Long ago, the sloth lineup was far more impressive. Some even walked the earth instead of hanging in the treetops – and they came in all shapes and sizes, from modest desert-dwellers to elephant-sized giants.
New research from the Florida Museum of Natural History and other institutions is offering fresh insight into this now-extinct diversity.
Scientists combined fossil measurements, DNA, and ecological data to uncover why some sloths evolved to become massive, while others remained small and nimble.
“They looked like grizzly bears but five times larger,” said Rachel Narducci, a co-author of the study and collection manager of vertebrate paleontology at the museum.
When sloths were giant Most extinct sloths didn’t live in trees because they were too big. The Megatherium, one of the largest species, was the size of an Asian elephant and weighed around 8,000 pounds.
These prehistoric sloths roamed the land, feeding on leaves they could reach with long tongues, much like a giraffe.
Others, like the Shasta ground sloth, were much smaller and occupied very different environments. They lived in the deserts of North America and were known for their fondness for cacti.
These sloths often made their homes in caves, where they also left behind fascinating fossilized clues.
In 1936, researchers discovered a massive pile of fossilized dung, bat guano, and packrat nests in Rampart Cave near Lake Mead. The deposit was over 20 feet thick.
Some sloths didn’t need to find caves – they made their own. Using enormous claws, they carved shelters directly into the earth. You can still find these ancient burrows today, complete with claw marks etched into the stone walls.
Why sloths are smaller today Tree-dwelling sloths today are not giant for a reason. Trees can only support so much weight. The average tree sloth weighs about 14 pounds.
Even those that split their time between ground and canopy rarely top 174 pounds. If a branch breaks, there’s little a sloth can do to stop a fall.
Although they’ve been known to survive 100-foot drops, that doesn’t mean falling is safe – especially when trees in the Amazon can reach 300 feet high.
But the puzzle scientists wanted to solve wasn’t why sloths in trees are small – it was why ground sloths varied so much in size.
Some sloths stayed relatively petite, while others evolved into lumbering beasts. The team behind the new study explored everything from predator avoidance to food access to climate and shelter options. To do this, they needed a lot of data.
Building a family tree The researchers examined over 400 fossils from 17 different museums. They collected data on sloth anatomy, location, diet, and lifestyle.
At the Florida Museum, Narducci measured 117 limb bones from their vast collection of North American and Caribbean sloths – the largest such collection in the world.
“We have the largest collection of North American and Caribbean-island sloths in the world,” she said.
By Eric Ralls