New research reveals how ancient giant sloths dug caves, thrived in the oceans and moved between diverse environments – until climate change and the arrival of humans led to their extinction
Long before they became symbols of calm and tree-hugging, sloths were giant creatures weighing about 3,600 kilograms, which roamed deserts, dug caves in cliffs, and even swam like manatees.
Now, scientists have managed to piece together the epic story of their evolution, using ancient DNA and hundreds of fossils, in order to understand how they grew to gigantic proportions — larger than most modern cars — and why they eventually shrank or disappeared altogether.
A strange evolutionary family tree
Most of us know sloths as slow-moving creatures that live in trees, take a month to digest a meal, and only come down to the ground to defecate once a week. They may look like little furry bears, but their closest relatives are actually ants and armored animals. This strange combination in evolution suggests a diverse and surprising past.
There are only six species of sloths today. But there used to be dozens, including ones with bottle-like snouts that specialized in eating ants, and others that looked like early versions of armadillos.
Fathers the size of elephants
Many of the early sloths didn't live in trees at all—they were simply too big. The largest of them, Megatherium, was the size of a modern Asian elephant and weighed about 3,600 kilograms.
“They looked like grizzly bears – but five times bigger,” said Rachel Narducci, director of the vertebrate paleontology collection at the Florida Museum of Natural History.
Narducci was a co-author of the study published in the journalScience , in which ancient DNA and more than 400 sloth fossils from 17 museums were analyzed to answer the question: How did sloths get so big?
Huge variety of sizes
Early ground sloths came in a variety of sizes. Some, like Megatherium, used long tongues to eat leaves from tall trees—similar to the role of giraffes today. Others, like the Shasta ground sloth, were smaller but still robust, feeding on cacti and roaming the deserts of the American Southwest.
Tree sloths have always been small. Species that live exclusively in trees weigh an average of about 6.5 kilograms, while those that divide their time between the ground and the canopy reach about 79 kilograms.
The reason for this is obvious: branches can only support a limited amount of weight, and falling from a height is a serious risk for a slow creature. Some modern sloths have survived falls from 30 meters, but in the Amazon rainforest, trees reach 90 meters, so evolution favors a smaller size at the top.
Why did they become so big??
Why did some sloths become giants, while others remained reasonably large? There could have been several reasons for this – such as searching for food, protection from predators, or adaptation to harsh living environments.
Many sloths loved caves, and their size helped them not only find shelter but also dig it themselves. The Shasta sloth, for example, preferred small natural caves in the rocks of the Grand Canyon—like geological lung caves. These caves also served as natural toilets: in 1936, paleontologists discovered a pile of fossilized sloth feces, bat droppings, and rat remains more than 6 feet thick.
Digging with their own claws
Large sloths were not content with existing caves. Using giant claws – some of the largest ever seen in a mammal – they dug their own caves in rock and hard soil. Evidence of this can still be seen today in claw marks on the interior walls of caves they dug with their hands.
Their size was also influenced by changes in climate, genetic closeness between different species, and metabolic rate.
The evolutionary tree of sloths
The researchers combined anatomical data, ancient and modern DNA, geographic location, diet, and lifestyle (climbing or walking) to create the sloths' evolutionary tree. Using measurements of hundreds of fossils, they also estimated their weight.
The Florida museum played a central role:
"We have the world's largest collection of North American and Caribbean sloths," Narducci said.
The information is collected, processed on a computer – and the result: the influence of the environment is the main factor in their size.
Volcanic heat and shrinking sloths
About 15 million years ago, a massive volcanic eruption in northwest America created a basalt layer that covered an area of about 2.5 million square kilometers. The eruption lasted about 750 years and led to a global warming period called the "Middle Miocene Climatic Optimum."
As a result, sloths became smaller. The heat brought more rain, which expanded forests and benefited small species. Maintaining body temperature in the heat also requires less body mass – a phenomenon known in mammals.
Swelling again in the cold
After a million years, an era of gradual cooling began. As the temperature dropped, the size of the sloths grew again.
Ground sloths lived everywhere—from the Andes to the deserts of America to the forests of Canada. Some even adapted to life in the sea. A species called Thalassocnus lived between the Andes and the Pacific Ocean, ate sea grass, and had dense bones like those of a manatee.
To survive in harsh environments, sloths developed armor—so they could conserve energy, move efficiently, and fend off predators. Some carried tiny bony plates (osteoderms) in their skin—like their armadillo relatives.
They reached their peak size during the ice age – the Pleistocene – before becoming extinct.
The spread of humans and the end of the sloths
About 15,000 years ago, as sloths began to disappear, humans also appeared in North America. The sloths' size, which initially protected them from predators, became a disadvantage. They were not fast or sufficiently protected from human hunters.
Sloths that lived in trees survived for a while. Two species in the Caribbean survived until about 4,500 years ago—the time the pyramids were built in Egypt—and went extinct shortly after human settlement there.
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