Japanese macaques are famous for soaking in hot springs during winter, but new research suggests this behavior may serve purposes beyond staying warm.
Japanese macaques, commonly known as snow monkeys, have become famous for their winter gatherings in natural hot springs. The image of monkeys relaxing in steaming water is often seen as a simple response to harsh cold, but new findings from Kyoto University suggest this seasonal habit has broader biological effects that extend beneath the surface.
“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.
Studying Macaques in the Wild
To test this idea under natural conditions, the team carried out long-term observations at Jigokudani Snow Monkey Park in Nagano prefecture. Over two consecutive winters, they monitored a group of female macaques, noting which individuals regularly entered the hot springs and which consistently stayed out.
These behavioral records were paired with parasite assessments and gut microbiome sequencing, allowing the researchers to examine whether bathing altered the macaque holobiont (an integrated biological system consisting of the host and its associated microbes and parasites).
The analysis revealed that hot spring use was linked to subtle but measurable biological differences. Macaques that bathed showed changes in where lice were found on their bodies, as well as differences in certain gut bacteria, suggesting that warm water exposure may disrupt parasite activity or affect where lice lay their eggs. Together, the findings point to a more complex role for bathing behavior, one that influences how animals interact with the microscopic life that surrounds and inhabits them.
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.
No Increase in Infection Risk
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.
Reference: “Of hot springs and holobionts: linking hot spring bathing behavior, parasitism, and gut microbiome in Japanese macaques” by Abdullah Langgeng, Wanyi Lee, Goro Hanya, Munehiro Okamoto and Andrew J. J. MacIntosh, 19 January 2026, Primates.
DOI: 10.1007/s10329-025-01234-z
Funding: Japan Society for the Promotion of Science
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