Japanese snow monkeys use hot spring baths to rebalance internal and external microbial ecosystems

Kyoto University researchers find that Japanese macaques' hot spring bathing rebalances gut bacteria and lice patterns without increasing parasite risks.

By: AXL Media

Published: Mar 4, 2026, 9:04 AM EST

Source: The information in this article was sourced from Kyoto University

Investigation into the macaque holobiont

Researchers from Kyoto University traveled to the Nagano prefecture to investigate how thermal bathing impacts the holobiont, a term describing the collective biological system of an animal and its associated microscopic organisms. Over the course of two winter seasons, the team monitored a group of female Japanese macaques, categorizing them based on the frequency of their hot spring use. The study utilized a combination of direct behavioral observations, physical parasite checks, and advanced gut microbiome sequencing to map the relationship between this unique cultural behavior and the monkeys' biological health.

Impact of thermal immersion on ectoparasites

The data collected during the study showed that time spent in hot springs directly correlates with changes in the patterns of lice found on the monkeys' bodies. Researchers noted subtle but distinct differences in how lice interacted with frequent bathers compared to those that remained dry. It is theorized that the immersion in hot, mineral-rich water may interfere with the activity levels of the lice or specifically disrupt their egg-laying processes. This suggests that the hot springs may act as a natural, behavior-led method of controlling external parasite loads during the harsh winter months.

Microbial shifts and shared water safety

Analysis of the macaques' gut microbiomes revealed that while the overall diversity of bacteria remained similar across the group, specific bacterial genera were more prevalent in monkeys that avoided the hot springs. Surprisingly, the research challenged long-held assumptions regarding disease transmission in shared water sources. Despite the close proximity of the monkeys in the pools, those that bathed showed no increase in the rate or severity of intestinal parasite infections. This finding indicates that for these primates, the benefits of thermal bathing do not come with a significant cost of increased internal parasite transmission.