Why did the Giant Ground Sloths Vanish?

Added: 2026-05-29

[00:00:00]In northern Brazil, not far from where the Amazon meets the sea, there is a vast underground structure just below the surface. A massive tunnel that predates human arrival in South America.

[00:00:12]The tunnel has multiple branches, totals around 1,500 m long, and reach as high as 2 m tall and 4 m wide in some places, with the walls heavily marked with lacerations from large claws. The scratch marks revealing the architects of these burrows. They were made by giant ground sloths the size of hippos.

[00:00:30]These tunnels are the largest burrows ever built by a non-human animal and some of the largest structures made by animals known to exist. The northern burrows are noteworthy for their sheer size. But the tunnel networks made by these creatures are better known from southern Brazil and Argentina where they are actually fairly abundant. Left behind by creatures that once upon a time were equally abundant. Not all of them borrowers. They were diverse and inhabited many ecosystems in every corner of North and South America, having as many as 80 species adapted to live in the desert, tundras, and forests, some growing to the size of African elephants. And they were seamlessly part of an ecosystem that contains animals still living today.

[00:01:09]Their remains are so abundant and large that they were some of the first extinct creatures to be discovered. But then, as little as 10,000 years ago, they vanished, just leaving their small climbing cousins in the treetops.

[00:01:28]Ground sloths, just like sloths living today, were incredibly slow. Or more accurately, they had a very slow metabolism, and this was their secret weapon. Sloths have one of the slowest metabolic rates of any known mammal.

[00:01:40]Study of the brownthroatated sloth has found their metabolism is somewhere between 40 to 74% of the speed of an average mammal of the same size. Sloths also live with much greater body temperature swings. The vast majority of mammals alive today are strict endtherms, where their metabolic heat and other features hold their temperature tightly within a very strict temperature range. Most mammals, including humans, have a body temperature that can't healthfully fluctuate more than about 1° centigrade.

[00:02:07]But it is completely normal for a sloth's body temperature to change by about 10° over the course of the day and optimally sits much lower than most mammals around 30°. Due to producing less metabolic heat, their surrounding temperature still has a decent amount of influence over their body temperature, and they even bask in the sun to warm up in a similar way to reptiles. They are what are known as heterrothermic. They are warm-blooded, but their body temperature can also be similar to their surroundings, usually when they are inactive. The practical advantage of this is that they require significantly less food than similarsized mammals.

[00:02:40]Howler monkeys have a very similar body size to three-toed sloths, share the same habitats, and have a similar diet of mostly leaves, yet have to eat almost three times as much food to survive. A 2025 study examined the teeth of various extinct ground sloth species to gauge their body temperature. When an animal builds its teeth, it incorporates rare isotopes that bond at certain temperatures. By examining these bonds, scientists can accurately calculate the exact temperature at which the minerals were formed. Tooth enamel is very robust and so the chemical makeup from inside a tooth can be unchanged for thousands of years. Meaning the body temperature of long dead creatures can be examined. The teeth of four different species of extinct sloth was studied. Aeromarium, megatherthetherium, promethereum, and notherops alongside the temperatures of the habitats they lived in. It was found their body temperatures were not too dissimilar to modern sloths, suggesting they may have been less endothermic than most mammals too and have slower metabolisms. The very largest ground sloths like meggathereum and aromatherium broached the size of elephants and in the case of aromatherium may have even surpassed the size of a very large African bull elephant being one of the largest land mammals that has ever lived. Due to their very large size, there is debate over what their fur covering would have been like. Patagonia was home to a cow-sized sloth named Milodon Darwini that was adapted to live in freezing temperatures that would have dropped well below zero when they were alive and have remains found in even the most southern parts of South America. Myodon sheltered in caves across Patagonia and the cold and dry conditions mean Myodon remains can be found mummified with soft tissues like skin and fur still surviving and they had thick fur that was reddish brown or a light sandy brown color. Although not as extensive, fur samples from other ground sloths have been discovered from species that lived in hot climates. Most sloths had fur, but it was thought that similar to animals like rhinos and elephants, the larger sloths may have adapted lighter fur covering due to losing heat at a slower rate. As they had slower metabolisms and the surrounding temperature played a bigger part in their core temperature, it means that they may have had more fur than modern animals of a similar size. Aramomthereum lived in tropical climates whereas meggathereum generally inhabited cooler, more open plain environments where thicker fur coverings may have been more likely. The lower metabolic rate of sloths was inherited from an incredibly ancient ancestor. Brown sloths and modern sloths are not very closely related to the vast majority of mammals living today and are survivors of an ancient South American lineage of mammals named the zenathans which include antiters and armadillos. All these creatures are unified by their strange joints. Xenos meaning strange and Arthuros referring to their joints.

[00:05:16]All of the Zenans have unique extra articulations in their lower spine that help reinforce their lower back and make it stiffer. Having a more robust lower back allows them to rear up and use their forlims more. Most likely originally evolving for digging.

[00:05:30]Interestingly, modern sloths have actually reduced their extra spine articulations, but they were still very prominent in ground sloths being more similar to others in Arthur and is one of the key differences in their skeletons. The vast majority of mammals are from a giant mamalian group named the Borathetheria that encompasses about 90 to 95% of all mammal species alive today, including bats, whales, rodents, primates, and carnivorous mammals, etc. And interestingly, this massive grouping of mammals aren't the closest living group to zen. The majority of genetic studies show they are actually more closely related to another very small group of mammals that separated from the other mammals tens of millions of years ago, known as the aphoria. The Afritheria mostly contains African animals like elephants, manatees, and arvarks. It seems strange these two small lineages of niche mammals on completely separate continents are related, but there is an explanation.

[00:06:22]The first fossils of Sanins aren't known in the fossil record until around 60 million years ago. And due to genetic studies, it is known that they separated from other mammals much longer ago. DNA analysis has shown that it is highly likely that Zenathans and the Afrotheria diverged around 90 to 105 million years ago.

[00:06:40]before eventually joining to North America and before a long period of complete isolation. South America's last connection to another continent was Africa. Geological evidence shows that the continent broke apart around 100 million years ago during the Cretaceous period and so coincides with the Zenathan's divergence. It is thought that the natural barrier of the ocean created in Vicarians with the Afrotheria evolving in Africa and the Zenathans evolving in South America in isolation.

[00:07:05]It isn't just sloths that have a slow metabolism. And this is actually something that is shared by ols and arans likely inherited from an ancient common ancestor. However, sloths have specialized into this even further as the three-toed sloth has the slowest metabolism of any mammals ever studied, including egg laying mammals like platypus and akidnas. Interestingly, the 2025 study shows that the metabolic rate of ground sloths, while extremely low for a mammal, was likely higher than modern sloths and probably more comparable to others like armadillos and anteers. Due to the relative safety of the trees, it is likely climbing sloths were able to evolve to be even slower and conserve even more energy. The earliest slots in the fossil record show up around 35 million years ago. But during the meioene 23 to 5 million years ago is when they became significantly more common and diverse as well as their population spreading continentwide. The earliest sloth that is known from extensive fossils was a small ground dwelling creature named hapalops that is mainly known from Argentina. It was one of the smallest ground sloths ever discovered, only being around 1 m long, walking on its knuckles, sharing a habitat with giant armadillos, the ancestors of gipadons, predatory marsupials, and halfton flightless bird apex predators. Fossils of some familiar animals like iguanas were also found in the same habitat. All living sloth species belong to two major groups. The bradapus or three-finger sloths and the calopus or two-fingered sloths. And these two groups are not closely related. In fact, DNA evidence shows they had already diverged long before Hapalops was living, most likely around 30 to 40 million years ago. They had ground dwelling ancestors and Bradypus and Calopus sloths adapted to life in the trees independently. Bradypus sloth being more closely related to certain ground sloth like megalonics and megthereum than they are to calop sloths. During the Mayene, sloths reached their most populous and most diverse, adapting to fill many different niches. Due to shifts in global ocean currents and some other factors, the meiosene saw a planetwide cooling trend.

[00:09:02]This was particularly problematic in South America and was accompanied by the forming of the Andes. The mountain range had already been forming for tens of millions of years, but the Meiosene epoch saw the most significant and rapid uplift. South America went from being a largely flat and tropical continent to a mountainous, more arid and dry land mass with a much smaller tropical area. This created a period of environmental instability, but also opened up new habitats for animals to colonize. Sloths were uniquely good at adapting to this new world compared to other animals, quickly evolving new forms that suited these new environments. For example, 5 to 10 million years ago, Peru was home to a large sheltered bay that housed an unusually productive marine ecosystem, able to support massive populations of whales, sharks, and seals. Evidence for this being shown by numerous marine animal remains in the desert. Among this fottage was also the remains of a marine adapted sloth named Thalosnus that had adapted to take advantage of the greenery that supported such a rich ecosystem. There are five recognized species of Thalosnus that show incremental transition to live in marine environments over the course of around 4 million years like having nostrils further up their snouts and a more specialized jaw for aquatic vegetation.

[00:10:12]Many marine animals like manatees and dugons have denser bodies to give them ballast in the water and study has shown these sloths had adapted in a similar way. Some of the earliest species bone density was not too different from other ground sloths but the later species show much greater bone density showing they had become more specialized in marine environments in the space of a few million years. The meioene is also when some of the earliest very large ground sloths evolved. There are fossils in Venezuela that show three very large one- ton plus sloths shared the same habitat coexisting together. This part of the world was a section of a very large wetland system that spanned across northern South America 10 to 20 million years ago. One of the smaller sloths that was common in this ecosystem named Pseudaporeitherum has fossil remains showing it was attacked by a giant alligator. A tibia bone from the sloth was discovered with puncture marks that match the jaws of Purosaurus, which was one of the largest crocodiles to ever live that also inhabited these wetlands.

[00:11:07]Interestingly, the giant sloths in this ecosystem weren't necessarily closely related and actually represent different families of sloths that evolved large body sizes independently. This is one of the things that made sloths so adaptable. They were able to adapt large body sizes very quickly and did it multiple times. A study in 2014 examining the remains of many different sloth fossils showed that on average they were able to evolve larger body sizes at a rate of around 100 kg every 1 million years which is some of the fastest evolutionary growth of any known mammal. This would have been in part because of selective brushes from their changing environments but also it is likely that sloth's slow metabolism would have helped them achieve this feat. They require less food per kilo of body weight and so evolving a large body size is less energetically expensive for them compared to other mammals. Sloughs have also evolved a massive four-chambered stomach that is very similar in function to hoofed animals like cows, deer, and goats, which makes them highly efficient at breaking down poor quality plant food like leaves, grass, and bushes that many other animals couldn't eat. This would have made them very competitive at filling herbivorous niches across the continent because they are so effective at digesting all sorts of different plant species, even tough hardy vegetation.

[00:12:18]For instance, manatees and dong have also independently evolved a four-chambered stomach to allow them to consume seaggrasses and aquatic sloths like would have been able to quickly adapt to eating foods like this as well.

[00:12:30]During the meiosene and later, the most common herbivorous animal in South America were hoofed animals named ntoangullets that are now completely extinct. DNA evidence has now confirmed they were related to living hoofed animals, but very distantly, most likely having a common ancestor just after the dinosaurs had gone extinct over 60 million years ago and then evolved in South America in isolation. They were extremely diverse, but often convergently evolved similar forms to hoofed animal species in the rest of the world. As South America was an isolated continent, there were many strange animals similar to notulates like giant predatory birds and carnivorous marsupials known as the sporacodons.

[00:13:06]However, South America's native animal groups declined in the pleosene around 5 million years ago. Historically, this was thought to be due to the formation of Central America connecting North and South America with northern mammals like cats, dogs, deers, and pigs etc. out competing the southern animals. However, more recent study has shown this is oversimplified and South American animals were already in decline before the continental connection. Global temperatures were decreasing throughout the pleosene as the earth was heading for an ice age. But this was a global phenomenon and the extinction rate was particularly bad in South America. The reasons for the decline in animals is uncertain. But what is for sure is that sloths were significantly less affected by it. Their diversity reduced a little bit from its peak in the meiosene. But by and large, their populations remained resilient well into the ice age. Even after ecosystems became increasingly colonized by northern animals like saber-tooth cats and various species of camelids, they successfully adapted to these new pressures and continued on.

[00:14:03]Not only did they survive, but they also traveled northward into North America and established populations in North America with the remains of at least 11 different species being found in North America. However, what's more interesting is that although many of these sloth species came over to North America through Central America during the interchange, some like megalonics didn't. Megalonics evolved in North America and its ancestors made it there around 9 million years ago, long before there was a landbridge between the continents. The earliest evidence of sloths in North America are from a group of creatures named the pleominastes that have fossils in the continent from the late Meiosene and are thought to be ancestral to megalonics.

[00:14:41]Ground sloths had actually already migrated to Caribbean islands as early as 35 million years ago, adapting and establishing a diverse community of distinct island species. Most famous was Megalochnus that was about the size of a pig and was actually the last known surviving ground sloth, not going extinct until around 4 to 5,000 years ago. In the past, it was thought that the early arriving North American sloths had come from the Caribbean. However, DNA tests have shown that Megalonics was actually more closely related to South American sloths. Before Central America formed a land bridge, it was actually a group of volcanic islands, particularly the parts that became modern-day Panama and Costa Rica. So, it is thought that the ancestors of Megalonics may have swam and island hopped their way to North America before it became fully connected.

[00:15:25]Sloths remained successful throughout the ice age and actually during the plea scene produced their largest species like megatherthetherum and aramtherium.

[00:15:33]This is also when the prolific burrowing sloths made their extensive tunnels in the ground. During the period where these burrows were being made, there were dozens of sloth species found across South America, often sharing the same habitats. So, it isn't actually known for certain which sloths made them. However, for the very numerous smaller tunnels found in southern Brazil and northern Argentina, the most likely candidate is a hipposized crown sloth named Glossthetherium. They were numerous in the parts of South America with the most burrows. They lived at the right time, and their claw size matched the scratch marks on most burrows. Plus, their bones show specialist features not shared by many other sloths that may have made them better at digging. A glossythereum sized animal fits most of the burrows quite well, but there were some smaller burrows that may have been made by smaller sloth species as well as larger burrows that some researchers have argued may have been made by a larger sloth like Lesteron. Why sloths built these sometimes massive structures is not fully understood, but for creatures with slow metabolisms, it makes sense they would seek shelter from predators and temperature extremes. Due to the sheer size of the burrows, it is likely they were built over generations by different sloths. From around 15,000 years ago, at the end of the last glacial period of the ice age, ground sloths started to decline and then completely vanishing on the mainland around 10,000 years ago. In a blink of an eye, dozens of highly diverse species went completely extinct. There may have been some environmental impact or climate change, but it is highly likely that human over hunting was the reason behind their extinction. There is evidence of hunting and butchery on the bones of ground sloths in both North and South America, including sloth and human footprints in New Mexico that seem to show a 3 m tall Harland's ground sloth being surrounded by human hunters. The sloth footprints show it turning in circles in some areas, potentially showing it was defending itself or trying to scare away the hunters. The extinction of the creatures correlate with the arrival of humans. For instance, South American sloth seem to have gone extinct 1,000 to 500 years later than they did in North America as humans were migrating their way south.

[00:17:32]Humans also didn't get to the Caribbean until much later, where the last ground bones can be found. It's likely that low metabolism and a corresponding slowmoving energyefficient lifestyle may have ended up being a weakness against human predators that use projectiles.

[00:17:47]So, one of their greatest assets ended up becoming the thing that potentially led to their extinction.

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