Do Communal Baths Not Increase Infections? The New Hygiene Norms Suggested by Japanese Macaque Hot Spring Bathing

The Charm of "Hot Spring Monkeys" Is Not Just Their Cuteness

Japanese macaques soaking up to their shoulders in the steam of hot springs are globally known as a winter spectacle. However, it seems their "bathing culture" offers more than just maintaining body temperature and reducing stress. A research team from Kyoto University has shown that hot spring bathing can also affect the monkeys' external parasites (lice) and gut microbiota (References 1-4).

What's interesting here is that the intuitive thought of "hot springs = everyone in the same bath = increased infection" didn't necessarily hold true. Instead of increasing "certain risks," lice distribution changed, and there were subtle differences in gut bacteria among individuals that bathed in hot springs.

Keyword: "Holobiont" — The "Team" of Host and Co-inhabitants

The concept central to the research team's study is "holobiont." This perspective views an animal's body not as a standalone entity but as an "ecosystem" functioning together with microorganisms living on the skin and in the gut, as well as external and internal parasites (References 1-4).

In other words, the act of bathing or not bathing in hot springs is not merely a lifestyle difference but potentially alters the way of assembling the health state, including the "co-inhabitants" inside and outside the body.

What Was Investigated: Tracking 16 Adult Females Over Two Winters in Jigokudani

The research was conducted at Jigokudani Monkey Park in Nagano Prefecture from December 2019 to March 2021, targeting 16 adult females (9 bathing / 7 non-bathing) (References 2-3).

The important point here is that the research aimed to see "what changes occur from bathing in hot springs itself." It combined observations with parasite monitoring and gut microbiota analysis using fecal samples to simultaneously compare behavior, parasitism, and microorganisms (References 1-4).

Result 1: Lice "Did Not Decrease" But "Changed Distribution"

The study used the frequency of "nit-picking," a type of grooming behavior, as an estimated indicator of lice load (References 1-4). It suggested that estimated lice load differed between the "water-immersed parts" and "non-immersed parts" of the body in bathing and non-bathing individuals, indicating a change in lice distribution on the body surface (Reference 3).

This is a point that can be easily misunderstood. The study did not assert that "bathing in hot springs eliminates lice." Rather, it suggested the possibility that "where lice (or eggs) tend to attach" might fluctuate (References 1-4). The research team indicated that immersion in hot springs might disrupt lice activities or the attachment and distribution of eggs (References 1-2).

Result 2: Intestinal Parasites Did Not Increase Even in Communal Baths

The suspicion that "sharing the same bath might make intestinal parasites more transmissible" is natural. However, this study found no significant difference in infection probability or intensity regarding gastrointestinal parasites (at least several worms and protozoa were detected) (Reference 3).

At least "under natural conditions," the simple equation that sharing hot springs immediately increases parasite risk was not supported (References 1-4).

Result 3: Gut Bacteria Showed "Same Diversity, But Slightly Different Composition"

Regarding gut microbiota, while there was no significant difference in overall diversity (alpha and beta diversity), several genera were found to be more abundant in non-bathing individuals (four genera mentioned in the paper) (Reference 3).

Again, this does not mean "hot springs = dramatic change in the gut." The overall picture is similar, but the "members of the composition" slightly differ. The behavioral differences might selectively influence part of the gut ecosystem (References 1-4).

Why Both Body Surface and Gut Change in Hot Springs (Hypothesis)

It's intuitive that hot spring bathing could affect external parasites. Getting wet, warming up, and changes in hair condition alone could alter parasite behavior or egg attachment conditions. The research team also suggested the possibility of disrupting activity and oviposition sites (References 1-2).

On the other hand, changes in gut bacteria seem distant. However, behavioral changes intertwine with feeding behavior, stress, physiological state, and contact patterns with other individuals. It makes sense to think that bathing in hot springs is incorporated as part of the life history and social behavior, subtly altering the "conditions" of the gut environment—though distinguishing causality remains a future challenge, as noted in the paper (Reference 3).

"Culture" Creates Health: Unexpected Commonalities with Human Bathing Habits

The charm of this study lies in overturning the view that "behavior is merely a response to the environment." Behavior might have the power to "reorganize relationships" with organisms inside and outside the body.

Moreover, the possibility that communal baths do not necessarily increase infections challenges the intuition surrounding "bathing," "hygiene," and "shared spaces" in human society (References 1-2). Of course, directly applying this to human public health is risky, but the perspective of not immediately assuming "sharing = increased risk" holds research value.

Reactions on Social Media (Summary of "Trends in Posts")

※Since the Phys.org article page itself had no comments at the time of publication (Reference 1), here we organize it asreaction patterns likely to emerge when such topics spread on social media.

• "Hot springs are not just for warmth but also for 'insect control'?"
  The strong image of hot spring monkeys makes the element of "changing lice distribution" easily striking as a "surprise."

• "It's interesting that parasite risk doesn't increase in communal baths, contrary to intuition."
  A type of reaction where the assumption of "same bath = unsanitary" is shaken.

• "I've never heard of the word 'holobiont' before."
  A "discovery of new words" type of reaction that often occurs when biological concepts are introduced to the general public.

• "Hot spring monkeys, cute and scientifically fascinating."
  Due to the subject's inherent photo and video appeal, it tends to circulate more as an "interesting topic" rather than a scientific news story (Reference 2).

• "Maybe human 'bathing culture' also changes the microbial world."
  A pattern where thoughts leap to human habits (bathing, sauna, communal baths), leading to more discussion-oriented comments (References 1-2).

References

1. Phys.org: Overview of the study (hot spring bathing affects lice distribution and gut bacteria, no increase in parasite risk confirmed), research site (Jigokudani), publication date, DOI, reaction status on the article page (shares/comments display).
   https://phys.org/news/2026-01-hot-doesnt-monkeys-disrupt-lice.html

2. Kyoto University Research News (equivalent to a press release): Research aims, methodological framework (behavioral observation + parasites + gut microbiota analysis), significance of hot spring bathing (holobiont, shared water source = implications for increased infection).
   https://www.kyoto-u.ac.jp/en/research-news/2026-01-20

3. Primates (Springer Nature, original paper page): Research period (2019/12–2021/3), subjects (16 adult females, 9 bathing / 7 non-bathing), results (differences in estimated lice indicators, no difference in gastrointestinal parasites, no difference in gut bacterial diversity, presence of more abundant genera in non-bathing individuals).
   https://link.springer.com/article/10.1007/s10329-025-01234-z

4. EurekAlert! (Research news release): Summary of research findings for the general public (effects of hot spring bathing on parasites and gut bacteria, observational study, paper information, DOI).
   https://www.eurekalert.org/news-releases/1113222