques did not exhibit higher rates or intensities of parasite infection than those that remained out of the water.
According to the team, these findings demonstrate how behavior can shape the parasites and microbes living on and inside an organism and is an important driver of animal health. It also highlights how complex behavior-health links are in wild animals, suggesting that hot spring bathing influences a few host-organism relationships, but not others.
“Behavior is often treated as a response to the environment,” Langgeng added. “But our results show that this behavior doesn’t just affect thermoregulation or stress: it also changes how macaques interact with parasites and microbes that live on and inside them.”
Showing that animal behavior can selectively determine which types of microorganisms live on their fur or in their guts can help researchers understand how actions that influence animal health evolved. It may also help us better interpret changes in the microbiome of social animals.
Although more research is needed, it also shows parallels with how human cultural practices such as bathing can affect microbial exposure. Although drinking water is essential for health, this work challenges the assumption that sharing water sources in natural conditions, such as hot springs, increases the risk of disease.
The best new PopSci 2025 releases
