I studied huddling groups of Japanese macaques (Macaca fuscata) in the Arashiyama E troop at the “Arashiyama Monkey Park Iwatayama” in Kyoto, central Japan. Japanese macaques made physical contact with other individuals and formed huddling groups when air temperatures were low. The 99-101 adult females and 26-36 adult males in the study troop formed 345 huddling groups during 42 scan samplings in the winter of 2001, and 376 huddling groups during 52 scan samplings in the winter of 2002. The average size of huddling groups was 2.34 (range: 2-7) individuals in 2001, and 2.31 (range: 2-6) individuals in 2002. There was no huddling group of two males. Females more frequently huddled with females than with males. Two maternal kin related females huddled more frequently than unrelated females did. Mother-daughter pairs huddled most frequently. Two individuals usually huddled ventrally-ventrally, ventrally-laterally, and ventrally-dorsally. The distribution of huddling group sizes shows that the approaching individuals did not choose a particular size of huddling. However, the approaching individuals chose locations where they simultaneously contacted with two individuals 1.5 times more frequently than locations where they contacted with only one individual. This choice made the shape of huddling groups triangular and diamond-shaped more frequently than expected. By decision making of each individual, specific patterns emerged in the shape, composition, and position of each individual in huddling groups. As well as huddling behaviors, two and more primate individuals were involved in various social interactions. During the interactions, primates make their decision based on complex cognitive mechanisms and non-linear functions, compete and cooperate with the same opponents in their troop, and predict and manipulate the opponent’s behavior. These traits in social interactions among primates might make their society more complex and interesting.

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