Unity Brings Mutual Benefits: The Huddling Strategies of Temperate-Living Primates in Cold Environments.

Behavioral thermoregulation is a key adaptive tactic shaped by natural selection and species evolution, enabling animals to cope with variable thermal environments, particularly facing cold challenges. Quantifying the thermal and energetic consequences of behavior in the wild provides key insights into how animals cope with environmental challenges. Our study examines huddling, a typical social thermoregulatory behavior in the golden snub-nosed monkey (Rhinopithecus roxellana), the northernmost distributed colobine primate, using behavioral observations and infrared thermography to assess its energetic benefits and social function. As environmental temperatures decrease, both the frequency and duration of huddling behaviors significantly increase. Notably, when temperatures fall below 5°C, the occurrence of larger huddles (comprising more than five individuals) increases, with a marked preference for the ground as the primary site for these aggregations. Individuals exhibit partner preferences in huddling. Furthermore, the monkeys in higher-ranking social units exhibited higher huddling frequency and longer durations, suggesting a link between social rank and huddling behavior. Thermal imaging results revealed 3.7°C escalation in average body surface temperature among huddling individuals compared to solitary counterparts, the principal contact areas (chest and abdomen), in particular, increased by 6.7°C. We calculated thermal dissipation in cold environments, demonstrating that huddling individuals experienced substantially reduced heat loss compared to solitary states: peripheral individuals reduced heat loss by 40.7%, while central individuals (primarily females and juveniles) achieved a dramatic 72.6% reduction. This study examines the social and ecological factors driving huddling behavior and quantifies its thermal benefits, advancing our understanding of thermoregulatory strategies in gregarious mammals facing cold environments.