Panama has plenty of strange animals, but few are as extraordinary as the pygmy three toed sloth. It looks like a miniature version of the familiar brown throated three toed sloths found elsewhere in Panama, moves with the same wonderfully unhurried attitude and has the same permanently relaxed expression, but there is one enormous difference. This little sloth lives in one tiny place on Earth: Isla Escudo de Veraguas, a small Caribbean island off the coast of Panama's Bocas del Toro region. It isn't simply a rare animal found in Panama. It is an island endemic, meaning the species' natural range is restricted to this single island.
A Sloth With Its Own Private Island
Isla Escudo de Veraguas is tiny, covering only about 4.3 square kilometres, and sits roughly 17.6 kilometres off Panama's Caribbean coast. The island became isolated from the mainland thousands of years ago as sea levels changed, creating the conditions for something remarkable to happen. Over time, the isolated sloths developed into a distinctly smaller form. The pygmy three toed sloth was formally described as a separate species in 2001. Scientists call this sort of evolutionary change insular dwarfism, where animals living in isolated island environments can evolve smaller body sizes than their mainland relatives.
The result is one of Panama's most unusual mammals. The pygmy sloth is considerably smaller than the mainland brown throated three toed sloth. One conservation source describes it as roughly 40 percent smaller, while documented measurements put adults at around half a metre in total length. Even by sloth standards, it is a remarkably compact animal.
But its small body isn't actually the most remarkable thing about it.
Its entire world is tiny.
It Doesn't Live All Over Panama
This is where the pygmy sloth story becomes fascinating.
If you visit Panama and see a small three toed sloth hanging from a tree, you have almost certainly not discovered a pygmy sloth. The ordinary brown throated three toed sloth is widespread across mainland Panama. The pygmy three toed sloth is restricted to Isla Escudo de Veraguas.
Even more remarkably, researchers have found the pygmy sloths overwhelmingly associated with red mangroves growing around the island's tidal areas. A 2012 study surveyed the island's mangrove thickets and found 70 pygmy sloths within the mangrove habitat, plus nine just outside the survey areas. All of those animals outside the mangroves were still within 20 metres of a mangrove thicket.
Think about that for a moment.
There are sloths that live in the forests of mainland Panama, sloths that live in trees around towns and parks and sloths that can occasionally be surprisingly easy to spot. But this particular species has evolved into a specialist living on a tiny Caribbean island, with its existence closely tied to a very specific type of tree.
It is almost like Panama accidentally created a sloth laboratory.
The Mangrove Sloth
Red mangroves are not what most people imagine when they picture sloth habitat.
When people think of sloths, they usually imagine lush rainforest canopy, huge trees and deep jungle. The pygmy sloth has something much stranger. Its preferred habitat is the tangled world of coastal red mangroves, where trees grow with their distinctive roots partly surrounded by seawater and tidal mud.
The mangrove habitat is so important that researchers have described the pygmy three toed sloth as a mangrove specialist. In the 2012 survey, the researchers mapped approximately 106,699 square metres of mangrove habitat on the island, or about 10.7 hectares. That means the animal's already tiny island home contains an astonishingly small amount of the habitat in which researchers actually found most of the animals.
This makes the pygmy sloth particularly vulnerable.
If you lose a little rainforest somewhere on a huge continent, animals may sometimes be able to move into another patch of forest. On a tiny island where the suitable habitat is already extremely limited, there isn't necessarily anywhere else to go.
The Number That Shocked Researchers
One of the most fascinating parts of the pygmy sloth story came when researchers actually went looking for them.
Before the detailed survey, estimates of the population were considerably higher. But when researchers systematically surveyed the island's mangrove patches, they located 79 individual pygmy sloths: 70 in mangroves and nine immediately outside them. Because the researchers could not guarantee that they had seen every animal, this was considered a minimum estimate rather than an absolute population count.
Seventy nine animals is an almost unbelievably small number for an entire mammal species.
Imagine trying to find every person living in a city and discovering there were only 79 people there.
And this isn't a species that happens to have 79 animals in one local population while thousands more live elsewhere. Its natural range is that tiny island.
That is why the pygmy three toed sloth is classified as Critically Endangered.
Why Are They So Small?
Scientists have proposed several explanations for the pygmy sloth's small size.
The obvious explanation is its isolated island environment and its specialized diet. But the exact evolutionary story isn't completely settled. Researchers have suggested that feeding on mangrove leaves may have played a role, while other hypotheses have looked at factors such as the ability of sloths to defend feeding territories and the ecological pressures of island living.
This is an important reminder that evolution isn't a simple process where an animal decides to become smaller because there is less food.
Over thousands of years, populations experience different environmental pressures, genetic changes accumulate and eventually isolated populations can become sufficiently different to be recognized as distinct species.
The pygmy sloth is essentially a living record of Panama's geological and evolutionary history.
The Island Wasn't Always an Island
The story gets even more interesting when you look at the island itself.
Isla Escudo de Veraguas became isolated from mainland Panama roughly 9,000 years ago. Once sea levels rose and separated it from the mainland, animals living there were effectively placed in an evolutionary experiment.
Isolation is incredibly important in evolution.
A population on the mainland is constantly interacting with neighbouring populations. Individuals move, mate and exchange genes with animals from other areas. Put a population on an island and suddenly those connections can disappear.
Over generations, island populations can become different.
Sometimes dramatically different.
The pygmy sloth is one of the most entertaining examples imaginable because the result is essentially a miniature Caribbean sloth species that exists on one little Panamanian island.
And Then Humans Arrived
Unfortunately, being fascinating doesn't make a species safe.
The pygmy sloth's tiny range means that habitat loss is particularly serious. Researchers have identified habitat fragmentation and loss as potential threats, while earlier scientific assessments have also identified hunting and tourism related disturbance among concerns.
The problem isn't simply the total number of trees.
It is which trees.
If these sloths really are strongly dependent on red mangrove habitat, destroying or fragmenting those mangrove patches could remove the very environment they need to survive.
And because the species has nowhere else on Earth to go, there isn't a convenient backup population living hundreds of kilometres away.
One of Panama's Most Extraordinary Animals
The pygmy three toed sloth is not the animal you are most likely to see while backpacking through Panama.
You don't wander around Panama City and suddenly discover one hanging from a tree. You don't go to Boquete expecting to see one. You won't find them distributed throughout the forests around Bocas del Toro.
You would have to reach Isla Escudo de Veraguas, an isolated island in the Caribbean, to enter the natural range of the species.
And that is exactly what makes it so fascinating.
Panama isn't simply a country where you can see sloths.
Panama is home to a sloth that evolved into its own species on a tiny island off the Caribbean coast.
It is smaller than its mainland relatives, specializes in mangrove habitat and has one of the most restricted distributions of any mammal on the planet.
It is also a perfect example of why Panama's wildlife is so much more interesting than simply making a checklist of monkeys, toucans and sloths.
Sometimes the really extraordinary animal isn't the one you see everywhere.
Sometimes it is the little creature hanging quietly in a mangrove tree on an island most people have never heard of.
And somewhere out there, surrounded by Caribbean water and red mangroves, one of Panama's rarest mammals is probably doing what sloths do best: absolutely nothing very quickly.