Simple Summary: Japanese macaques, also known as snow monkeys, are known for their ability to huddle together in cold weather to stay warm. However, the size of these huddling groups can vary greatly across different regions, with exceptionally large clusters observed on Shodoshima Island compared to smaller groups in colder areas. This study used computer simulations to understand the factors influencing these huddling behaviours. By modelling individual decisions to join or leave a huddle based on temperature, group size, and other simple rules, we found that environmental conditions and local social dynamics play key roles in determining huddle size. Larger groups tend to form more clusters, but individual preferences and social tolerance also shape these behaviours. Our findings highlight how simple behaviours at the individual level can lead to complex group patterns. Understanding these dynamics not only helps us learn more about how animals survive in extreme conditions but also provides insights into the social structures and adaptability of Japanese macaques, which could inform conservation strategies and studies on animal behaviour.