Hot spring bathing doesn't just keep snow monkeys warm

Kyoto, Japan -- Japanese macaques, colloquially referred to as snow monkeys, famously soak in steaming hot springs during winter. It's easy to see that this helps them stay warm in cold temperatures, but a team of researchers at Kyoto University recently discovered that this iconic behavior does more than keep the monkeys warm.

"Hot spring bathing is one of the most unusual behaviors seen in nonhuman primates," says first author Abdullah Langgeng. The researchers suspected that bathing may play a significant role in influencing the macaques' associated parasites and microbial communities.

To investigate, the team headed to Jigokudani Snow Monkey Park in Nagano prefecture. Across two winters, the researchers followed a group of female macaques, comparing individuals that regularly bathed in hot springs with those that did not. By combining behavioral observations, parasite monitoring, and gut microbiome sequencing, the team tested whether bathing influences the macaque holobiont, an integrated biological system consisting of the host and its associated microbes and parasites.

The results revealed that hot spring bathing subtly reshapes the monkeys' relationships with parasites and gut microbes. Macaques that bathe showed altered lice distributions and gut bacteria, suggesting that soaking may disrupt louse activity or egg placement.

The team also observed subtle shifts in gut microbes. Overall microbiome diversity was similar between bathers and non-bathers, but several bacterial genera were more abundant in non-bathing individuals. And despite concerns that shared hot springs might increase exposure to intestinal parasites, bathing macaques did not show higher parasite infection rates or intensities.

Altogether, this study demonstrates how behavior can shape the animal holobiont and act as an important driver of animal health. It also underscores the complexity of behavior-health links in wild animals, suggesting that hot spring bathing influences some host-organism relationships while leaving others unchanged.

"Behavior is often treated as a response to the environment," says Langgeng, "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 study is among the first to link a natural animal behavior to changes in both ectoparasites and the gut microbiome in a wild primate. By showing that behavior can selectively shape components of the holobiont, the research has implications for understanding the evolution of animal behaviors that influence health, and for interpreting microbiome variation in social animals.

Beyond that, this study draws parallels to how human cultural practices such as bathing affect microbial exposure, and thus also challenges the assumption that shared water sources necessarily increase disease risk, at least under natural conditions.

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The paper "Of hot springs and holobionts: Linking hot spring bathing behavior, parasitism, and gut microbiome in Japanese macaques" appeared on 19 January 2026 in Primates, with doi: 10.1007/s10329-025-01234-z

About Kyoto University

Kyoto University is one of Japan and Asia's premier research institutions, founded in 1897 and responsible for producing numerous Nobel laureates and winners of other prestigious international prizes. A broad curriculum across the arts and sciences at undergraduate and graduate levels complements several research centers, facilities, and offices around Japan and the world. For more information, please see: http://www.kyoto-u.ac.jp/en