Drones, spider monkeys and Los Arboles Tulum

Advancing the use of technology to answer an age-old question.

One of the biggest questions in spider monkey conservation in Mexico and abroad is: how many monkeys are there? A simple question that time and again proves very difficult to answer. The most honest answer is “we don’t yet know”. This comes down to several things. For one, the species of spider monkey that is found in Mexico, the Geoffroy’s spider monkey (scientific name: Ateles geoffroyi) is the same species that you can find in all Mesoamerican countries (from Mexico to Panama), so that means that there is a lot of area to cover when trying to count them! Secondly, some of the methods that are commonly used to count monkeys, such as walking long distances in the forest, are both time consuming and labor-intensive, which means that they are costly, and you only cover small areas at a time. So, to try and overcome some of these hurdles we paired up with drone experts in the UK to try and design better methods to count monkeys and find areas where they occur. 

Over the past three years, we trialed several types of drones in Los Arboles Tulum to help improve the ways that we can survey spider monkeys. We started with a custom-made drone that was fitted with a thermal camera. We flew over the spider monkey sleeping sites and were able to successfully detect and count spider monkeys on the thermal footage. A thermal camera, unlike a regular camera (for instance the one on your cellphone) captures a heat signature. The spider monkeys appear as white-yellow blobs against a backdrop of purple-to-black forest. To ensure that the spider monkeys can be observed clearly in thermal imagery, you need to ensure maximum contrast between the monkey and the forest. To do this, you fly at times when the forest is coldest, so that the monkeys appear warmer than their surroundings. If you were to fly at midday for instance, it is likely that the spider monkeys would be camouflaged by the heat given off by the leaves and trees and you would not be able to spot them in thermal imagery. This also means that the flying time for the thermal camera is limited to the night or the very early morning. However, flying the custom-made drone was no easy feat! 

 Above) The custom-made drone with thermal camera. Below) On the left we see a large fig tree that is used by the spider monkeys as a sleeping tree. It is extremely difficult to see any monkeys. A thermal image taken of the same tree in the same moment shows that 12 monkeys are in that tree.

To overcome the limited flying time and the difficulty of flying such an advanced system, we performed a follow-up study in Los Arboles Tulum in 2019 and early 2020 in which we used an off-the-shelf drone with a high-definition visual spectrum camera. This is the equivalent of a very good camera on your phone. This type of drone had not been tested for detection of monkeys before. We were therefore entering unchartered waters but were hopeful that the quality of the camera would allow us to see the monkeys on the video footage. As we expected, we were able to spot spider monkeys, but because the monkeys are often found between the different layers of the forest canopy, this method did not allow us to count the monkeys. We can therefore use this method to find monkeys in areas where we are not sure they exist, but will not be able to count how many monkeys there are in these areas. 

Spider monkey in the tree canopy observed with an off-the-shelf drone with a high-definition camera. 

Then early this year, our hopes came true and the same company that developed the user-friendly off-the-shelf drone, developed a drone that has both a thermal camera and a high-definition visual spectrum camera. This means that we can finally use thermal imagery but without all the caveats of the custom-made drone. Our collaborator Dr. Serge Wich will come to Los Arboles Tulum this month to trial this new drone to not only detect spider monkeys, but also be able to count them! 

After years of following the monkeys through the forests of Los Arboles Tulum, we now know exactly how many monkeys are part of the “Coba monkey group” which uses the western half of Los Arboles Tulum. This means that we can actually compare the number of monkeys observed during drone flights to the real number of monkeys that should be using that area, to help find out how many monkeys are missed and what might cause us to miss monkeys during flights. We would like to perform flights at night over the sleeping sites when the forest is cold to cover as many sleeping sites in a single night as we can using the thermal camera. On the thermal camera, people and pets (dogs, cats) appear as yellow blobs, just like the spider monkeys do. No individual features or faces can be distinguished, and the thermal camera cannot see through surfaces or into homes. On the thermal footage, the roofs of houses appear as shapes of different sizes and colors, depending on how much heat they give off. 

On top of flying at night we would like to perform flights during the day to map some of the forest of Los Arboles Tulum and be able to use this information to better understand the travel routes used by the spider monkeys. 

All the information we obtain during these different studies are shared with other research groups and the general public through webinars and scientific publications. This way, collectively, we can start to answer the question of how many Geoffroy’s spider monkeys there are left in the wild. We could not have undertaken any of these studies without your support and interest. We are as always extremely grateful for your help, encouragement and support to conduct cutting-edge research that will help the conservation not only of spider monkeys in LAT, but throughout Mesoamerica. 

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