Nighttime behaviors of white-headed langurs (Trachypithecus leucocephalus): The role of predation avoidance and thermoregulation

We collected nighttime behavioral data by observing a group of white-headed langurs (Trachypithecus leucocephalus), comprising 16 individuals in a limestone forest in Guangxi Chongzuo National Nature Reserve, China, from October 2016 to August 2017. We recorded various aspects of the langurs’ nighttime behavior, including the duration of entering/leaving sleeping sites, the duration of remaining in sleeping sites, the duration and frequency of movement, and the number and size of sleeping clusters in sleeping sites. The primary aim was to investigate thermoregulation strategy in these primates at night. By applying thermoregulation hypotheses, we aimed to elucidate the underlying factors driving seasonal changes in white-headed langurs’ nighttime behavior. All individuals spent 7.53 ± 1.16 min on entering sleeping sites, and spent 9.70 ± 3.05 min on leaving sleeping sites, which could be a response to predation risk. Movement throughout the night was observed in most individuals. Sleeping clusters were formed by white-headed langurs during the night, with larger clusters sizes occurring in the cold season (4.68 ± 0.69) compared with the hot season (3.59 ± 0.77). Additionally, langurs prolonged their stay in sleeping sites during the cold season (12.36 ± 0.63 h) compared with the hot season (11.20 ± 0.63 h), potentially as a thermoregulation strategy to combat low temperature. Our findings indicate that temperature, rainfall, day length and food availability influence the nighttime behavior of white-headed langurs, highlighting the importance of ecological variables in determining primate nocturnal behavioral responses. The observed changes in white-headed langurs’ nighttime behavior strongly support their inclination toward behavioral strategies that minimize thermoregulation costs in limestone forests.