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Japanese macaques, known as snow monkeys, are celebrated for their habit of soaking in natural hot springs during bitterly cold winters. While this behavior clearly provides warmth, researchers have now investigated whether it offers other physiological benefits. In 2026, a team from Kyoto University announced findings that the hot spring baths influence not only the monkeys themselves but also the parasites living on them and the microscopic communities within their digestive systems. These factors are critically linked to individual health and immune function.
The research was officially published in the peer-reviewed scientific journal Primates on January 19, 2026.
Scientists studied a group of female Japanese macaques at Jigokudani Wild Snow Monkey Park across two winter seasons. They carefully compared individuals that bathed regularly with those that rarely or never entered the hot springs. The investigation went beyond observing the monkeys' external behavior; it also examined the tiny creatures living on and inside them. The team documented bathing frequency, checked for external parasites like lice, and conducted detailed genetic analysis of the bacterial communities, or microbiome, within the monkeys' intestines.
Researchers employ the term holobiont to describe an animal and the complete collection of microorganisms and parasites that coexist with it. This concept frames each monkey not as a solitary organism, but as a complex, integrated living ecosystem.
The study's results revealed small but potentially meaningful biological differences between the two groups. Monkeys that regularly used the hot springs exhibited altered distributions of lice across their bodies. This suggests that immersion in warm water may disrupt the normal activity or life cycle of these external parasites. Internally, the overall diversity of gut bacteria was generally similar for both bathing and non-bathing monkeys. However, the relative abundance of certain specific bacterial types varied. Notably, some bacterial groups were more commonly found in monkeys that did not bathe.
A particularly surprising discovery was that the communal use of the hot springs did not lead to a higher prevalence of parasite transmission. Bathing monkeys did not carry a greater parasite load than their non-bathing counterparts, challenging a common assumption about shared water sources.
These findings illustrate that an animal's instinctive or learned behaviors can impact its health in nuanced ways, extending beyond their primary, obvious purpose. For snow monkeys, hot spring bathing serves more than a thermoregulatory function; it subtly alters the ecological dynamics of the parasites and microbes inhabiting their bodies. Even minor shifts in these microscopic communities can have cascading effects on digestion, immunity, and overall physiological state. Lead researcher Abdullah Langgeng of Kyoto University emphasized the broader implications of their work.
Behavior is frequently interpreted as a basic response to environmental conditions, but our results demonstrate that this particular behavior does not merely affect temperature regulation or stress reduction. It also modifies how the macaques interact with the parasitic and microbial organisms that live on and within them.
The study further suggests that, by extension, behavioral and cultural practices in other species—including human customs like communal bathing or sauna use—may similarly shape our exposure to and relationships with microorganisms. It contests the simplistic notion that sharing water necessarily elevates disease risk, at least within certain natural contexts.
Snow monkeys reside in large, multi-generational social groups and are the only wild primates documented to habitually use geothermal hot springs. At Jigokudani, the monkeys traverse deep snow to access open-air pools where water temperatures can approach 104° Fahrenheit (40° Celsius). Given that winter air temperatures frequently fall below freezing, these springs provide a vital and highly attractive source of warmth. As Langgeng noted, this behavior is exceptional in the animal kingdom.
Hot spring bathing represents one of the most distinctive and unusual behaviors observed in non-human primates.
This habit is not innate but is socially learned through observation and imitation. Field studies consistently show that young monkeys closely watch and mimic the actions of older group members, particularly their mothers. Participation in bathing is also influenced by age, sex, and social hierarchy within the troop. Adult females and juvenile monkeys typically bathe most often. Conversely, adult males and lower-ranking individuals tend to bathe less frequently, often remaining on the periphery or on dry land. Because the hot spring pools have limited capacity, dominant individuals can monopolize the prime soaking spots, forcing others to wait or forgo the experience entirely.
The now-famous tradition of snow monkeys bathing in hot springs originated unexpectedly in the early 1960s. A young monkey at Korakukan, a traditional Japanese inn located in the Jigokudani Valley, was observed reaching into an outdoor bath used by human guests. The monkey was likely attracted by food scraps that had fallen into the water. Other monkeys soon began to copy this novel behavior and ventured into the springs themselves. They quickly discovered the exceptional comfort of the warm water during the harsh winter months. Over time, more members of the troop adopted the practice, transforming it into a stable social custom. The behavior became so consistent that in 1964, a dedicated hot spring pool was constructed specifically for the monkeys. This area is now internationally recognized as Jigokudani Monkey Park.
A logical question arises: do the monkeys experience a dangerous chill when they emerge from the hot water into freezing air? Unlike humans, who rely on sweat evaporation for cooling and can suffer rapid heat loss in cold conditions, snow monkeys possess very few functional sweat glands and do not utilize sweating as a primary cooling mechanism. This physiological difference means they avoid the sudden, drastic drop in core body temperature that a wet human might experience.
Their dense, layered winter fur provides superb insulation, trapping warm air close to the skin. Furthermore, their bodies are exquisitely adapted to extreme cold. Blood vessels in their extremities, such as hands and feet, constrict significantly in low temperatures. This vasoconstriction reduces blood flow to the surface, minimizing heat loss from these vulnerable areas. These combined adaptations enable snow monkeys to walk barefoot on snow or exit a hot spring without a substantial risk of frostbite or acute thermal shock.
Thus, while humans scientifically study and debate the therapeutic benefits of hot baths and spa therapies, snow monkeys appear to have stumbled upon some advantages through natural behavior and social learning. For these primates, a soak in a geothermal spring is not merely a strategy for thermoregulation. It is an integral component of a lifestyle that, through subtle interactions with their internal and external ecosystems, helps shape their collective and individual health with each steamy immersion.
In summary, snow monkeys utilize hot spring baths primarily for warmth during winter. Emerging scientific research indicates these baths also produce measurable effects on their parasite loads and gut microbial communities. It is a comforting social behavior that may confer additional, hidden health benefits.