Giant ground sloths are extinct now, but scientists uncover what environmental factors helped them evolve in the first place.
By Brianna Cieszynski
The lovable sloth hasn’t always been limited to the branches of tropical rainforests. Giant terrestrial sloth species once roamed the ground and overshadowed their tree-dwelling relatives, until one earth-shattering event changed everything.
The life of a sloth is an uneventful one. They spend their days upside down lounging high up in the trees of tropical rainforests, eating leaves from a select few trees and moving as slow as 0.15 miles (0.24 kilometers) per hour. There are only two living types of sloths in the world—the two-toed and three-toed sloth—and only six species between these two families. Historically, however, dozens of species of sloth have existed over the course of more than 35 million years. Most of these didn’t live in trees at all due to their large size!
The largest of these terrestrial giants were in the genus Megatherium, reaching the size of an Asian bull elephant and weighing a staggering 8,000 pounds. How, then, did they turn into the tree-dwelling slowpokes we know now, and how did they get so big in the first place? Researchers have found answers to the mystery of giant ground sloths, discovering the secrets of their largely unknown history.
Life on the ground
Arboreal (tree-dwelling) sloths are known to be small, lightweight animals with an average weight of 14 pounds, and this is a necessity for staying alive in the fragile and easily breakable branches of large trees. Though tree sloths have reportedly survived falls of up to 100 feet, a tumble from the towering trees of the Amazon rainforest spells out certain disaster for the lethargic creatures. Ground sloths didn’t have this requirement, so in some ways, their large stature is no surprise.
“They looked like grizzly bears but five times larger,” said Rachel Narducci, collection manager of vertebrate paleontology at the Florida Museum of Natural History. She is also the coauthor of a new study aiming to discover why there were such excessive size differences across extinct sloth species. Researchers studied their history using the museum’s access to “the largest collection of North American and Caribbean-island sloths in the world,” according to Narducci.
Giant ground sloths could use their long, prehensile tongues to eat leaves from the tops of trees the way giraffes can. They had gigantic claws that they used to carve out their own caves as shelters, many of which are still left behind today as a relic of their incredible abilities. Their large size may have assisted them in finding food and avoiding predators, giving them an evolutionary advantage, or it could have been affected by the climate or their metabolic rates. Through collecting information on the shapes of skulls across extinct and living sloth species as well as taking measurements on hundreds of museum fossils, the scientists discovered the real cause: climate change. More specifically, sloth evolution is tied to a cataclysmic event that transformed the Earth’s atmosphere for a million years after.
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Sudden growing pains
After 20 million years of relatively unchanged sloth behavior—some growing an affiliation with tree-climbing while the largest roamed the ground—one volcanic event changed everything. A massive eruption of the Columbia River flood basalt resulted in magma spilling out of a wound in the ground, spanning across 600,000 cubic miles over the Pacific Northwest. The event lasted 750,000 years, emitting high levels of greenhouse gases linked to a period of global warming called the Mid-Miocene Climatic Optimum. The rising temperatures caused a shift in the sizes of sloths, as they reacted to the new levels of heat stress by becoming smaller. More habitat for small sloths was also being created due to the increased precipitation caused by higher temperatures, expanding forests and possibly allowing an advantage for sloths that could fit in trees.
As the Earth gradually cooled down at the end of this period, sloths started to grow once again to compensate, creating a separation between tree and ground sloths. While arboreal and semi-arboreal sloths stuck to the trees, giant ground sloths traveled through forests, mountains, deserts, savannahs. There were even semi-aquatic sloths that were strong swimmers and got their food from the ocean. The changes in Earth’s climate accurately explains the size differences between these species and the ability for some to grow to such impressive heights.
Being bigger in such varied habitats allowed them to “conserve energy and water and travel more efficiently across habitats with limited resources,” said Narducci. “And if you’re in an open grassland, you need protection, and being bigger provides some of that.” So if sloths benefited from their continuous growth spurt, what caused their ultimate extinction?
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The end of giant ground sloths
Even with the advantage of an increased variety of habitats and optimized energy conservation, giant ground sloths eventually disappeared, with the drop-off occurring about 15,000 years ago. Their extinction lines up with the migration of early humans to North America, making the likely cause of their demise clear. Their large size, which had kept them protected and warm for so many years, made them an easy target. Though the exact cause of their disappearance is still debated, it is probable that they were hunted to extinction by humans.
In the end, only the tree sloths persisted, remaining high up in the trees and narrowly avoiding extinction. Only a handful of sloth species exist in the current day, all tree-dwelling and much smaller than their terrestrial counterparts.
This study was published in the peer-reviewed journal Science.
Reference
Bargo, M. S., Boscaini, A., Cantalapiedra, J. L., Casali, D. M., De Iuliis, G., Gaudin, T. J., Langer, M. C., Narducci, R., Pujos, F., Soto, E. M., Soto, I. M., Toledo, N., Vizcaíno, S. F. (2025). The emergence and demise of giant sloths. Science, 388(6749), 864–868. https://doi.org/10.1126/science.adu0704
Featured image: Illustration by Diego Barletta, captioned “Ancient sloths lived in trees, on mountains, in deserts, boreal forests and open savannahs. These differences in habitat are primarily what drove the wide difference in size between sloth species.” Licensed under CC BY.
About the Author
Brianna Cieszynski is a senior year Zoology student at Michigan State University aiming to educate others about environmental conservation and news. She works as a Community Engagement and Inclusion intern at a local zoo and spends her free time either at the library or the theater. Follow her on Instagram @briecieszynski.
