Lost world of giant sloths: Why did they shrink and when did it happen?

Sloths are famous for their laid-back lifestyle. They cling to trees, digest food at a glacial pace, and only use the bathroom once a week. But today’s tree-dwelling sloths are just a tiny branch of what was once a vast and varied family.

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.

With fossil measurements and genetic data in hand, the team created a “sloth tree of life” stretching back more than 35 million years. They then layered in other key information, like whether the animals were climbers or walkers.

Finally, the researchers used this framework to estimate sloth body weights and analyze how these changed over time. Their conclusion: habitat and climate were the strongest forces driving sloth size.

Earth’s climate shaped sloth evolution

Early sloths likely looked more like Great Danes than elephants. The earliest known sloth, called Pseudoglyptodon, lived in Argentina about 37 million years ago.

For the next 20 million years, sloth size stayed fairly stable, and then there was a dramatic shift.

Volcanic eruptions in the Pacific Northwest released huge amounts of lava and greenhouse gases. These eruptions aligned with a period of global warming known as the Mid-Miocene Climatic Optimum. Forests spread. Temperatures rose. Sloths shrank.

“It makes sense,” Narducci explained. Smaller bodies lose heat more easily and are better suited for warm, wet climates. More trees also meant more food for smaller, tree-loving species.

But the world didn’t stay hot. After about a million years, the Earth began cooling again. Sloths responded by bulking up.

They expanded into new environments – mountains, grasslands, deserts, and forests stretching all the way to Alaska. Some species even adapted to life in the sea.

“They developed adaptations similar to those of manatees,” Narducci said. “They had dense ribs to help with buoyancy and longer snouts for eating seagrass.”

Benefits and risks of bigger bodies

Being big had its perks. It helped conserve water and energy, especially in dry or cold places.

“This would’ve allowed them to conserve energy and water and travel more efficiently across habitats with limited resources,” Narducci said. “And if you’re in an open grassland, you need protection, and being bigger provides some of that.”

“Some ground sloths also had little pebble-like osteoderms embedded in their skin,” noted Narducci. These bony growths, shared with armadillos, likely offered extra protection.

Sloths reached their peak size during the Pleistocene ice ages. But their giant size also made sloths vulnerable. When humans arrived in North America about 15,000 years ago, they began to vanish.

“About 15,000 years ago is when you really start to see the drop-off,” Narducci said. They were too slow to escape and too large to hide. Even sloths in the Caribbean, who lived in trees, eventually disappeared after humans arrived around the time the Egyptian pyramids were built.

From giant sloths to tree huggers

This study shows how animals adapt – or don’t – when the world around them changes.

Sloths grew and shrank in response to Earth’s climate shifts, much like many species do today. Their story is a reminder of how closely life on Earth is tied to the environments we live in.

As researchers continue to study extinct sloths, we get a clearer picture of what ancient ecosystems looked like – and how life has always had to find new ways to survive.

The full study was published in the journal Science.

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