Inside the Science of Japan's Snow Monkey Spa

The photographs are irresistible: Japanese macaques sit shoulder-deep in steaming water while snow collects around Jigokudani in Nagano. The scene looks like a winter holiday, but researchers have found that hot spring bathing serves a practical biological purpose. [1][2]

The behavior began relatively recently. Kyoto University reports that a young female macaque entered a hotel hot spring in 1963, and other monkeys copied her. The park later built a pool specifically for the animals. By 2003, about one in three females in the group regularly bathed during winter. [1][3]

To test the benefits, a 2018 study followed 12 adult females during spring and winter. The team recorded how often each animal bathed and collected fecal samples that allowed them to estimate levels of glucocorticoid metabolites, which are used as indicators of physiological stress. [1][2]

The pattern was strongly seasonal. Monkeys entered the hot spring more often during winter and especially during colder weeks. Bathing reduces heat loss, so the animals can conserve energy that would otherwise be spent maintaining body temperature. [1]

The researchers also found that bathing was associated with lower stress-hormone levels. Social rank complicated the picture: dominant females gained more access to the pool, but their status also involved more aggressive encounters and higher energy costs. [1][2]

Importantly, the hundreds of tourists watching the monkeys each day did not appear to raise the measured stress levels. The study therefore points more strongly toward cold weather and social life as important pressures in this group. [1]

Hot spring bathing is a good example of behavioral flexibility. The macaques did not evolve a new organ for winter; they learned to use a feature of their environment. A habit that began with one adventurous monkey became a practical winter strategy for part of the group.