Example of preliminary ground survey data taken with the thermal camera unmounted from the drone. Two spider monkeys are visible in this figure. Temperature scale for false colour thermal image is shown on the right-hand side. Using these data, we expected the surface temperature of spider monkeys to be around 30 °C.
Wildlife conservationists rely on accurate and precise counts of animal populations for conservation related decisions and actions; however, availability of such data for majority of arboreal species lacks due to difficulties and costs of performing ground surveys. To overcome these challenges, studies were carried out by a group of researchers using thermal infrared (TIR) camera equipped drones to monitor primate populations of howler monkeys and spider monkeys and orangutans in Los Arboles Tulum, a residential development located about 14 km from the municipality of Tulum, Quintana Roo, Mexico. In this area houses are allowed to occupy only 5% of each 2-hectare plot hence as of now 98% of the 400-hectare area remains forested.
The project demonstrated that a TIR camera fitted drone can be successfully employed to determine the presence and count the number of spider monkeys in a closed-canopy forest. The vegetation is classified as medium-evergreen forest, trees are around 30 m tall and the terrain is relatively flat.
Methods
This technology could be applied to determine the distribution and population size of spider monkeys in areas where other species of similar size spend large amounts of time in the forest canopy can be accurately distinguished from spider monkeys in the TIR images. Spider monkeys were easy to detect in the TIR footage due to their relatively high temperature compared to their surroundings. Since they spend a large amount of time at the top of the forest canopy and tend to sleep on terminal branches and an aerial viewpoint offers distinct advantages over ground-based observations. The group of spider monkeys living in Los Arboles Tulum has been studied since November 2016 and is habituated to the presence of humans.
Examples of TIR images obtained from the Fusion camera whilst mounted on the drone recorded during a flight at 19:30 on 20th June 2018 at sleeping site A (left) and 05:45 on 20th June 2018 at sleeping site B (right). Drone height for both flights was 70 m above ground level, approximately 40–45 m above the canopy. Temperature scale corresponding to false colour thermal image is indicated on the right-hand side. Locations of spider monkeys are indicated with arrows. The circle indicates a human observer counting monkeys on the ground. Some sources of possible confusion when examining thermal infrared data are also visible in these images, particularly in the evening survey (left). Several branches have been heated by the sun during the day and appear as bright thermal sources.
Equipment
The drone used was a 550 mm quadcopter with an extruded aluminium frame constructed from arms and glass fiber plates to provide optimal strength to weight performance and was powered by a 14.8V Lithium Polymer battery which provided a flight time of approximately 10 min when the drone was fitted with the TIR camera. A Pixhawk 2.1 autopilot was used to run the open-source ArduCopter firmware, providing a high degree of flexibility in configuring and operating the system. The autopilot was configured to allow either manually piloted or automatic (preprogramed) flight. The camera was facing straight down at an angle of 90 degrees during all flights of the present study.
To compare counts of spider monkeys between ground surveys and drone surveys, the team first determined whether spider monkeys were present at their sleeping sites between 17:30 and 19:00. Once confirmed they flew the drone over the sleeping site and performed ground counts simultaneously. Then both drone and ground surveys were conducted at the same sleeping site the following morning before sunrise to compare evening and morning counts from drone and ground surveys.
Conduct of Tests & Related Analyses:
For aerial survey of monkeys pre-programmed grid flights were conducted over sleeping sites with drone flight range between 60 and 70 m above ground level. Grid flight duration ranged between 4 and 8 min depending on the area of the site surveyed, the flying height, and the percentage of overlap and side-lap selected for the footage. Additionally, 4 hover flights were flown over a single sleeping tree for 5 min. A total of 33 drone flights (29 grid flights and 4 hover flights) with the TIR camera were conducted at the three selected sleeping sites between the 10th and the 23rd of June 2018. The most common reaction to the drone was for the monkeys to change position or move down a few meters. Once the drone survey finished (usually after 6–10 min from the start), many individual monkeys returned to their original positions. More than once, a monkey let out a scream in response to the drone flying overhead. Still, several individual monkeys showed no change in their behaviour in response to the drone. For instance, juveniles that played before the drone started continued playing for the entire duration of the flight.
Ground counts (i.e., the number of monkeys counted by ground observers) were carried out by a five experienced observers equipped with binoculars to aid in counting each monkey and a GPS unit to mark the position of monkey subgroups. To test the agreement between counts from drone and ground surveys, a concordance analysis was performed. For making the comparison a subsample of the TIR footage from the drone flights was edited into 20 videos of 30 s in length each.
Conclusion
In conducting these drone based species counting trials and by comparing a substantial dataset obtained by flying the drone while simultaneously counting monkeys from the ground, the researchers were able to demonstrate that the use of a TIR camera as a feasible and valid method of obtaining data on the presence and number of spider monkeys at their sleeping sites in a closed-canopy forest. We performed grid and hover flights to determine whether both methods could be used to detect and count spider monkeys.
Additionally, data from TIR footage obtained by drones can be used to improve species distribution models and calculate population density. Researchers opine that the use of TIR imaging can therefore help in the urgent quest to fill the gaps in existing knowledge on the distribution and abundance of spider monkeys across their range.
This research was funded by the National Geographic Society (WW-198EC-17), the UK Science and Technology Facilities Council Global Challenges Research Fund (ST/R002673/1), and Los Arboles Tulum
Citation: Thermal Infrared Imaging from Drones Offers a Major Advance for Spider Monkey Surveys, Drones 2019, 3(2), 34; doi:10.3390/drones3020034 – https://www.mdpi.com/2504-446X/3/2/34
Denise Spaan, Claire Burke, Owen McAree, Filippo Aureli, Coral E. Rangel-Rivera, Anja Hutschenreiter, Steve N. Longmore, Paul R. McWhirter and Serge A. Wich