Effectiveness of thermal infrared drone surveys in detecting the diurnal primate community in Cat Tien National Park, South Vietnam

Thermal infrared (TIR) imaging and drone technology have recently emerged as valuable tools in wildlife monitoring, offering advantages over traditional survey methods. We conducted manual and systematic TIR drone surveys from April 2022 to May 2023 in Cat Tien National Park, Dong Nai Province, southern Vietnam, to assess their effectiveness in detecting and counting six diurnal sympatric primate species at their sleeping sites. Our study revealed successful detections of black-shanked douc langurs (Pygathrix nigripes), Annamese silvered langurs (Trachypithecus margarita), southern yellow cheeked gibbons (Nomascus gabriellae), long-tailed macaques (Macaca fascicularis), and pig-tailed
macaques (Macaca leonina). However, detection reliability varied among species, with arboreal langurs showing higher reliability compared to semi-terrestrial macaques or gibbons. Ecological and behavioral factors influenced species’ detection effectiveness, with large-bodied, arboreal species being more reliably detected. For instance, black-shanked douc langurs exhibited distinct thermal signatures, facilitating their detection and count. In contrast, gibbons showed avoidance behavior during drone surveys, posing challenges for detection. We developed a scoring system based on these factors to assess thermal detection reliability, which could assist in evaluating detectability in multispecies studies and the reliability of TIR drone surveys. The low resolution of TIR drone imagery
also limited species differentiation, particularly for species with similar morphology or behavior. We recommend combining TIR drone surveys with ground-truthing methods, camera trap surveys, and passive acoustic monitoring for comprehensive primate monitoring research. Our findings underscore the potential of TIR drone technology for detecting and monitoring large arboreal species like langurs, while highlighting the need for developing global scoring system to assess TIR detection reliability for different primate species. This study contributes to understanding sympatric primate behavior and
ecology, aiding conservation efforts, and emphasizes the importance of innovative technologies in primate monitoring.