Japanese ‘Snow’ Monkeys. After bathing in a hot spring, how much heat do they lose?

In the days before central heating who can remember having to get out of a warm bath into the cold air of a bathroom in winter? It was a case of getting dry and into clothes as quickly as possible. That thought never left me as I watched the Japanese Macaques (Macaca fuscata) sitting for several hours at a time in the hot spring of Jigokudani Monkey Park in Nagano, Japan. I was reminded again when I looked through the video and photographs we had taken in 2009. We wondered at first if the thick coats of these monkeys did not wet easily and water therefore did not penetrate to the skin. However, we saw—and this can be seen in the video where there is grooming going on—that their coat is absolutely saturated. They really do get soaked to the skin.

In the depths of the Japanese winter it is easy to see that monkeys would save a considerable amount of energy needed to keep warm by sitting in volcanically warmed hot spring or onsen for part of the day. But even with a good shake after emerging from their soak in the onsen, there seemed a lot of water left to evaporate and therefore cool the body in air temperatures well below freezing.

Regardless of the length of time in the water, the cost of keeping the body warm after coming out into the cold air in terms of energy consumption would be the same. Therefore, in order to ensure that the bathing experience does not result in a net energy loss, one might expect that monkeys would stay in the hot water for relatively long periods, and that is what they do. Some sit, sometimes with their eyes closed, with their heads on a surrounding rock; others groom or are groomed while some dive to the bottom to retrieve grains of corn (which looked like wheat) that are scattered throughout the park during the day.

While I have seen research on thermoregulation in Japanese monkeys I have seen no studies or calculations on their energy balance as a result of their getting warm in onsen and then getting out with saturated thick fur, although potential problem was raised in the 1970s. The question is important because of the demonstration that the conception rate of females is higher in those with higher energy reserves and body fat. Any mechanism to reduce energy expenditure in the cold will be of advantage so there must be some sort of balance being achieved between time spent foraging and time spent immersed in hot water—and of course the downside of emerging sodden into cold air. I have been unable to find out if bathing is related to supplementary feeding in the park. A well-fed monkey that does not have to forage for so long could be one that can take time out in an onsen and thus help its energy balance.

The importance of bodily condition for reproduction in female Japanese Macaques is apparently reflected in the use of the onsen: it is mainly dominant females (who defend the pool) and they pass the habit to their daughters. Subordinate females rarely get the chance. Some adult females spend up to 10 hours in the water and some have been seen to stay there overnight. The young (who would get even colder with their relatively large surface area after emerging sodden) rarely enter the water voluntarily, although as the last sequence in the video shows, do sometimes get very wet when trying to reach their mothers in the pool.

The bathing behaviour is said to have been first seen in the early 1960s with a female immersing itself to reach some floating beans. I have not been able to find out whether bathing is confined to the one troupe which, as I noted above, receives supplementary food, or if the behaviour has been observed elsewhere where the range of the monkeys and hot springs coincide.

Here is the video from 2009:

Garcia C, Huffman MA, Shimizu K, Speakman JR. 2011. Energetic consequences of seasonal breeding in female Japanese Macaques (Macaca fuscata). American Journal of Physical Anthropology 146, 161-170.

Zhang P, Watanabe K, Eishi T. 2007. The habit of hot-spring bath in a free ranging group of Japanese macaque in the Jigokudani, Nagano Prefecture. American Journal of Primatology 69, 1425-1430.