Most monkeys live in warm rainforests. Japanese macaques live in mountain forests where winter brings deep snow and freezing wind. They have thick brown fur that grows thicker in winter, and pink faces that turn redder in the cold. A grown-up macaque is about the size of a four-year-old child.
In one valley in central Japan, called Jigokudani, the monkeys discovered something special. The local hot springs - bath-warm water bubbling up from underground - feel amazing on a freezing day. So they climb in. The babies stay snug on their mums' backs, just their faces poking out of the steam.
Macaques live in big family groups of up to 100 monkeys. They look after each other in clever ways: older sisters help mind the babies, and friends spend hours 'grooming' each other - carefully combing through fur with their fingers to clean it. It is also how they show they like each other.
Japanese macaques are very smart. In one famous group, a young female called Imo learned to wash dirty sweet potatoes in the sea before eating them. Other monkeys watched her, copied her, and now the whole group does it. Scientists think Imo's idea spread the way a song or a joke spreads in a school.