By Natalia Mesa Published May 23, 2025 | Comments (1) | Giant ground sloths Megalocnus rodens and Megalonyx wheatleyi at the American Museum of Natural History © Dallas Krentzel Giant sloths with razor-sharp claws and as large as Asian bull elephants once roamed the Earth, snacking on leaves at the tops of trees with a prehensile tongue. Now, scientists have figured out why they became so huge—and why these massive sloths didn’t stick around—according to a new study published in Science. Today, two sloth species dwell in Central and South America. But long ago, dozens of sloth species populated the Americas, all the way from Argentina to Canada. Like modern-day sloths, the smaller species were tree-dwelling. But the larger sloths? “They looked like grizzly bears but five times larger,” Rachel Narducci, collection manager of vertebrate paleontology at the Florida Museum of Natural History and coauthor of the study, in a statement. The larger sloths didn’t do much tree climbing, at risk of falling to their deaths. Instead, they survived by being terrifying; the largest sloths had long, sharp claws that they used to carve their own caves out of raw earth and rocks. But exactly why they got so large remained a mystery. To figure out how these sloths got so massive, researchers analyzed ancient sloth DNA and compared more than 400 fossils from natural history museums to create a sloth tree of life. The researchers traced the sloths’ origin to 35 million years ago. And, because the scientists were particularly interested in how sloths got their size, they estimated their weights by taking fossil measurements. The researchers concluded that the Earth’s past climate was a big factor. Thirty-five million years ago, the first ancestor of modern-day sloths, which lived in what is now Argentina, was roughly the size of a large dog. Sloths hardly changed in size for 20 million years, and lived on the ground. Then, during a warming period around 16 million years ago, sloths adapted by evolving smaller physiques due to their need to keep cool. Then, as Earth cooled down again—which it’s been doing on and off for the past 50 million years—sloths started to get bigger and bigger. They also started to migrate, fanning out from Argentina throughout North and South America, and even up to Alaska and Canada. These new habitats presented challenges that the sloths met, in part, by bulking up. This new size also helped them keep warm and stay safe from predators. “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.” They reached their most massive size during the Pleistocene Ice Ages, which spanned roughly 3 million to 12,000 years ago, shortly before they disappeared. Scientists aren’t completely sure why sloths went extinct, but they do have some guesses. Early humans migrated to the Americas around 20,000 years ago. Larger ground-dwelling sloths likely became a prime, meaty target for early humans, and being on the ground became a liability. Larger sloths were the first to go, but tree sloths didn’t escape unscathed. Over time, more and more species of tree-dwelling sloths went extinct, too. Two species survived in the Caribbean until around 4,500 years ago—until humans wiped them out. Now, sloths mostly keep to Central and South America, but thankfully aren’t on the menu anymore. Daily Newsletter

The sloth family tree once sported a dizzying array of branches, body sizes and lifestyles, from small and limber tree climbers to lumbering bear-sized landlubbers.  Why sloth body size was once so diverse, while today’s sloths are limited to just two diminutive tree-dwellers, has been a long-standing question. Scientists have proposed that sloths’ body size might be linked to a wide variety of factors: habitat preferences, diets, changes in global temperature, or pressure from large predators or humans. Sign up for our newsletter We summarize the week's scientific breakthroughs every Thursday.

Sloths once came in a variety of sizes and lived in multiple settings in many parts of the world. A study in the journal Science examined sloth evolution over the past 35 million years, investigated multiple factors driving their growth and expansion throughout the world, and concluded that human hunting starting around 15,000 years ago drove their dramatic decline.Today, only six species within two genera remain. All are relatively small (especially compared to their largest ancestors) tree-dwellers that primarily live in the tropical rainforests of South and Central America.“These species are a tiny remnant of a once diverse American clade that was mostly made up of large-bodied species,” according to an editorial summary that accompanied the paper. Ancient Sloths Were Once WidespreadThat’s a huge contrast to sloth life during the late Cenozoic. During that period, more than 100 genera of sloths lived in a wide range of habitats and a variety of sizes, topping out at nearly 20 feet tall and weighing several tons.To investigate this diversity — and to track where, when, and why it collapsed — a team of scientists examined fossil measurements, DNA and protein sequences, and advanced evolutionary modeling. In doing so, they reconstructed sloth evolutionary history across 67 genera. They then investigated whether evolutionary changes in size were linked to habitat, diet, climate, predation, or other ecological pressures.Habitat Drove Sloth SizeThe findings show that habitat appeared to be a major driver in shaping their body size evolution. The earliest sloths were large and grazed on the ground. Some species adapted to tree dwelling and developed smaller body sizes. However shifts in both sloth size and dwelling didn’t happen in a straight line. The species size grew or shrunk as the climate warmed and cooled, and as ecosystems shifted from grasslands to woodlands.The species thrived for tens of millions of years, exhibiting the most variety in body sizes in the Pleistocene, which began about 2.6 million years ago.Ancient Humans Caused Dramatic DeclineThen, starting about 15,000 years ago, the creature experienced “a sudden and dramatic decline,” according to a press release.The researchers report that decline doesn’t mesh with any major known climate events. "Size disparity increased during the late Cenozoic climatic cooling, but paleoclimatic changes do not explain the rapid extinction of ground sloths that started approximately 15,000 years ago,” according to the paper. However, it does coincide with the expansion of humans into the Americas. The likely conclusion is that human hunting drove the extinction of the larger, ground-based sloths, while the smaller ones related to today’s creatures escaped by taking to the trees.Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.

Ancient sloths came in a variety of sizesDiego Barletta A cooling, drying climate turned sloths into giants – before humans potentially drove the huge animals to extinction. Today’s sloths are small, famously sluggish herbivores that move through the tropical canopies of rainforests. But for tens of millions of years, South America was home to a dizzying diversity of sloths. Many were ground-dwelling giants, with some behemoths approaching 5 tonnes in weight. Advertisement That staggering size range is of particular interest to Alberto Boscaini at the University of Buenos Aires in Argentina and his colleagues. “Body size correlates with everything in the biological traits of an animal,” says Boscaini. “This was a promising way of studying [sloth] evolution.” Boscaini and his colleagues compiled data on the physical features, DNA and proteins of 67 extinct and living sloth genera – groups of closely related species – to develop a family tree showing their evolutionary relationships. Unmissable news about our planet delivered straight to your inbox every month. Sign up to newsletter The researchers then took this evolutionary history, which covered a span of 35 million years, and added information about each sloth’s habitat, diet and lifestyle. They also studied trends in body-size evolution, making body mass estimates of 49 of the ancient and modern sloth groups. The results suggest sloth body-size evolution was heavily influenced by climatic and habitat changes. For instance, some sloth genera began living in trees – similar to today’s sloths – and shrank in body size as they did so. Meanwhile, three different lineages of sloths independently evolved elephantine proportions – and it seems they did this within the last several million years, as the planet cooled and the growth of the Andes mountains made South America more arid. “Gigantism is more closely associated with cold and dry climates,” says team member Daniel Casali at the University of São Paulo, Brazil. Many of these diverse sloths disappeared during two stages: one around 12,000 years ago and the other around 6000 years ago, says Boscaini. “This matches with the expansion of Homo sapiens, first over the entire American supercontinent, and later in the Caribbean,” he says — which is where some giant sloths lived. Notably, the only surviving sloth species live in trees so are much harder for humans to hunt than massive ground sloths. The idea that humans were the death blow for ancient megafauna is well-supported, says Thaís Rabito Pansani at the University of New Mexico, who wasn’t involved in the study. “However, in science, we need several lines of evidence to reinforce our hypotheses, especially in unresolved and highly debated issues such as the extinction of megafauna,” she says. The new evidence shores up this story. “Sloths were thriving for most of their history,” says Casali. “[The findings] teach us how a very successful [group] can become so vulnerable very quickly.” Journal reference:Science DOI: 10.1126/science.adu0704 Topics:evolution

Going for a stroll Carnivorous crocodile-like monsters used to terrorize the Caribbean While low sea levels helped sebecids spread, rising waters left them isolated. Elizabeth Rayne – May 16, 2025 1:10 pm | 21 Credit: By Ghedoghedo, CC BY-SA 3.0 Credit: By Ghedoghedo, CC BY-SA 3.0 Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more How did reptilian things that looked something like crocodiles get to the Caribbean islands from South America millions of years ago? They probably walked. The existence of any prehistoric apex predators in the islands of the Caribbean used to be doubted. While their absence would have probably made it even more of a paradise for prey animals, fossils unearthed in Cuba, Puerto Rico, and the Dominican Republic have revealed that these islands were crawling with monster crocodyliform species called sebecids, ancient relatives of crocodiles. While sebecids first emerged during the Cretaceous, this is the first evidence of them lurking outside South America during the Cenozoic epoch, which began 66 million years ago. An international team of researchers has found that these creatures would stalk and hunt in the Caribbean islands millions of years after similar predators went extinct on the South American mainland. Lower sea levels back then could have exposed enough land to walk across. “Adaptations to a terrestrial lifestyle documented for sebecids and the chronology of West Indian fossils strongly suggest that they reached the islands in the Eocene-Oligocene through transient land connections with South America or island hopping,” researchers said in a study recently published in Proceedings of the Royal Society B. Origin story During the late Eocene to early Oligocene periods of the mid-Cenozoic, about 34 million years ago, many terrestrial carnivores already roamed South America. Along with crocodyliform sebecids, these included enormous snakes, terror birds, and metatherians, which were monster marsupials. At this time, the sea levels were low, and the islands of the Eastern Caribbean are thought to have been connected to South America via a land bridge called GAARlandia (Greater Antilles and Aves Ridge). This is not the first land bridge to potentially provide a migration opportunity. Fragments of a single tooth unearthed in Seven Rivers, Jamaica, in 1999 are the oldest fossil evidence of a ziphodont crocodyliform (a group that includes sebecids) in the Caribbean. It was dated to about 47 million years ago, when Jamaica was connected to an extension of the North American continent known as the Nicaragua Rise. While the tooth from Seven Rivers is thought to have belonged to a ziphodont other than a sebacid, that and other vertebrate fossils found in Jamaica suggest parallels with ecosystems excavated from sites in the American South. The fossils found in areas like the US South that the ocean would otherwise separate suggest more than just related life forms. It's possible that the Nicaragua Rise provided a pathway for migration similar to the one sebecids probably used when they arrived in the Caribbean islands. Walking the walk So how did sebecids get from one land mass to the other on foot? They were made for it. Sebecids evolutionarily diverged from crocodiles during the Jurassic period. They had skulls similar to those of theropod dinosaurs, with a high rostrum (which holds the teeth along with the palate and nasal cavity) that was long and narrow. Their mouths were full of ziphodont teeth, which are compressed along the sides and have a serrated edge made for tearing flesh. Most important among the adaptations that made sebecids terrestrial animals were legs longer than their crocodilian brethren—legs made for walking on land. “Considering their terrestrial adaptations, their dispersal may have been either facilitated by some ephemeral terrestrial connection or string of large and closely spaced islands or occurred on a natural raft,” the research team said in the same study. Though they have been found across South America, earlier specimens of sebecids are best documented in the south of the continent, while later specimens surfaced in the north and tropical zones. Both the ziphodont teeth and concave vertebrae are found among the fossils found in the Dominican Republic, Cuba, and Puerto Rico, giving them away as sebecids. The locations of the fossils are consistent with the types of environments these carnivores were thought to inhabit as the Eocene gave way to the Oligocene. After they ended up in the Caribbean, the original population of sebecids eventually became isolated as sea levels rose, leaving the sub-populations on islands surrounded by water. The sebecids were apex predators in South America and are thought to have stayed at the top of the food chain in their new hunting grounds. Some sebecid remains have been found with fossils of terrestrial and semiaquatic vertebrates, such as sloths and turtles, that supposedly were their prey. Not only did sebecids get around, but they also lasted 5 million years longer in the Caribbean than they did in South America. This might have been because certain plant and animal species that died out on the mainland continued to survive on the islands. Crocodiles and predatory birds took over as apex predators after the sebecids died out. Even with a mouth full of knives, you can’t be at the top forever. Proceedings of the Royal Society B, 2025.  DOI: 10.1098/rspb.2024.2891 Elizabeth Rayne Elizabeth Rayne is a creature who writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. She lurks right outside New York City with her parrot, Lestat. When not writing, she is either shapeshifting, drawing, or cosplaying as a character nobody has ever heard of. Follow her on Threads and Instagram @quothravenrayne. 21 Comments