Seeing one sloth in a leafy tree is a lovely wildlife moment. Working out how many sloths live across an entire region is a much harder scientific problem. They are quiet, camouflaged and usually high above the people trying to count them.
That missing information matters. Conservationists need reliable population data to learn whether sloth numbers are stable, falling or recovering. Without a baseline, it is difficult to identify the most important habitats, measure whether canopy bridges and tree planting are helping, or spot a decline before it becomes a crisis.
The Great Sloth Census is tackling that problem in Costa Rica by combining three very different ways of finding signs of sloths: skilled people looking from the ground, drones looking for heat in the canopy, and a specially trained dog searching for sloth droppings.
Why are sloths so difficult to count?
Sloths have spent millions of years becoming difficult to notice. Their slow movement helps them avoid attracting attention, while their fur can take on a greenish appearance that blends into the rainforest canopy. They may remain tucked among leaves and branches for long periods, far above a researcher's eye level.
A simple walk through the forest therefore cannot reveal every sloth present. One observer may spot an animal that another misses. Dense vegetation, weather, time of day, tree height and the experience of the survey team can all affect the result.
Researchers also need more than a collection of exciting sightings. A census uses repeatable survey areas and methods so that results can be compared between places and over time. That is how scattered observations begin to form an estimate of abundance, density and distribution.
Method one: experienced human observers
The most familiar method is still extremely valuable. Trained observers walk planned routes, or transects, and scan the canopy for sloths. People can notice shapes, movement and behavioural clues that an automated system might miss, while recording the habitat around each sighting.
Early results reported by The Sloth Conservation Foundation found that human observers detected the most sloths during its comparison: 0.75 sloths per half hectare. That does not mean people will always be the best method in every forest. It shows why methods must be tested in the habitats where they will actually be used.
Ground surveys are also demanding. Teams may spend many hours walking through heat, mud and thick vegetation, and their view stops at the underside of the canopy. Covering a large country entirely on foot would take an enormous amount of time.
Method two: thermal drones
Thermal cameras detect differences in heat rather than relying only on visible colour and shape. Mounted on a drone, they give researchers another view of the upper canopy and can cover areas that are difficult to inspect from below.
In the census comparison, thermal drones detected 0.5 sloths per half hectare. That was fewer than the human teams, but drone surveys required much less labour. The balance between detection rate, time and cost becomes important when a project expands from a few study plots to many regions.
Thermal imaging is not magic. Sun-warmed leaves, weather conditions, canopy thickness and an animal's position can affect what the camera sees. Researchers still need trained pilots, careful flight plans and a way to check possible detections. The tool is useful because it adds a different stream of evidence, not because it makes field biology effortless.
Method three: follow the poo
Sloths leave another clue on the forest floor. Their droppings can reveal that an animal has used an area even when nobody saw it in the trees. This inspired an unusual part of the project: Keysha, a dog trained to locate sloth scat.
The foundation's account of Keysha's work explains how she searched for samples as part of the field comparison. Scat can support more than presence records. Carefully analysed samples may contain information relevant to diet, parasites, genetics and stress.
Keysha's work was pioneering, but the comparison found that scat detection was the least effective and least efficient of the three counting methods. That is not a failed experiment. Science needs honest tests, including results that show an appealing idea is not the best tool for a particular job. The team learned where the method's limits were and chose to continue with human observers and drones.
The first stage is complete
In April 2026, the foundation announced that the first stage of the Great Sloth Census had been completed after three years of work. The initial region had been surveyed, with hundreds of hours of drone flights and long days spent walking forest transects. The next ambition is to expand into other parts of Costa Rica and refine the methods through collaboration.
That expansion matters because sloth habitat is not uniform. A technique that works well in dense lowland rainforest may perform differently in a fragmented town, plantation, coastal forest or mountain habitat. Comparing methods across landscapes helps researchers avoid assuming that one survey recipe fits everywhere.
What can a sloth census change?
A reliable count does not protect an animal by itself. It gives conservation decisions a stronger foundation. Population maps can help identify important habitat, reveal isolated groups and guide where restored tree corridors or canopy crossings may have the greatest value.
Repeated surveys can also test whether an intervention is working. If sloth abundance changes after habitat is reconnected, researchers have evidence to investigate. If numbers continue to decline, they can adapt rather than simply hoping a well-intentioned project is enough.
The Great Sloth Census is a good reminder that conservation often begins with a deceptively simple question: how many are there? Answering it takes boots on the ground, cameras in the sky, patient analysis and, in this case, one very determined dog. The sloths may be experts at hiding, but better evidence can help ensure they are not overlooked.
Sources and image credit
- The Sloth Conservation Foundation: The Great Sloth Census
- The Sloth Conservation Foundation: Great Sloth Census, The Dog Days Are Over
- The Sloth Conservation Foundation: Tales from the Jungle, April 2026
Image: The Sloth Conservation Foundation, from its Great Sloth Census project page.