Japanese macaques, known as snow monkeys, bathe in hot springs not only for warmth but also to influence their internal ecosystems. Researchers at Kyoto University discovered that regular bathing leads to differences in lice patterns and certain gut bacteria among the monkeys. Importantly, sharing the pools does not increase parasite infection risks.
Japanese macaques, commonly called snow monkeys, are renowned for their habit of soaking in hot springs during winter in Japan's Nagano prefecture. A study conducted by researchers from Kyoto University at Jigokudani Snow Monkey Park over two winters examined how this behavior affects the monkeys' parasites and gut microbiome.
The team, led by first author Abdullah Langgeng, observed a group of female macaques, comparing those who frequently bathed with those who rarely or never did. They used behavioral observations, parasite examinations, and gut microbiome sequencing to assess the macaque holobiont—the combined system of the animal and its associated microbes and parasites.
Results revealed that bathing monkeys exhibited different lice patterns on their bodies compared to non-bathers, possibly due to hot water interfering with lice activity or egg-laying sites. In terms of gut bacteria, overall diversity remained similar between the groups, but certain bacterial genera were more prevalent in monkeys that avoided the springs.
Notably, the study found no elevated risk of intestinal parasites among bathers, countering expectations that shared water might spread infections. Infection rates and severity showed no increase in those using the hot springs.
"Hot spring bathing is one of the most unusual behaviors seen in nonhuman primates," Langgeng stated. He added, "Behavior is often treated as a response to the environment, but our results show that this behavior doesn't just affect thermoregulation or stress: it also alters how macaques interact with parasites and microbes that live on and inside them."
This research, published in the journal Primates in 2026, is among the first to link a wild primate's natural behavior to changes in both external parasites and internal microbiomes. It suggests that such behaviors can selectively shape health-related elements in social animals and draws potential parallels to human practices like bathing, which may influence microbial exposure without necessarily heightening disease risks in natural settings.
