Hot monkey, cold reality: surveying rainforest canopy mammals using drone-mounted thermal infrared sensors

Abstract

Animals of the rainforest canopies are often endangered by deforestation or hunting but are difficult to survey and study because of the inaccessibility of the treetops, combined with the visual camouflage of many species. Drone-based thermal sensors have the potential to overcome these hurdles by rapidly scanning large forested areas from above, detecting and mapping wildlife based on the contrast between their warm body temperatures and the cool tree canopies. We tested this concept by flying a drone-mounted thermal infrared radiometric sensor over the wildlife-rich rainforests of Barro Colorado Island, Panama. Arboreal mammals had body temperatures around 27°C and were conspicuous in the thermal infrared imagery at night and early morning when the forest canopy was cool (23-25°C), but were difficult to detect by mid-morning, by which time the direct sunshine had heated up canopy vegetation to over 30°C. Species were difficult to identify from thermal infrared imagery alone, but could be recognized from synchronized visual images taken during the daytime. Simultaneous drone and ground-based surveys of the same area proved that the aerial thermal camera did detect highcanopy species missed by the terrestrial observer, but that substantially more animals were detected by the human than by the drone. Because animal detection was so much better at night, when species ID was difficult, we suggest that future work could combine automated detection of animals from thermal infrared imagery with flash photography or IR illumination to enable species ID during nocturnal surveys. We conclude by discussing various logistical challenges that limit the utility of drone-based thermal infrared today, but that could be overcome by continued improvement of technology and collaboration with permitting agencies.

Thermal Infrared Imaging from Drones Offers a Major Advance for Spider Monkey Surveys

“…Conversely to our findings, Kays et al, 2019 [23] found that ground counts were higher than drone counts. However, they compared counts from only two flights, whereas we compared counts from 28 flights.…”

“…In addition to spider monkeys, the Yucatan Peninsula is home to black howler monkeys (Alouatta pigra), but they are very rare in Los Arboles Tulum. Other animals of similar size to spider monkeys (e.g., coatis, small cats) occupy lower vegetation layers and will therefore be more difficult to detect in TIR drone footage [23]. None of these species were observed near the monkeys during our ground observations.…”

“…Accurate and precise counts of animal populations form the basis of conservation decisions and actions; however, they are lacking for the majority of arboreal species due to difficulties and costs of performing ground surveys. To overcome these difficulties, recent studies have fitted drones with TIR cameras to monitor primate populations (howler monkeys and spider monkeys [23]; orangutans [42]). Here, we demonstrated that a TIR camera fitted to a drone can be successfully employed to determine the presence and count the number of spider monkeys in a closed-canopy forest.…”

“…Ground surveys may therefore be more accurate than TIR drone surveys during hours of the day when primates are active, but their surroundings have started to heat up, minimising thermal contrast. Kays et al, 2019 [23] performed surveys when spider monkeys were more active and thus more difficult to detect on TIR footage, potentially explaining differences in results between the studies.…”

“…Thus, for species that are nocturnal or species where detection of animals at dawn and dusk is important, there is a need to explore detection opportunities with other sensors. Thermal infrared (TIR) cameras, which use a different part of the electromagnetic spectrum (8-13.5 µm), have recently been used to detect a variety of animals (e.g., deer, seals, koalas, kinkajous, monkeys, and rhinoceros [19][20][21][22][23]).…”