Hot spring soaking reshapes parasite and microbe balance in Japanese macaques

by Riko Seibo
Tokyo, Japan (SPX) Jan 21, 2026

Japanese macaques, often called snow monkeys, are well known for soaking in hot springs during harsh winters in central Japan. Researchers at Kyoto University have now shown that this behavior not only helps macaques stay warm but also subtly reshapes their relationships with parasites and gut microbes.

A team led by first author Abdullah Langgeng conducted a two-winter field study at Jigokudani Snow Monkey Park in Nagano prefecture, focusing on a group of free-ranging female macaques. The researchers compared individuals that regularly used hot springs with others that did not, combining behavioral observations, estimates of lice load via grooming and nit-picking behavior, monitoring of intestinal parasites, and sequencing of gut microbiome samples.

The study tested whether hot spring bathing influences the macaque holobiont, an integrated biological system consisting of the host and its associated microbes and parasites. The results indicated that bathing females showed altered lice distributions and differences in gut bacterial composition relative to non-bathers. The team suggests that repeated soaking may disrupt louse activity or egg placement on the fur, changing how ectoparasites persist on frequently bathing animals.

Gut microbiome analyses revealed that overall microbial diversity in fecal samples was similar between bathing and non-bathing macaques. However, several bacterial genera were more abundant in non-bathers, indicating that hot spring use is associated with selective shifts in gut microbial communities rather than wholesale restructuring. Despite the potential for shared hot water to transmit intestinal parasites, bathing macaques did not exhibit higher parasite infection rates or intensities than individuals that avoided the springs.

Taken together, the findings show that behavior can act as a driver of holobiont structure and animal health, even when the effects appear subtle at first glance. The results illustrate that a single, conspicuous behavior like hot spring soaking can influence certain host-organism relationships, such as lice and specific gut bacterial taxa, while leaving other components of the parasite and microbiome assemblage largely unchanged.

"Hot spring bathing is one of the most unusual behaviors seen in nonhuman primates," says Langgeng. The team emphasizes that such behavior should not only be viewed as a passive response to environmental conditions. "Behavior is often treated as a response to the environment," Langgeng adds, "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 work is among the first to link a naturally occurring behavior in wild primates to changes in both ectoparasites and the gut microbiome. By demonstrating that behavior can selectively shape different components of the holobiont, the study has implications for understanding how health-related behaviors evolve and how scientists interpret microbiome variation in social animals that share space, water, and close contact.

The researchers also draw parallels to humans, where cultural practices such as bathing and communal bathing influence patterns of microbial exposure. Their results challenge the assumption that shared water sources necessarily increase disease risk under natural conditions, at least for the parasites and microbes examined in this monkey population. Instead, hot spring use by Japanese macaques appears to be compatible with maintaining parasite levels while modestly restructuring microbe-host interactions in ways that may benefit or fine-tune host health.

Research Report:Of hot springs and holobionts: linking hot spring bathing behavior, parasitism, and gut microbiome in Japanese macaques