Thermal infrared imaging from drones can detect individuals and nocturnal behavior of the world’s rarest primate

Escalating anthropogenic pressures now threaten ~60% of primate species across the world with extinction. Developing effective evidence-based conservation for threatened primate species requires accurate and precise information on their population abundance. However, standard ecological field techniques are costly in terms of time, resources and manpower, meaning that the effectiveness of alternative survey and monitoring methods must be investigated. Thermal infrared imaging using drones may be able to improve ability to detect individuals and accuracy of population abundance estimates for primate species at lower cost. Here we use a drone with a thermal infrared sensor to survey the largest social group (Group C) of the Hainan gibbon (Nomascus hainanus), the world's rarest primate species, which survives as a remnant population in Bawangling National Nature Reserve, Hainan, China. Group C is known to currently contain nine Hainan gibbon individuals based on regular visual monitoring. Drone surveys conducted over two consecutive days and nights in April 2019 demonstrated that thermal infrared imaging can detect the presence of different gibbon individuals in this social group, with comparable group size estimates to regular visual monitoring, and provides the first information about Hainan gibbon sleeping behavior and the range of nocturnal body temperatures for the species. This method can therefore be used to monitor other Hainan gibbon groups in the future, and can also be used to survey individuals and study nocturnal behaviors in other threatened or cryptic primate species.