Hot Spring Bathing Doesn’t Just Keep Japanese Monkeys Warm

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LLike humans, some other primates like to snuggle up in hot springs. Japanese macaques (Macaca fuscata), also called “snow monkeys,” gather in natural hot springs to bathe together for hours. They prefer smoldering temperatures of around 106 degrees Fahrenheit which contrast sharply with the air temperatures in their winter mountain habitats.

Humans recognize the warming and stress-relieving benefits of soaking, but scientists question the implications on parasite load. On the one hand, hot springs can expose bathers to pathogenic bacteria, such as Acanthamoeba spp. which cause encephalitis. On the other hand, hot water can kill lice or strengthen the immune system to better fight pathogens.

A study published in Primates Researchers from Kyoto University, led by a wildlife conservation biologist from the Wilder Institute in Canada, studied the parasite loads of macaques at the Jigokudani Monkey Park in Nagano, Japan. Researchers collected fecal samples from 16 adult females (9 bathers and 7 non-bathers) to compare their internal parasite loads. Observations of their stinging behavior served as an indicator of external lice load.

Read more: »How a hurricane brought the monkeys together»

Overall, the abundance of intestinal parasites, which included protozoa and helminth worms, was similar between bathing and non-bathing macaques. But four types of bacteria (Methanobrevibacterium, Granulacatelle, FusobacteriumAnd Acinetobacteria) were significantly more prevalent in monkeys that chose not to soak. As for lice, whether the macaques bathed or not, they picked up more nits on their upper bodies than on their lower bodies. But, for swimmers, this difference was more pronounced, suggesting that the hot water could have brought on the lice.

Crucially, although hot spring soaking did not statistically change the likelihood of parasite infection, it appeared to cause changes in the relative abundance of internal and external parasites.

“Behavior is often treated as a response to the environment,” explained first author and Kyoto University wildlife biologist Abdullah Langgeng, “but our results show that this behavior doesn’t just affect thermoregulation or stress: it also changes the way macaques interact with the parasites and microbes that live on and inside them.”

At least in these natural hot springs, some of us may be relieved that communal baths don’t increase the risk of illness.

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Main image: AarenChenPS2 / Shutterstock

• Devin Reese

  Published on January 27, 2026

  Devin Reese is the editor-in-chief of Natural history and a science writer based in Alexandria, Virginia.