Utilizing thermal imaging drones to investigate sleeping site selection in an arboreal primate

Sleeping site selection is essential for understanding primate behavioral ecology and survival. Identifying where species sleep helps determine priority areas and critical resources for targeted conservation efforts. However, observing sleeping sites at night is challenging, especially for species sensitive to human disturbance. Thermal infrared imaging (TIR) with drones is increasingly used for detecting and counting primates, yet it has not been utilized to investigate ecological strategies. This study investigates the sleeping site selection of the Critically Endangered black-shanked douc langur (Pygathrix nigripes) in Cát Tiên National Park, Vietnam. Our aim is to assess the feasibility of using a TIR drone to test sleeping site selection strategies in non-nesting primates, specifically examining hypotheses related to predation avoidance and food proximity. Between January and April 2023, we conducted 120 drone flights along 22 transects (~1-km long) and identified 114 sleeping sites via thermal imaging. We established 116 forest structure plots along 29 transects in non-selected sites and 65 plots within douc langur sleeping sites. Our observations reveal that douc langurs selected tall and large trees that may provide protection against predators. Additionally, they selected sleeping sites with increased access to food, such as Afzelia xylocarpa, which serves as a preferred food source during the dry season. These results highlight the effective use of TIR drones for studying douc langur sleeping site selection with minimal disturbance. Besides offering valuable insights into habitat selection and behavioral ecology for conservation, TIR drones hold great promise for the noninvasive and long-term monitoring of large-bodied arboreal species.