This story originally appeared in bioGraphic, an online magazine about nature and sustainability powered by the California Academy of Sciences.
Story by Hillary Rosner | Photographs by Suzi Eszterhas
In the balmy waters of a tiny island off Panama’s Caribbean coast, a pygmy three-toed sloth, (Bradypus pygmaeus) takes a dip. Just discovered at the start of the 21st century, and found exclusively on this island roughly 17 kilometers (10 miles) from the Panamanian mainland, this is the only sloth known to swim in salt water. And these diminutive tree-dwellers seem to swim far more frequently than their larger cousins, placidly paddling with just their flat-snouted, hairy heads protruding from the turquoise sea.
“If they have to change trees, they just plop into the water,” says Becky Cliffe, a British zoologist and founder of the Sloth Conservation Foundation, or SloCo. “They’d rather swim than crawl on the ground.” Because sloths float easily, swimming is an efficient form of locomotion. In fact, sloths can move three times faster in the water than they can through the trees. Roughly a third of a sloth’s body mass comes from leaves in its stomach, which generate a fair amount of gas as they’re being digested. “They’re like big balls of air,” Cliffe says.
Cliffe believes pygmy sloths also likely swim longer distances than other sloths – though, like many aspects of the species’ ecology and behavior, there is no conclusive evidence.
Studying sloths – pygmy and otherwise – requires ingenuity, time, and a whole lot of patience. “With a lot of animals, you can do a few weeks or months in the field and collect a lot of data,” says Cliffe, who is based on the east coast of Costa Rica and began working with sloths a decade ago while studying zoology as an undergraduate. She landed a year-long research position at a sloth sanctuary, and, as she describes it, “slowly discovered nobody really knows anything about sloths.”
What little is known only adds to the intrigue surrounding these animals. Sloths, which live only in Central and South America, belong to the order Pilosa, along with anteaters; they are also distant cousins of armadillos. Arguably some of nature’s strangest creatures, their hairy coats host entire ecosystems of invertebrates and algae; the latter lend the sloths’ coats their greenish tint. Sloths move at the astonishingly slow pace of about 36 meters (120 feet) per day, typically climbing down from their arboreal perches just once a week to poop.
Nearly everything about sloth behavior and physiology proceeds in slow motion. Cliffe has discovered, for example, that it takes the animals around 30 days to simply digest a leaf. Based on findings like these, she knew that “a one-month research project was not going to cut it.” She opted for a seven-year Ph.D. project, during which she set out to learn about sloth genetics, behavior, and biochemical processes. As part of that research, she outfitted sloths with tiny data-logger “backpacks” – technology designed by her advisor at Swansea University – that record thousands of biological measurements throughout the day. Cliffe’s findings from that research, as well as a lengthy genetic study, will be published later this year.
Cliffe also hopes to shed light on some of the mysteries of pygmy sloths specifically – including whether they are truly a distinct species. There are six known species of sloths in total, including both two-toed and three-toed varieties. Scientists described B. pygmaeus as a new species in 2001, based mainly on its size: Pygmy sloths are roughly 40 percent smaller than the average sloth, and lighter, too, with more modest skulls. They eat the leaves of red mangrove (Rhizophora mangle) trees, which no other sloths are known to eat. And pygmy sloths live solely on Isla Escudo de Veraguas, the outermost island in the Bocas del Toro archipelago, a chain that separated from mainland Panama as sea levels rose some 9,000 years ago. Further supporting this separate-species hypothesis, Escudo de Veraguas is also home to endemic species of hummingbirds and bats.
On four of the chain’s other islands, the resident sloths are smaller than average as well. But only those on Escudo de Veraguas are different enough that scientists have designated them as a separate species. Distinct genetic differences have yet to be proven, though, and Cliffe believes pygmy sloths are actually just a smaller-bodied population of brown-throated three-toed sloths (Bradypus variegatus) – the same species found throughout Central America.
In 2015, Cliffe and colleagues visited Escudo de Veraguas for five days with the hope of determining just how many sloths live on the island, how much inbreeding is occurring, and how distinct the sloths are from their mainland cousins. The researchers collected hair samples for DNA, and also recorded body size and weight.
The results of the study are not yet complete, but for now the dwarf sloth is listed as critically endangered, based on its isolated habitat and what some researchers believe is a decline in numbers from hunting and potential tourism development on its island home. Although Escudo de Veraguas has no resident population of humans, indigenous families from Panama’s Ngäbe-Buglé Comarca frequently set up fishing camps there, cutting down trees for huts and firewood and purportedly eating sloths whenever their fishing nets come back mostly empty.
A scientific survey several years ago found that the island’s mangrove forests were being fragmented by tree felling, and that the resulting gaps in the canopy were interrupting the previously continuous treetop highways. This is an important loss for sloths, which are at a much greater risk of predation by feral cats, among a few other predators, when traveling on the ground.
During that study, the researchers also noted two dead pygmy sloths whose carcasses were still intact, suggesting that they weren’t killed by predators. That could signal “a high rate of death through disease, habitat loss, or natural causes,” they wrote.
Shrinking genetic diversity caused by inbreeding within small populations could pose a threat to the pygmy sloths as well. The island covers only about 4 square kilometers (1.5 square miles), and just how many sloths live there is a question that has vexed scientists since the species was first described. In research published in 2012, scientists surveying the island found 79 of the animals, the vast majority of them living in the easily accessible mangroves at the island’s edges. But many more sloths may inhabit the interior forests, where they are much harder to see and track.
“Censusing a cryptic animal such as a sloth is challenging,” wrote Bryson Voirin, a biologist at the Max Planck Institute for Ornithology, in a 2015 study of the pygmy sloth. “Standard methods such as footprint tracks and camera traps are ineffective, and visual searches likely miss an overwhelming majority of sloths, especially in the dense rainforest.” Voirin concluded that the population was far higher than previously thought, likely somewhere between 500 and 1,500.
Ongoing research continues to turn up intriguing facts about the pygmies—like what resides in the teeming cities of their fur. One team of researchers analyzed exactly what was living in the pygmy sloths’ coats and discovered that while they host just one variety of green algae, they provide habitat for nearly 40 other kinds of tiny critters—more than any other species of sloth—“including ciliates, apicomplexans, dinoflagellates, and fungi,” according to a study published in 2010.
Cliffe’s findings, when published, will provide another window into these curious creatures’ biology. And as scientists begin to see pygmy sloths more clearly, they’ll also gain new insights into how to protect the pint-sized swimmers and their lush little island home.
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