Why giant sloths got so big—and went extinct

Today, just six species of sloth remain, split between two families: two-toed and three-toed. But in prehistoric times, sloths were far more diverse—and some were massive compared to the tree-dwellers we know now. If you’ve been to the Natural History Museum in London, you might have seen the striking replica skeleton* of Megatherium americanum, one of these ancient giants.

New research from a team of scientists from the Americas sheds light on how these now-extinct sloths reached such impressive sizes—and why they eventually vanished.

Bigger sloths, grounded lives

Unlike modern sloths, which live exclusively in trees, the largest prehistoric species were ground-dwellers. Some were tall enough to pull leaves from treetops while standing upright. Megatherium, for example, lived in the grasslands of South America until about 10,000 years ago. Fossil footprints from Argentina suggest it walked on hind legs, using its tail for balance.

The scientists analyzed ancient DNA and more than 400 fossils from 17 museums to investigate how and why some sloths became giants, while others adapted to life in the trees. The findings were published recently in the journal Science.

Tree-dwelling sloths—then and now—tend to be small, averaging about 14 pounds. But those that split their time between ground and canopy were much larger, often around 174 pounds. Fully terrestrial sloths grew even bigger.

Caves, claws and climate

Size had clear advantages. It likely helped ground sloths find food, avoid predators, and survive harsh climates. Many took shelter in natural caves—like the Shasta ground sloth, which preferred wind- and water-carved alcoves in the Grand Canyon. These caves often doubled as communal latrines. In 1936, scientists discovered a 20-foot-thick mound of fossilized dung, bat guano, and packrat middens in Rampart Cave near Lake Mead.

Other sloths didn't wait for nature to provide shelter—they dug their own. Their claws, among the largest known in any mammal, could carve caves into earth and rock. Some of these ancient dwellings still survive, their walls etched with claw marks.

Habitat, climate, metabolism, and evolutionary lineage all played roles in shaping sloth size. To untangle these influences, researchers combined fossil shapes, ancient and modern DNA, ecological data, and hundreds of bone measurements. This helped them build a sloth family tree stretching back more than 35 million years.

Evolution on a changing planet

The team concluded that habitat—closely tied to climate—was the primary driver of sloth size. The oldest known sloth ancestor, Pseudoglyptodon, lived 37 million years ago in Argentina. It was likely a small, ground-dwelling species, roughly the size of a Great Dane. Some later sloths took to the trees, but not all stayed there. The largest species, like Megatherium and Mylodon, appear to have evolved from arboreal ancestors that returned to a terrestrial lifestyle.

For about 20 million years, sloth size remained relatively stable. Then came a turning point.

A massive volcanic event cracked open the Earth between modern-day Washington and Nevada, releasing lava that formed the basalt cliffs seen today along the Columbia River. This slow, 750,000-year eruption coincided with a period of global warming known as the Mid-Miocene Climatic Optimum. The greenhouse gases it released likely drove the warming.

In response, sloths shrank. Warmer temperatures brought more rainfall and expanded forests—ideal for smaller, tree-dwelling species. Shrinking body size is also a known biological response to heat stress, documented in many species over evolutionary time.

After about a million years, global temperatures began to fall again. And as the climate cooled, sloths bulked up once more.

Giants of the Ice Age

Image: Biodiversity Heritage Library

As forests gave way to open landscapes, ground sloths spread far and wide—climbing the Andes, crossing deserts, reaching North American forests, even adapting to marine environments. In these varied habitats, large body size helped conserve energy and water, and improved mobility in sparse terrain. Some species even developed armor-like osteoderms—bony plates in their skin, shared with armadillo relatives and recently found in spiny mice as well.

By the Pleistocene ice ages, sloths had reached their peak size. But around 15,000 years ago, their numbers began to crash.

The causes remain debated, but the timing is telling. Human arrival in the Americas coincided closely with sloth extinction. The bulk that had once protected them from predators became a liability. While ground sloths vanished, their smaller, tree-dwelling cousins observed from above.

Even they weren’t entirely safe. In the Caribbean, two species of arboreal sloths held on until about 4,500 years ago—around the time the pyramids were being built in Egypt. Their extinction followed soon after humans arrived on the islands.

* Fun fact: The sloth skeleton on display in the Natural History Museum is a plaster replica made in 1848 from casts of several partial skeletons. It has been on show since 1850—making it one of the museum’s longest-standing exhibits.

Disappointingly, the Science paper is not open access but this story is based on a press release from the Florida Museum of Natural History which contributed to the study.