Oct. 20 is International Sloth Day, but every day is a good day to learn more about one of the coolest animals on the planet.
They might be giants (or at least they used to be)
The small cuddly sloths of today were not so adorable a few million years ago. The most famous giant ground sloth of South America, Megatherium, was the size of an elephant and grew to over 8,000 lbs on a steady diet of plants. It was the discovery and observation of Megatherium fossils from South America among others that provided some of Darwin's evidence for his theory of natural selection. The evolutionary history of sloths is actually quite complicated—even though today there are only 6 living sloth species, over the past 35 million years there have been at least 50 different species that inhabited land, trees, and water.
Megatherium and other ground sloths were too big for trees and lived on the ground unlike their fully arboreal modern relatives. A variety of ground sloths lived in the Americas up until a few thousand years ago, which is fairly recent in terms of evolutionary history. Noted explorer and former U.S. President Theodore Roosevelt donated his own find—ground sloth poop—to the American Museum of Natural History. Sometimes ground sloth dung is found in caves in South America that nearly looks fresh due to the cold conditions, but these animals went extinct around 10,000 years ago.
Sloths eat leaves, except when they eat human waste
Speaking of poop—back in 2001 some field biologists in Peru noticed an unexpected visitor in their latrine. Sloths are folivores that live in trees and rarely come down to the ground, so they were shocked to find a two-toed sloth in their toilet “scooping with one hand from the semi-liquid manure composed of faeces, urine and toilet paper and then eating from the hand.” In the following years the crew noted this behavior 25 more times until they finally decided to fence of the latrine and prevent more sloth visitors.
There isn’t one agreed upon explanation for this previously unknown coprophagy (poo-eating) but they posit the sloths could be seeking out some nutrients or minerals for their diet.
Sloth hair is its own micro-habitat and it could save us
Sloths are known for being slow—so slow in fact, a wide variety of organisms can set up shop in their thick hair. Research has shown there is a species of green algae known as Trichophilus welckeri that inhabits the hair of all living sloth species. This relationship is thought to be symbiotic as the algae gets a sheltered place to grow and the sloth gets green camouflage for its life in the tree canopy.
Algae isn’t the only thing in a sloth's coat though. Other bacteria, moths, insects and cockroaches all like to call sloth hair home. Remarkably, in 2014, researchers found that sloth hair is actually a novel source of a fungi from the Ascomycota phylum that is able to fight off the parasites that cause malaria and Chagas disease.
They are pretty good swimmers
Most people picture sloths in the trees, so when they see them in the water they are typically shocked—but sloths are actually decent swimmers. People sometimes take the sloths out of the water but they can hold their breath and swim just fine, so leave 'em alone.
They are weird and wonderful and need our protection
Sloths move so slowly they have a difficult time escaping deforestation. They have an evolutionary history spanning 35 million years, but the living species are under serious threat from humans. The Brazilian three-toed sloth is vulnerable to forest clearing in the Atlantic forests of eastern Brazil which threatens the genetic diversity of the species as a whole. Not all sloth species are currently threatened, but continuing habitat degradation will not be helpful for promoting population growth. In some areas, two-toed sloths are even caught for sale in the illegal pet trade.
Shaena Montanari is a paleontologist and science communicator. Follow her on Twitter at @DrShaena for the latest natural history and fossil news.