“20-foot sloths” is a popular label, not a reliable measurement of every giant ground sloth. The best-known giant, Megatherium americanum, stood about 3.5 metres (12 feet) tall when upright and weighed as much as four tonnes, according to the Natural History Museum. Those figures describe standing height and mass—not a 20-foot upright height. Its extinction, like that of other ground sloths, cannot be pinned on one proven cause: evidence points to human interaction and environmental change, with the details varying by species and region.
What were the “20-foot sloths”?
They were giant ground sloths: extinct relatives of living sloths, not bears or dinosaurs. The phrase usually evokes Megatherium americanum, one of the largest known species. The Natural History Museum reports that it could weigh up to four tonnes and reach about 3.5 metres (12 feet) standing on its hind legs. Those are separate measurements; neither supports calling it 20 feet tall.
The scale of extinct sloths varied widely. A 2025 University of Florida report on a study published in Science described researchers analyzing ancient DNA and comparing more than 400 fossils from 17 natural history museums to investigate how and why extinct sloths evolved large bodies. That collection-wide figure does not mean every fossil was Megatherium. Florida Museum of Natural History vertebrate paleontology collection manager Rachel Narducci put the giant’s appearance this way: “They looked like grizzly bears but five times larger,”
How did Megatherium live?
It was a plant-eater despite its claws
Chemical analysis of its teeth indicates that Megatherium was herbivorous. Its enormous claws may look like weapons, but their presence is not evidence that it hunted other animals. The animal could stand and walk bipedally, bringing its great size to bear as it reached for vegetation.
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Its fossils place it in South America
The Natural History Museum reports Megatherium fossils from Argentina, Uruguay and Bolivia. Its account gives a fossil time range of around 400,000 years ago to the start of the Holocene, around 8,000 years ago. That range applies to reported evidence for Megatherium americanum, not to every ground sloth species.
What caused giant ground sloths to go extinct?
There is no established single explanation for the extinction of all ground sloths. The evidence is better understood by separating direct signs of human contact from region- and species-specific evidence of environmental change.
Humans interacted with at least some giant sloths
People and Megatherium overlapped in time. Cut marks on fossils suggest that humans butchered or ate these animals. That is evidence of interaction, but it does not establish that hunting alone drove the species to extinction—or that the same explanation applies to every ground sloth.
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Climate and vegetation changes mattered, but not uniformly
The Natural History Museum says the exact causes of Mylodon darwinii’s extinction are not fully understood, while identifying climate change and its effects on vegetation as almost certainly partial contributors. That statement concerns Mylodon, not Megatherium or all ground sloths.
Other studies reinforce the need to keep conclusions tied to place and taxon. A 2019 Nature Communications analysis found patterns consistent with mixed causes across megafaunal taxa; its result for the Shasta ground sloth did not appear linked to Clovis-period hunting. A 2023 Science study of Rancho La Brea in southern California found that camels and ground sloths disappeared there earlier than several other large mammals, alongside aridification and tree loss. That is a regional sequence, not a universal account of South American Megatherium extinction.
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- Size: The Natural History Museum’s figures for Megatherium americanum are up to four tonnes and about 3.5 metres (12 feet) upright. They do not establish a 20-foot standing height.
- Human role: Cut-marked fossils support contact and possible butchery, not a sole-cause hunting verdict.
- Environmental role: Climate and vegetation change are plausible contributors, with direct institutional guidance here applying specifically to Mylodon and separate regional findings from Rancho La Brea.
- Overall conclusion: Ground sloths were diverse, and extinctions unfolded across different places and taxa. The available findings do not justify one cause for all of them.
Where to see ground sloth fossils and models
The Natural History Museum’s Megatherium explainer covers its size, range and fossil evidence. Its Darwin’s ground sloths collection guide discusses several taxa, including Mylodon, and points to digitized 3D specimen models available through the museum’s data portal and Sketchfab.
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