Posted on 06/06/2025 5:44:17 AM PDT by Red Badger
New research shows 8,000-pound sloths once dug caves, adapted to oceans, and roamed widely—until climate and humans brought their downfall. (Artist’s concept.) Credit: SciTechDaily.com
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Long before they became tree-hugging symbols of chill, sloths were 8,000-pound giants that roamed deserts, dug caves into cliffs, and even swam like manatees.
Scientists have now pieced together the epic story of their evolution, using ancient DNA and hundreds of fossils to explain how sloths once grew to mammoth proportions—bigger than most cars—and why they eventually shrank or vanished altogether.
Sloths’ Strange Family Tree
Most of us know sloths as the slow-moving, tree-loving creatures that take an entire month to digest a meal and only poop once a week. They might look a bit like small, furry bears, but their closest living relatives are actually anteaters and armadillos. If that seems like an odd evolutionary match, you’re not alone—and there’s a fascinating reason why.
Today, there are just six species of sloths. But in the past, there were dozens, including some with bottle-shaped snouts that munched on ants, and others that looked like early versions of armadillos.
The Elephant-Sized Ancestors
Many of these ancient sloths didn’t live in trees at all. They were simply too large. The biggest of them, known as Megatherium, reached the size of modern-day Asian elephants and tipped the scales at around 8,000 pounds.
“They looked like grizzly bears but five times larger,” said Rachel Narducci, collection manager of vertebrate paleontology at the Florida Museum of Natural History.
Narducci co-authored a new study published in Science, where researchers analyzed ancient DNA and over 400 sloth fossils from 17 different museums to answer a long-standing mystery: how did some sloths get so enormous?
Scientists analyzed ancient DNA and compared more than 400 fossils from 17 natural history museums to figure out how and why extinct sloths got so big. Credit: Florida Museum of Natural History photo by Kristen Grace
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A Spectrum of Sloth Sizes
It turns out ancient ground sloths came in a wide range of sizes. Some, like the gigantic Megatherium, used long tongues to grab leaves from tall trees, filling a role similar to that of today’s giraffes. Others, like the Shasta ground sloth, were more modestly sized but still robust, roaming deserts in what is now the southwestern United States and feasting on cacti.
Tree-dwelling sloths, on the other hand, have always been small. Species that spend their entire lives in the treetops average about 14 pounds. Those that split their time between ground and canopy weigh closer to 174 pounds.
There’s a clear reason for this size limit. Trees can only support so much weight before branches snap. Sloths aren’t built for quick escapes, so falling is a serious risk. Some modern sloths have survived tumbles from as high as 100 feet. But with Amazonian trees stretching nearly 300 feet tall, evolution favors staying small when you live high off the ground.
The Mystery of Megasize
What’s less clear is why some ground sloths grew to such excessive sizes while others seemed content with being merely large. There may have been several reasons, which is why it’s been so hard for scientists to answer the question with confidence.
Larger sizes might have been advantageous for finding food or avoiding predators, for example. Ground sloths had a special fondness for caves, and their size undoubtedly played a role in their ability to find and make shelters. The moderately sized Shasta ground sloth favored small, natural caves bored by wind and water into the cliffsides of the Grand Canyon, like the alveoli of a gigantic, geologic lung. These also doubled as convenient latrines; in 1936, paleontologists discovered a mound of fossilized sloth poop, bat guano and packrat middens more than 20 feet thick in Rampart Cave, near Lake Mead.
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. Credit: Diego Barletta, edited Digging Their Own Dens
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Larger sloths weren’t restricted to pre-existing caves. Using claws that are among the largest of any known mammal, living or extinct, they could carve their own from bare earth and rock. Many of the caves they left behind are still around with claw-mark décor along the interior walls, evidence of their ancient nesting excavations.
Other factors that may have contributed to their size discrepancy include climate, the degree of relatedness among sloth species, and metabolic rates. The ability to accurately discriminate between these several possibilities required a substantial amount of and various types of data.
Building the Sloth Tree of Life
The authors combined information about the shape of fossils with DNA from living and extinct species to create a sloth tree of life that traced the sloth lineage all the way back to their origin more than 35 million years ago. With this scaffold in place, they added results gleaned from decades of research about where sloths lived, what they ate and whether they were climbers or walkers. Because the authors were specifically interested in the evolution of size, they collected data for the final analytical ingredient by measuring hundreds of museum fossils, which they used to estimate sloth weight.
This is where the Florida Museum played a special role. “We have the largest collection of North American and Caribbean-island sloths in the world,” Narducci said. She carefully took several measurements of 117 limb bones and shared the numbers with her colleagues.
The authors mixed all this information together, computationally stirred it, and got back a fully baked answer.
Habitat Drove Sloth Size
The result: Size differences among sloths have been primarily influenced by the types of habitats they lived in and, by extension, climate change.
“Including all of these factors and running them through evolutionary models with multiple different scenarios was a major undertaking that had not been done before,” Narducci said.
The sloth dynasty coincided with significant, life-altering changes in Earth’s climate. The oldest thing that scientists can reasonably consider to be a sloth is called Pseudoglyptodon, which lived 37 million years ago in Argentina. Analyses from the study indicate the earliest sloths would have likely been small ground dwellers, about the size of a great Dane. At various points throughout their evolutionary history, sloths adopted a semi-arboreal lifestyle. Not all of them stayed in the trees, however. The largest sloths, including Megatherium and Mylodon, likely evolved from a tree-adapted sloth that ultimately decided to stay firmly planted on the ground.
Against this background of indecisive climbers and walkers, the size of sloths hardly changed at all for about 20 million years, irrespective of their preferred method of locomotion. Then something earth-shattering occurred.
Volcanic Heat and Shrinking Sloths
A giant wound opened up between modern-day Washington state and Idaho down through parts of Oregon and Nevada, and magma boiled out of it. This left a nearly 600,000 cubic mile scab over the Pacific Northwest. It’s still visible in some places along the Columbia River, where millions of years of running water have cut through and polished a colonnade of basalt. These rock pillars have a distinct hexagonal shape caused by the way in which the magma hardened and cracked as it cooled. The volcanic event that made them was a slow burn that lasted roughly 750,000 years and aligned with a period of global warming called the Mid-Miocene Climatic Optimum. The greenhouse gasses emitted by the volcanic eruption are currently considered the likeliest cause of the warming.
Sloths responded by getting smaller. This may be because warmer temperatures brought increased precipitation, which allowed forests to expand, thereby creating more habitat for smaller sloths. Size reduction is also a common way for animals to deal with heat stress and has been documented in the fossil record on several different occasions.
Bigger Bodies for Colder Climates
The world remained warm for about a million years after the volcano fell silent. Then, the planet resumed a longstanding pattern of cooling that has continued in fits and starts to the present. Sloths reversed course too. The more temperatures fell, the bulkier they became.
Arboreal and semi-arboreal sloths had the obvious limitation of having to live near trees, but ground sloths lived just about anywhere their feet would take them. They climbed the Andes Mountains, fanned out through open savannahs, migrated into the deserts and deciduous forests of North America and made a home for themselves in the boreal forests of Canada and Alaska. There were even sloths adapted to marine environments. Thalassocnus lived in the arid strip of land between the Andes and the Pacific. They survived in this harsh region by foraging for food in the ocean.
“They developed adaptations similar to those of manatees,” Narducci said. “They had dense ribs to help with buoyancy and longer snouts for eating seagrass.”
Natural Armor and Cold Resistance
These varied environments presented unique challenges that ground sloths met, in part, by beefing up. “This would’ve allowed them to conserve energy and water and travel more efficiently across habitats with limited resources,” Narducci said. “And if you’re in an open grassland, you need protection, and being bigger provides some of that. Some ground sloths also had little pebble-like osteoderms embedded in their skin,” Narducci said, referencing the bony plating that sloths had in common with their armadillo relatives, a trait that was also recently discovered in spiny mice.
Equally as important, larger bodies helped sloths contend with cooling climates. They reached their greatest stature during the Pleistocene ice ages, shortly before they disappeared.
“About 15,000 years ago is when you really start to see the drop-off,” Narducci said.
Humans and the Final Fall
There’s still debate about what happened to sloths, but given that humans arrived in North America at about the same time sloths went extinct in droves, it’s not hard to speculate. Paradoxically, the large size that kept them safe from most predators and insulated from the cold became a liability. Neither fast nor well-defended, ground and semi-arboreal sloths were easy pickings for early humans.
Arboreal sloths watched the carnage unfold below them from the safety of the treetops, but even there, they didn’t escape without losses. Long after their ground-dwelling relatives had gone extinct everywhere else, two species of tree sloth in the Caribbean held out until 4,500 years ago. Humans arrived in the Caribbean about the same time that Egyptians were building the pyramids. Caribbean tree sloths went extinct not long after.
Reference:
“The emergence and demise of giant sloths”
by Alberto Boscaini, Daniel M. Casali, Néstor Toledo, Juan L. Cantalapiedra, M. Susana Bargo, Gerardo De Iuliis, Timothy J. Gaudin, Max C. Langer, Rachel Narducci, François Pujos, Eduardo M. Soto, Sergio F. Vizcaíno and Ignacio M. Soto, 22 May 2025, Science.
DOI: 10.1126/science.adu0704
Alberto Boscaini, Néstor Toledo François Pujos, Eduardo Soto, Sergio Vizcaíno and Ignacio Soto of the Consejo Nacional de Investigaciones Científicas y Técnicas, Daniel Casali, Susana Bargo of the Universidad Nacional de La Plata, Max Langer of the Universidade de São Paulo, Juan L. Cantalapiedra of the Universidad de Alcalá, Gerardo De Iuliis of the University of Toronto and Timothy Gaudin of the University of Tennessee at Chattanooga are also co-authors of the study.
Funding for the study was provided in part by the Fondo para la Investigación Científica y Tecnológica (grant no. PICT 2020-0851), the Consejo Nacional de Investigaciones Científicas y Técnicas (grant no. PIBAA 28720210101086CO), the National Geographic Society (grant no. EC-44712R-18), São Paulo Research Foundation (grant no. 2022/00044-7), Madrid Government Talent Attraction (grant no. 2017-T1/AMB5298, the European Union NextGenerationEU/PRTR (grant no. CNS2023-144573), the Universidad Nacional de La Plata (project N997) and the Fondo para la Investigación Científica y Tecnológica (grant no. PICT 2021-0426).
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Mmmm...sloth
OMG! How awful, those mean old nasty humans meant the demise of these poor sloths. And to think the caveman’s invention of fire led to glowbull warming.
There is a solution to all of this we must embrace. Humans, in particular the white race, must be eliminated. The only allowable exceptions will be the wealthy elites that want global power. They may also have people of color as their servants. /s/
I’m making that sarcastic remark because of the way the thread title is written.
It was POC’s that made them extinct...............
Tastes like.......................MAMMOTH!..............
Giant Sloth = GIANT BBQ’s..........................
sooo, native populations hunted it to extinction? you don’t say?
Invaders....................
The pictures don’t show armor.
Humans and their spears killed killed all the sloths and every other species over 150 lbs in the Holocene. And it happened all at once. Could not possibly have been anything else. Humans really are that bad
They were so slow they didn’t show up for the photo shoot................
CoVid-0000001........................
Michelle? Is that yo hairy as.....
Flood, Fire, and Famine
in the History of Civilization
by Richard Firestone,
Allen West, and
Simon Warwick-Smith
They sold it on eBay.
And they probably let a lot of it go to waste. A couple of tons of leftover sloth might require refrigeration.
And what was the co2 level when these giant sloth roamed?
The natives were not that good servants of the land as they are described.
They caused massive extinctions all around the world.
E.g. Maori landed at the shores of New Zeeland only like 1300. Yet they managed massive extinction of New Zeeland species like Moa or Haast Eagle.
Very quickly!
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