Currently, only two species of sloths exist, but in the past, numerous species roamed the Earth, including one with a nose resembling a bottle and another akin to the ancestors of modern armadillos. Most of these extinct sloths were too massive to inhabit trees. The largest of them all – a member of the genus Megatherium – reached the size of an Asian bull elephant, weighing around 3.63 tons (8,000 pounds).

The ancient sloths inhabited a variety of environments – trees, mountains, deserts, northern forests, and open savannas – leading to wide differences in size among sloth species. Image credit: Diego Barletta.

“They resembled Grizzly Bears but were five times larger,” noted Dr. Rachel Nalducci, collection manager for vertebrate paleontology at the Florida Museum of Natural History.

In a recent study, Dr. Nalducci and her team examined ancient DNA and compared over 400 fossils from 17 museums to explore how and why extinct sloths achieved such sizes.

The size difference ranges significantly between the massive Megatherium and the more modestly sized Shasta Sloth, which thrived on cacti in the arid regions of North America.

Conversely, the sloths adapted for tree climbing were uniformly small, averaging around 6 kg (14 lbs), with ground-dwelling sloths like them averaging about 79 kg (174 lbs).

Ground sloths had a strong affinity for caves, where their size played a crucial role in finding and creating shelters.

The moderately sized Shasta Sloths ventured into the depths of geological lungs formed by wind and water, creating magnificent canyons.

The larger sloths were not limited to pre-existing caves; using their sizeable claws, the largest known mammalian claws (whether extinct or extant), they could carve out their shelters from bare earth and rock. Evidence of their nests can still be found today, with claw marks adorning the cave walls.

Other factors influencing size variations likely include climate, interspecies interactions, and metabolic rates.

To accurately analyze these factors, a substantial amount of diverse data was required.

The authors integrated fossil shape data with DNA from both living organisms and extinct species to construct sloth lineages dating back over 35 million years.

With this foundation established, they incorporated findings from decades of research about the habitats sloths occupied, their diets, and their behavioral patterns.

Paleontologists maintain a keen interest in the evolution of size, collecting extensive data from numerous museum fossils for their analyses.

Sloth size differences are significantly shaped by the habitats they inhabited, which were also influenced by climate change.

“This research, employing evolutionary models across various scenarios and accounting for all these factors, is a considerable undertaking unlike anything previously attempted,” remarked Dr. Nalducci.

The sloth lineage witnessed dramatic life-altering changes as Earth’s climate evolved.

The earliest recognized sloth, Pseudoglyptodon, inhabited Argentina around 37 million years ago.

Analysis from the research team suggests that early sloths were likely small, ground-dwelling creatures, about the size of a Great Dane.

Throughout evolution, sloths occasionally adopted semiarboreal lifestyles.

However, not all sloths remained tree dwellers; the largest species, including Megatherium and Mylodon, likely evolved from tree-adapted ancestors but ultimately thrived on the ground.

Amidst this mixture of arboreal and terrestrial creatures, sloth size remained relatively stable for about 20 million years, regardless of their activity preferences until a significant geological event occurred.

A great rift opened between present-day Washington and Idaho, extending through parts of Oregon and Nevada, releasing magma.

This process left a staggering 600,000 cubic miles of volcanic material in the Pacific Northwest, still observable today along the Columbia River, where flowing water has sculpted basalt into distinct columns.

These formations exhibit a unique hexagonal shape, resulting from the magma’s cooling and cracking processes.

The volcanic activity that formed these structures occurred over a slow burn spanning approximately 750,000 years, aligning with a period of global warming known as the mid-Miocene climate optimum.

Greenhouse gases released by these volcanic eruptions are presently thought to be a primary factor in that period of warming.

As a response, sloths began shrinking in size, possibly due to warmer temperatures leading to increased rainfall, which expanded forest habitats, offering more room for smaller species.

Size reduction is a common adaptive strategy among animals coping with heat stress, as recorded in various fossil records.

After volcanic activity ceased, the world remained warm for nearly a million years before returning to a longstanding cooling trend that persists today. Sloths also adapted, growing bulkier as temperatures decreased.

Arboreal and semiarboreal sloths were inherently limited by their need for tree proximity, but ground-dwelling sloths roamed freely in diverse terrains.

They trekked up the Andean mountains, traversed open savannas, migrated through deciduous forests in North America, and built homes in Canada and Alaska’s boreal forests.

There were even sloths adapted to marine environments. Thalassocnus thrived on dry land between the Andes and the Pacific Ocean, sourcing food from the ocean to survive in such harsh conditions.

“They developed adaptations analogous to those of the Manatee,” Dr. Nalducci explained.

“Their dense ribs aided buoyancy, and elongated noses facilitated foraging for sea grass.”

These varied environments posed unique challenges that ground sloths creatively navigated.

“Such adaptations would have conserved energy and water, enabling more efficient movement through resource-limited habitats,” Dr. Nalducci remarked.

“In open grasslands, protection is crucial, and increased size offers some degree of safety.”

“Some ground sloths also had sparse bone structures embedded in their skin, akin to pebbles.”

Moreover, larger body sizes played a vital role in helping sloths endure the cooling climate.

They reached their peak size during the Pleistocene Ice Age, just before their eventual disappearance.

“Around 15,000 years ago is when notable population declines began,” Dr. Nalducci observed.

While debates continue regarding the reasons for sloth extinction, it coincided with human arrival in North America around the same time, leading to significant declines in sloth populations.

Ironically, the very size that provided protection from typical predators and insulated them from cold climates also contributed to their downfall.

Fast but well-defended, ground sloths proved difficult prey for early humans.

Tree-dwelling sloths observed the devastation unfold below from the safety of their heights, yet they too faced losses.

Two Caribbean sloth species managed to survive until about 4,500 years ago, after their terrestrial relatives had vanished elsewhere.

Humans reached the Caribbean around the same time the Egyptians were constructing pyramids, resulting in the quick extinction of Caribbean sloths.

“Paleoclimate changes cannot solely account for the rapid extinction of ground sloths beginning around 15,000 years ago,” the researchers asserted.

“Their sudden disappearance indicates human-driven factors were likely at play in the decline and extinction of sloths on Earth.”

Study published in the May 22nd, 2025 edition of the journal Science.

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Alberto Boscani et al. 2025. The appearance and end of giant sloths. Science 388 (6749): 864-868; doi: 10.1126/science.adu0704

This article is based on a press release from the Florida Museum of Natural History.