Monkey see, monkey do.
About 50 years ago, Japanese macaques watched humans bathing in a hot spring near an inn in the mountains. The monkeys decided to get out of the cold too, and eventually joined the humans. The monkeys, not being well potty-trained, started using the bathroom while soaking, much to the displeasure of the hotel guests. A new monkey-only pool was constructed, but in winter months when the hot water is in high demand, some of the macaques will come down and bathe with the people again.
In addition to being a source of heat, the hot springs have turned into social sites. The macaques spend a great deal of time relaxing, socializing, and grooming one another.
Photo credit: Asteiner