Study Finds Monkey Huddles Affected by Ranking among Males

The Yomiuri Shimbun

17:21 JST, February 9, 2021

Dominant male Japanese macaques tend to benefit from the heat generated by monkey huddles in winter by having better access to thermally optimal positions, Kyoto University researcher Shintaro Ishizuka has found. He has recently released his findings in an electronic version of an international journal.

Ishizuka, working at the Primate Research Institute, Kyoto University, in Inuyama in Aichi prefecture, specializes in behavioral ecology and said that the results show how ranking may affect monkeys’ survival.

For many animals, including the Japanese macaque, protection from lower temperatures is important as colder conditions can lead to a decrease in reproductive function and other problems. Monkey huddles are thought to be for the purpose of maintaining warmth levels.

In December 2017, Ishizuka studied a free-ranging troop of about 150 monkeys, including six adult males, at the Choshikei Shizen Dobutsukoen “Osaru no Kuni” (Choshikei Nature Zoo “Monkey Country”) on Shodoshima island in Tonosho, Kagawa Prefecture.

He confirmed 56 large monkey huddles comprised of 21 monkeys or more and analyzed photos of them.

Higher-ranking males occupied non-peripheral positions more often than lower-ranking males in large huddles, with the alpha macaque occupying an inner position of the huddles 29 out of 46 times — accounting for about 60% of the huddles Ishizuka observed.

The second-ranked monkey occupied the inner positions 5 out of 12 times, or 40% of the huddles.

Ishizuka also confirmed that the higher a macaque ranked in the group tended to relate to more monkeys that would huddle around it.

“The ranking of males has been thought to be linked to their ability to obtain food and to mate preferentially with females,” said Chukyo University Prof. Hideshi Ogawa, who specializes in primatology. “This is a very interesting finding because it shows that ranking also affects monkey huddling aimed at keeping them warm.”