Japanese macaques occupy snow country where winter temperatures fall well below freezing, and their dense winter coat does most of the thermal work. Hot-spring bathing is not general to the species. It is confined largely to the troop at Jigokudani in Nagano Prefecture, where records trace the behaviour to a juvenile female who entered a hotel pool around 1963 and to the troop members who copied her.
Measurements at the site indicate that bathing does lower stress hormone levels in winter, so the habit carries a physiological benefit once acquired. The pattern of spread, from young individuals outwards through kin and playmates, matches other documented macaque traditions such as the potato-washing described on Koshima Island.
The often-repeated claim that the monkeys taught themselves to survive the cold this way overstates it. The population survived Japanese winters long before any hot-spring pool was accessible, and most Japanese macaques never bathe at all.