© Heather Angel
This photograph of Japanese macaques relaxing in a hot bath was taken on a winter morning by wildlife photographer and zoologist Heather Angel. The pool is in the Joshinetsu Kogen National Park, Nagano, Japan, in a place known as Hell's valley because of its steep cliffs, dense forests and the boiling water that spurts from the frozen ground. It was built 30 years ago when it was discovered that the monkeys, like us, like to bathe in hot water.

Japanese macaques - also known as snow monkeys - live further north than any other non-human primate. Snow covers the ground in Hell's Valley for four months of the year and temperatures often drop to -20 °C. Thermal pools - both natural and man-made - help the monkeys survive the harsh climate.

Snow monkeys are not only renowned for their love of hot baths, but for using tools and washing their food. In 1953, an 18-month-old female called Imo was spotted washing sweet potatoes to remove the mud before eating them. Ten years later the rest of the troop were also doing this, and had passed on the skill to the next generation. It was the first report of a learned tradition in a non-human species.

Angel has spent many hours observing snow monkeys and has noticed some other interesting habits. Like human children, the juveniles tend to leap straight into the water, submerging their heads, while the older ones climb in sedately, keeping their heads dry. If you want to see this for yourself, the best time to visit is early June when the mothers are with their newborn babies.

Find out more about snow monkeys in our gallery.