Wikimedia Commons // Public Domain
Wikimedia Commons // Public Domain

10 Big Facts About Giant Ground Sloths

Wikimedia Commons // Public Domain
Wikimedia Commons // Public Domain

Today, the six living species of sloths are usually found dangling from tree branches, or going viral on YouTube. But sloths used to be a lot more diverse—and a lot bigger. The extinct ground sloths pursued all sorts of different lifestyles and came in just about every imaginable shape and size. Some were cow-like grazers; others might have been accomplished burrowers; and, believe it or not, a few even dined beneath the ocean waves.


Megatherium (above) means “giant beast”—a fitting name for a creature that weighed several tons, reached 20 feet in length, and—when reared up on its hind legs—stood over 12 feet tall. The biggest sloth of all time, Megatherium americanum, occupied South America between five million and eleven thousand years ago. Above the equator, its slightly-smaller cousin, the 6000-pound Eremotherium, managed to spread as far north as New Jersey.


All ground sloths were predominantly quadrupedal. While they were more than capable of standing up on two legs (more about this later), the animals preferred to get around on four—but individual species differed widely from each other in terms of limb posture.

Scientists have divided ground sloths up into four recognized families, and only one—the megalonychids—stood flat on their rear feet like humans do. Because of the shapes of their ankle and/or hind claws, sloths from the megatheriid, mylodontid, and nothrotheriid families had to trudge along by putting weight on the outer sides of their feet.


Buried in the skin of the mylodontid ground sloths—including the Harlan’s ground sloth, whose range extended from Florida to Washington state—were a series of small bony discs. Known as “osteoderms,” these little knobs (nickel-sized in Harlan’s ground sloth) were mostly clustered around the back, shoulders, and neck and would have acted like protective chainmail.

This trait isn’t all that unusual. A few modern animals, including armadillos and crocodilians, also have osteoderms of some kind—as did many dinosaurs.


For these animals, standing up on two limbs required some extra stability. Whenever a ground sloth did this, its muscular tail would act like another leg, helping to support its considerable body weight.


The Sage of Monticello’s importance to American paleontology cannot be understated. In 1796, Jefferson—a respected armchair naturalist—received some curious bones from western Virginia (modern West Virginia). This find wasn’t all that unusual—similar-looking fossils had also emerged in Kentucky and other parts of Virginia. Still, Jefferson spoke at length about the big-clawed mystery animal at a 1797 meeting of the American Philosophical Society in Philadelphia. The future president dubbed this creature Megalonyx, or “great claw.” Though we now know that it was a large, flat-footed sloth, Jefferson originally mistook the animal for an enormous lion or tiger-like carnivore.

Currently, four different species of Megalonyx are recognized; the most famous, Megalonyx jeffersonii, was named in Jefferson's honor. On March 8, 2008, West Virginia recognized the animal as its official state fossil.   


What killed off the woolly mammoth, the scimitar cat, and North America’s other ice age mega-mammals? Homo sapiens usually gets a good chunk of the blame. Scientists have long speculated that humans killed and devoured ground sloths—but, for many years, there was no physical evidence to support this idea. Then, in 2008, incriminating scars were found on the femur of an Ohio Megalonyx. The 13,000-year old fossil is riddled with 41 unusual cuts that appear to have been left by manmade tools.  

As archaeologist Haskel Greenfield points out, we’ll likely never know if early Americans killed this animal or merely scavenged its remains. “The only thing that is clear,” he said in 2012, “is that there are disarticulation marks: they were separating the limbs from each other; they were cutting the joints. And some marks show that they were filleting the meat off the bone.” 


Imagine a sloth that’s trying hard to be a marine iguana. You’ve just pictured a member of the Thalassocnus genus. These Peruvian herbivores, which lived 8 to 4 million years ago, dove into the ocean for their supper. Hooked claws helped them latch onto submerged, seaweed-covered rocks; once anchored, a Thalassocnus could consume marine algae. Over time, evolution fitted the amphibious sloths with increasingly dense ribs and limb bones. Therefore, younger species were less buoyant—and probably more aquatic—than their ancestors had been.


Rivaling a black bear in size, Nothrotheriops would have been dwarfed by behemoths like Megatherium. Still, we know more about it than any other ground sloth thanks to one amazing find. Eleven thousand years ago, a New Mexican Nothrotheriops stumbled into a volcanic gas vent and died. Then, in either 1927 or 1928 (sources differ), a group of explorers happened upon the incredibly well preserved body. Not only were almost all of its ligaments and bones intact, but this Nothrotheriops also came with a few muscle fibers. Even more interestingly, the specimen retained some original skin—covered by rough, yellowish hair. The cherry on top was an accompanying dung ball, which helped confirm that Nothrotheriops ate a diverse array of plants—including cacti fruit, yuccas, and saltbushes. 

In 1928, Yale’s Peabody Museum of Natural History acquired the mummy, and today, visitors can find the creature on display in Mammal Hall.


Your average ground sloth was—in all likelihood—a browsing herbivore, pulling down tree branches with its strong forelimbs. However, the mylodontids may have also gathered food by digging for it. Experts argue that their wide, flattened claws look like ideal tools for excavating roots and tubers [PDF].

Believe it or not, mylodontids might have even been burrowers. Several huge, prehistoric tunnels have been found in Argentina [PDF]. Made sometime during the Pleistocene epoch (between 2.6 million and 11,700 years ago), these were natural marvels, with the longest stretching 130 feet from end to end. What could have possibly dug them? Two top suspects are Scelidotherium and Glossotherium: a pair of mylodonts with claws that match scratch marks found inside the burrows. 


The Caribbean islands seem like an unlikely place for ground sloths to have made their last stand—but that’s exactly where it happened [PDF]. Mainland North America lost all of its indigenous species around 11,000 years ago, and half a millennium later, South America, too, became a ground sloth-free continent.

But despite these extinctions, some ground sloths didn't die out until much later. Hispaniola and Cuba were home to assorted dwarf species. Descended from their full-sized counterparts on the mainlands, these mammals were resilient. Ultimately, Megaloncus rodens was the last ground sloth standing: Radiocarbon dating indicates that this 200-pound vegetarian waddled across Cuba as recently as 4200 years ago

Big Questions
Why Do Cats 'Blep'?

As pet owners are well aware, cats are inscrutable creatures. They hiss at bare walls. They invite petting and then answer with scratching ingratitude. Their eyes are wandering globes of murky motivations.

Sometimes, you may catch your cat staring off into the abyss with his or her tongue lolling out of their mouth. This cartoonish expression, which is atypical of a cat’s normally regal air, has been identified as a “blep” by internet cat photo connoisseurs. An example:

Cunning as they are, cats probably don’t have the self-awareness to realize how charming this is. So why do cats really blep?

In a piece for Inverse, cat consultant Amy Shojai expressed the belief that a blep could be associated with the Flehmen response, which describes the act of a cat “smelling” their environment with their tongue. As a cat pants with his or her mouth open, pheromones are collected and passed along to the vomeronasal organ on the roof of their mouth. This typically happens when cats want to learn more about other cats or intriguing scents, like your dirty socks.

While the Flehmen response might precede a blep, it is not precisely a blep. That involves the cat’s mouth being closed while the tongue hangs out listlessly.

Ingrid Johnson, a certified cat behavior consultant through the International Association of Animal Behavior Consultants and the owner of Fundamentally Feline, tells Mental Floss that cat bleps may have several other plausible explanations. “It’s likely they don’t feel it or even realize they’re doing it,” she says. “One reason for that might be that they’re on medication that causes relaxation. Something for anxiety or stress or a muscle relaxer would do it.”

A photo of a cat sticking its tongue out

If the cat isn’t sedated and unfurling their tongue because they’re high, then it’s possible that an anatomic cause is behind a blep: Johnson says she’s seen several cats display their tongues after having teeth extracted for health reasons. “Canine teeth help keep the tongue in place, so this would be a more common behavior for cats missing teeth, particularly on the bottom.”

A blep might even be breed-specific. Persians, which have been bred to have flat faces, might dangle their tongues because they lack the real estate to store it. “I see it a lot with Persians because there’s just no room to tuck it back in,” Johnson says. A cat may also simply have a Gene Simmons-sized tongue that gets caught on their incisors during a grooming session, leading to repeated bleps.

Whatever the origin, bleps are generally no cause for concern unless they’re doing it on a regular basis. That could be sign of an oral problem with their gums or teeth, prompting an evaluation by a veterinarian. Otherwise, a blep can either be admired—or retracted with a gentle prod of the tongue (provided your cat puts up with that kind of nonsense). “They might put up with touching their tongue, or they may bite or swipe at you,” Johnson says. “It depends on the temperament of the cat.” Considering the possible wrath involved, it may be best to let them blep in peace.

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John James Audubon, Wikimedia Commons // Public Domain
10 Tragic Stories of Extinct Animals
Drawing depicting the Great Auk, from the book 'Birds of America' by John James Audubon.
Drawing depicting the Great Auk, from the book 'Birds of America' by John James Audubon.
John James Audubon, Wikimedia Commons // Public Domain

The tale of the dodo is one of the most famous stories of extinction in all natural history. Native only to the tiny island of Mauritius in the Indian Ocean, the birds had never learned any reason to be fearful of humans, so when European explorers first began to visit the island in the 17th century, the dodos were apparently so unsuspecting they could be picked up by hand straight from the wild and killed. Although the dodo was never a particularly numerous species (the fact that it was flightless made it susceptible to floods and forest fires, which apparently kept its population naturally low), within less than a century of its discovery, interference by humans had led to its extinction. But it's by no means alone—the stories behind the disappearance of 10 other creatures are listed here.


A Roman mosaic of the extinct Atlas bear.
A Roman mosaic of the extinct Atlas bear.
The Picture Art Collection / Alamy Stock Photo

The Atlas bear was the only species of bear native to Africa, and once inhabited the area around the Atlas Mountains in the far northwest of the continent. The bear's lengthy demise can be traced all the way back to the time of the Roman Empire, when the animals were not only hunted for sport but captured, brought back to Rome, and made to battle gladiators and execute criminals in a gruesome spectacle known as damnatio ad bestias. Numbers continued to fall throughout the Middle Ages, when great swaths of forest in northern Africa were felled for timber, until finally the last surviving wild Atlas bear was shot and killed in the mid-1800s.


A mounted Carolina parakeet
James St. John, Wikimedia Commons // CC BY 2.0

The Carolina parakeet was once the only species of parrot native to the United States, found across a vast expanse of the country from New York in the north to the Gulf of Mexico in the south and the Rocky Mountains in the west. Excessive hunting and trapping meant that the birds had already become rare by the 19th century, but large, isolated flocks were still being recorded until as recently as the early 1900s. Sadly the birds were known for their altruistic habit of flocking to attend to dead or dying members of the same flock—so if only a few birds were felled by hunters, many of the rest of the flock would remain nearby, making themselves easy targets. The last known specimen died in the Cincinnati Zoo in 1918, and the species was finally declared extinct in 1939.


A Dusky Seaside Sparrow outside on a branch
U.S. Fish and Wildlife Service, Wikimedia Commons // Public Domain

In 1963, a decision was made by NASA to flood a vast area of marshland on Merritt Island in eastern Florida as a means of controlling the mosquito population around the Kennedy Space Center. Sadly, Merritt Island was also one of the last strongholds of the dusky seaside sparrow, a small dark-colored songbird, and when the land was flooded, so too was the sparrows’ main breeding ground. Drainage of the marshes around the St. Johns River for a highway project also contributed to habitat loss. The birds' population collapsed, and in the years that followed, the species struggled to regain its numbers. By 1979, only five birds—all males—remained in the wild, and the sparrow was finally declared extinct in 1990.


A drawing of a gravenche, an extinct freshwater fish
Wikimedia Commons // Public Domain

The gravenche was a species of freshwater fish native only to Lake Geneva, one of the Alpine lakes that straddle the border between France and Switzerland. The fish were apparently once so common in the lake that it alone accounted for two-thirds of all of the fish caught in Lake Geneva. Due to overfishing, the population of gravenche (Coregonus hiemalis) began to decrease rapidly in the early 20th century; the last known sighting was in 1950, and the species is now considered extinct.


Study of a great auk, circa 1910.
Hulton Archive, Getty Images

The penguin-like great auk was a large, flightless seabird once native to the entire North Atlantic Ocean, from Greenland and eastern Canada to the British Isles and the westernmost coasts of Europe. The birds were highly prized for their light and fluffy down, which was used as a stuffing for pillows and mattresses. And like the dodo, the fact that the birds were flightless made hunting and capturing them easy. The European population was almost entirely eradicated by the late 1600s, leading to one of the earliest environmental protection laws in history, passed by the British Parliament in 1770s, that prohibited killing the auks in Great Britain. Sadly, it was too late. As the birds became scarcer, demand for their feathers, meat and pelts increased, and the last two breeding birds were unceremoniously strangled to death on their nest by a pair of Icelandic hunters in 1844, while a third man stamped on the single egg that the female had been incubating.


Three Heath Hens
Game Birds, Wild-Fowl and Shore Birds of Massachusetts and Adjacent States, Massachusetts State Board Agriculture, Wikimedia Commons // Public Domain

Like the great auk, the North American heath hen was also the subject of an early protective bill, introduced to New York State legislature in 1791, but it too failed to save the species from extinction. Heath hens were once native to much of the northeast United States, and were so plentiful that their meat eventually gained a reputation for being "poor man's food." Nonetheless they continued to be hunted in such vast numbers that by the mid-1800s there were no hens at all left on the entire American mainland. The bird's final stronghold was Martha's Vineyard, Massachusetts, but illegal poaching, diseases carried by domestic poultry, and predation from feral cats caused numbers on the island to fall to less than 100 by the mid-1890s. A hunting ban and a specialized Heath Hen Reserve was introduced in 1908, and in response the population swelled to over 2000 in the years that followed. But a fire during the 1916 breeding season undid all of the reserve's hard work, and by 1927 there were only 12 birds—including just two females—left alive. The last lone male, nicknamed "Booming Ben" by the locals, died in 1932.


The 8-foot-long Japanese sea lion—an even larger cousin of the Californian sea lion—was once native to the Sea of Japan and bred in vast numbers along the beaches of the Japanese islands and the Korean mainland. Sadly, the animals were hunted in enormous numbers, but not for the reason you might think: their meat was poor quality and bad-tasting, so they weren't hunted for food, but rather for their skins (which were used to make leather), their bones (which were used in traditional medicines), their fat (which was rendered to make oil for oil lamps), and even their whiskers (which were used to make brushes and pipe cleaners). As recently as the early 1900s, more than 3000 sea lions were being killed every year in Japan, until the population collapsed to less than 50 individuals in 1915. Numbers remained low until the 1940s, when the maritime battles of the Second World War destroyed the last remaining colonies and much of their natural habitat. The last recorded (but unconfirmed) sighting was in 1974.


A stuffed passenger pigeon up for auction.
Rob Stothard, Getty Images

Until as recently as the early 1800s, the passenger pigeon was still considered the most numerous bird in all of North America. Individual flocks could contain in excess of a billion individual birds, and would take more than an hour to fly overhead. But as a hugely plentiful source of cheap meat, the birds were hunted in unprecedented numbers: at one nesting site in Michigan in 1878, as many as 50,000 birds were killed every day for nearly five months, and the last surviving flock of 250,000 birds was killed in its entirety by one group of hunters in a single day in 1896. The final individual bird—a female named Martha, who was being held in captivity at the Cincinnati Zoo—died in 1914.


Wikimedia Commons // Public Domain

Stephens Island is a tiny half-mile islet lying in the seas between the two main islands of New Zealand. After a lighthouse was built there in 1892, the local lighthouse keeper's cat, Tibbles, caught a bird that the keeper didn't recognize. He sent the specimen to a renowned New Zealand ornithologist named Walter Buller, and the bird was soon declared a new species—the Stephens Island wren—and identified as one of only a handful of flightless perching birds known to science. Sadly, within just three years of its discovery, the species was extinct. According to popular history, Tibbles the cat was singlehandedly responsible for killing off the entire population of the wrens (in which case, Tibbles would be the only individual creature in history responsible for the extinction of an entire species) but in reality, by the late 1890s, Stephens Island was so overrun with feral cats that it is impossible to say that Tibbles alone was responsible: In February 1895, the lighthouse keeper wrote in a letter that "the cats have become wild and are making sad havoc among all the birds."


The warrah, or Falkland Island wolf or fox
Wikimedia Commons // Public Domain

The warrah, or Falkland Islands wolf, was a unique species of wolf that was once the only mammal species native to the Falkland Islands in the South Atlantic Ocean. It's thought that the species became trapped on the islands during the last Ice Age, when the Falklands were connected to the South American mainland by an ice bridge that left the animals isolated when it melted. After the Falkland Islands were first settled by humans in the 1760s, the wolves were seen as a threat to livestock and were quickly hunted into extinction. The warrah was already rare by the time Charles Darwin visited the Falklands in 1833, and he ominously predicted that, "within a very few years … this fox will be classed with the dodo as an animal which has perished from the face of the earth." Like the dodo, the warrah had never had to learn to be fearful of humans, and with no trees or forests on the island in which to hide, the wolves proved easy targets. The last individual was killed in 1876.


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