Sleeping cluster patterns and retiring behaviors during winter in a free-ranging band of the Sichuan snub-nosed monkey

Little information is available on the sleeping cluster pattern and retiring behavior of Sichuan snub-nosed monkeys (Rhinopithecus roxellana). Here, we provide observational data on a provisioned free-ranging band in the Qinling Mountains, central China. The results suggest that winter night activity of R. roxellana is a compromise between antipredator and thermoregulatory strategies and an adaptation to ecological conditions of their temperate habitat. Monkeys retired between 1804 and 1858 h in winter. In support of the antipredation hypothesis, all monkeys slept in trees at night, whereas 18.8% of individuals slept on the ground during the day. Also, the study band was more spatially cohesive at night than in daytime, with shorter distances between one-male units. Keeping warm is critical for survival in freezing temperatures. Monkeys often slept in the lower stratum of the tree canopy, avoiding the upper canopy where it is cold and windy. They formed larger sleeping clusters at night than in daytime. The most common types of night-sleeping clusters were adult females and juveniles, followed by adult females with other adult females. These accounted for 60.2% of the total records. The frequency of female-male clusters is two times greater, and that of adult male-juvenile clusters was four times less at night than during the day. The variations in composition of sleeping clusters suggest affiliative patterns at night-sleeping sites differ from those during the day.