Sloths may seem like quirky tree-dwellers today, but their past reveals a dynasty of giants that once roamed the Americas.

Most people know sloths as slow-moving, bear-like creatures that dangle from trees, take nearly a month to digest a single meal, and defecate only once a week. Their closest relatives are anteaters and armadillos, which may sound like an unusual connection, but evolutionary history explains the link. Today only two sloth species exist, but in the past there were dozens, including one with a bottle-shaped snout specialized for eating ants and another that closely resembled early armadillos.

Many of these ancient sloths were far too large to live in trees. The giants of the group, belonging to the genus Megatheriumgrew to the size of Asian bull elephants and weighed 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 study published in Science in which researchers examined ancient DNA and analyzed more than 400 fossils from 17 natural history museums to understand how and why some sloths reached such massive proportions.

Ground-dwelling sloths displayed an extraordinary range of body sizes. At one extreme was the enormous Megatherium, capable of stripping leaves from tall trees with its long, flexible tongue, serving as an ecological counterpart to giraffes. At the other was the comparatively smaller Shasta ground sloth, which thrived in the deserts of North America by feeding on cacti.

Tree-dwelling sloths, however, followed a different evolutionary path. Those that lived exclusively in the forest canopy have always been small, averaging about 14 pounds. Species that split their time between the ground and the trees were somewhat larger, weighing an average of 174 pounds.

You don’t have to be a scientist to puzzle out why trees enforce a strict weight limit. It’s the same reason why modern tree sloths have a strange elastic quality to them: Branches break when put under too much strain, and sloths are not generally known for their ability to swiftly avert sudden disaster. Tree sloths have reportedly survived falls of up to 100 feet. However, given that falls from even moderate heights can cause severe damage and some trees in the Amazon Rainforest top out at just under 300 feet, it makes evolutionary sense to be as small as possible when going out on a limb.

Why Did Ground Sloths Get So Big?

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.

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 and various types of data.

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.

Climate and Habitat as Key Drivers

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 Pseudoglyptodonwhich 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 Mylodonlikely 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.

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.

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.

Adaptations for Survival

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.”

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.

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 them 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

Funding: Fund for Scientific and Technological Research, National Council for Scientific and Technical Research, National Geographic Society, São Paulo Research Foundation, Madrid Government Talent, European Union Newgenerationu/PRTR, National University of La Plata

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This article is republished from scitechdaily.com on 2025-09-27 07:21:00. Content and views belong to the original publisher, not UAE Today News.