Handheld Thermal Devices Can Facilitate Population Monitoring of the Critically Endangered Delacour's Langur <scp> <i>Trachypithecus delacouri</i> </scp> in Difficult Terrains

The Delacour's langur (<i>Trachypithecus delacouri</i>) is a Critically Endangered primate, restricted to a small region in northern Vietnam. In view of its very small population and existing threats, frequent population monitoring programs are urgently needed for this species. In this study, we evaluated the utility of handheld thermal imaging devices as a complementary tool to conventional ground-based visual surveys for primate population monitoring efforts. Based on results of past studies, we conducted field surveys in Kim Bang Protection Forest, Ninh Binh Province, Vietnam, where the second most important population of the Delacour's langur inhabits. While we followed protocols from previous ground-based visual surveys, we also used thermal monoculars to facilitate langur detections. By integrating thermal handheld devices, we documented at least 18 langur groups with around 116 individuals, an increase of about 11.5% in total population size compared to the most recent extensive study at the same site. When comparing to the 2022 drone survey in Kim Bang, our results also showed that while the drone platform demonstrates superior performance, the integration of thermal imaging devices substantially reduces survey effort relative to conventional ground-based visual techniques. Given the recent regulations on flying drones in remote areas in Vietnam, our findings suggest that thermal imaging devices offer a viable option to improve the efficacy of ground-based primate population monitoring surveys. Furthermore, when properly deployed, handheld thermal devices may provide key advantages for certain primate research topics.