KYOTO -- A researcher at Kyoto University in western Japan has found that monkeys dominant in their group may get more warmth than others when huddling in the cold.
Shintaro Ishizuka, a member of Kyoto University's Primate Research Institute specializing in behavioral ecology, observed Japanese macaques huddling to ward off the cold, and found that male dominant macaques took a position inside the huddle. Japanese macaques are known to have strict hierarchical systems.
"Learning the merits and demerits of hierarchies deepens our understanding of how animals live in groups," Ishizuka said.
Ishizuka conducted his research on one of the two Japanese macaque groups at Choshikei Monkey Park on Shodo Island in Tonosho, Kagawa Prefecture, in western Japan, in December 2017. In this group, consisting of about 150 monkeys at the time, there were six male adults whose hierarchy was already known. Ishizuka took pictures of huddles every five minutes, and studied the positions of males in the huddles and the numbers of monkeys they were in contact with.
Analysis of 100 photographs revealed that the highest-ranking male was in contact with about 5.5 macaques on average, followed by other males who were in contact with some 3.2 to 3.6 monkeys on average. The higher the ranking, the more individuals they were in contact with. Furthermore, observation of 56 huddles consisting of 21 or more monkeys each revealed that the higher the ranking a male had, the less often he took a peripheral position in the huddle. The highest-ranking male was also seen forcing his way into the middle of the huddle.
Ishizuka is considering analyzing females' behavior and taking temperature measurements using a small thermographic camera to see how much warmth the macaques actually get.
His study was published in the British online journal "Behavioural Processes."
(Japanese original by Satoshi Fukutomi, Kyoto Bureau)