Sleeping-Site Decisions in Tibetan Macaques: Social and Seasonal Drivers

Sleeping-site selection is a critical decision-making process in animals, influenced by evolutionary pressures. However, the key factors controlling this choice under group demography, and how these vary seasonally, remain poorly understood. This study investigated the selection of arboreal versus terrestrial sleeping sites and the underlying decision-making processes in a free-ranging group of Tibetan macaques (Macaca thibetana) at Huangshan, China, across mating (July-January) and non-mating (February-April) seasons. Generally, Tibetan macaques slept arboreally during mating season (July-September), switched to terrestrial sites afterward (October-March), and returned to trees in the following April. As for the decision-making, females consistently played a central role, initiating collective movements to arboreal sites more frequently and attracting more followers during the mating season, and joining the collective movements earlier across all seasons. Decision-making rules also varied seasonally. Females and middle-aged/older individuals mainly initiated movements to arboreal sites during the mating season, whereas terrestrial movements were led primarily by older individuals, with high social centrality attracting more followers. In the non-mating season, no specific social traits predicted initiators across sleeping sites, though females consistently attracted more followers and joined movements earlier. In terrestrial movements specifically, older individuals joined later, whereas highly socially central individuals joined earlier. Our research reveals that the sleeping-site selection decisions of Tibetan macaques are influenced by their matrilineal group structure. This study provides insights into the ecological adaptability of primates, demonstrating how dynamic decision-making supports survival in seasonal environments among social animals.