Developing a new method using thermal drones for population surveys of the world's rarest great ape species, Pongo tapanuliensis

Accurate population estimates are crucial for conservation, but surveying arboreal species can be costly and challenging. Unoccupied aerial vehicles (a.k.a drones) equipped with thermal infrared (TIR) and visible spectrum (RGB) cameras are increasingly used to survey rare primates, though their accuracy still requires ground-truthing. This pilot study evaluates the effectiveness of drones with TIR and RGB sensors to detect and count Tapanuli orangutans in South Tapanuli, Sumatra, in an unprotected area classified as an “Area for Other Use”. 37 drone flights, including seven grid and 30 manual flights, were conducted between June 18 and July 1, 2023. Tapanuli orangutans displayed body temperatures of approximately 26°C on the TIR sensor and were prominent in the infrared imagery during the evening and morning when the forest canopy was cooler (between 10 and 16 °C), but became more challenging to distinguish by mid-morning when sunlight elevated the vegetation cover's temperature to over 30°C. Our study demonstrates that a drone with a TIR camera can accurately detect and count Tapanuli orangutans and other canopy animals in multiple land uses. There was strong agreement between ground and drone counts for Tapanuli orangutans (6 vs 5 individuals), indicating no significant difference between these survey methods. Furthermore, the TIR drone footage allowed for the detection of additional individuals, owing to its broader coverage compared to ground surveys. Our comparative analyses illustrate how various drone methods can effectively detect Tapanuli orangutans and other canopy animals, with the potential to support fine-scale population abundance and habitat mapping through spatially explicit detections.