Primates census with thermal drones: A New Horizon?

Methods of monitoring threatened primate species with minimal disturbance and reduced
human-primate interfaces are becoming outstanding requirements in the post-Covid era. Thermal Infrared imaging (TIR) with Uncrewed Aerial Vehicles (UAVs) or “drones” are a promising tool to detect and count primate species, but systematic validation is still needed to explore their full potential. We aim to evaluate the feasibility of using drones fitted with thermal sensors to conduct primate census and to test reliable methods to monitor diurnal primate communities in Cát Tiên National Park, located in southern Vietnam. From January to February 2023, we combined diurnal ground-based surveys and thermal drone surveys at night over N=29 1km line-transects (covering an area of 100km²). So far, We conducted N= 80 ground-based surveys and N=61 thermal drone surveys and between both surveys detected six species including Pygathrix nigripes, Trachypithecus margarita, Nomascus gabriellae, Macaca fascicularis, Macaca leonina, and Macaca arctoides. During this time, new sub-population locations were identified of several species. By analyzing their thermal and physical signature (including temperature, body size, and shape) and sleeping behavior, we were able to differentiate between various species. Our preliminary results suggest that thermal imagery with drone can accurately identify and count the critically endangered Pygathrix nigripes without the need for ground-truthing. This method could be particularly useful for long-term monitoring of large-bodied and highly arboreal species such as douc langurs. To estimate the abundance and distribution of other primate species, complementary methods such as camera traps and passive acoustic monitoring are recommended.