The Mystery Behind the Biggest Bears of All Time

Hi everyone, Kallie here with our new t-shirt that YOU can get at DFTBA.com, link in the description. It's comfortable, pretty and this is a real pocket! So if you want one, go get it and help us! But now, let's talk about some big

bears

... no, like... REALLY big

bears

In the cloud forests of the Andes lives a secretive little animal known as the spectacled bear. bear, the only bear native to South America. With its distinctive markings, small size, and robust snout, it has a rather unique appearance compared to bears. And so are their habits. It spends much of its

time

in trees, and unlike most modern bears, the spectacled bear feeds almost exclusively on plants.

It turns out that while this bear has some unusual traits, he treats them honestly. It is the last surviving member of a subfamily called Tremarctinae, which includes the group of extinct bears commonly known as "short-faced bears." The name comes from early researchers who thought bears had short-looking snouts, although it is considered a misnomer today. In any case, their snouts were not their most distinctive quality. A pair of the little spectacled bear's ancestors were downright huge. For example, in North America during the Early Pleistocene, Arctodus simus existed. Standing on all fours, a large member of this species would have been tall enough to look an adult person directly in the eyes.

And in South America there was the surprising Arctotherium angustidens. One specimen, discovered in Buenos Aires in 1935, had upper arm bones that were twice as long as those of an adult human and would have stood over 3 meters tall on its hind legs! With its paws down, it was the largest bear the world has ever known. The story of how these bears came to be (and how they are related to the small, plant-eating spectacled bear) is a surprising story about new frontiers, rival creatures, and... continental drift. Faced with each of these challenges, short-faced bears proved remarkably adaptable, undergoing radical changes to their diets (and body sizes) to meet the demands of two changing continents.

And yet, for reasons we don't fully understand, their adaptability was not enough to keep them from going extinct. The first short-faced bears were not big, scary giants. In fact, the oldest known genus, a bear called Plionarctos, was about the size of a small spectacled bear. Plionarctos first appear in the North American fossil record about 7 million years ago in the Miocene epoch, and their fossils have been found from coast to coast. And some researchers think that it was the ancestor of all the Tremarctines that lived in America. Among their descendants was the so-called “lesser short-faced bear,” which first appeared about 2.5 million years ago and was almost as large as the largest American black bears that exist today.

And this supposedly "lesser" bear is believed to be the direct ancestor of the largest bear North America has ever seen: the enormous Arctodus simus. It first appears in the fossil record approximately 1.6 million years ago. It measured around 1.5 meters at the shoulder and the largest individuals may have weighed more than a ton. And this animal was not only large, but widespread. Bones of Arctodus simus have been unearthed at more than one hundred sites across North America, from the caves of the Ozark to the La Brea Tar Pits, and from the banks of an Alaskan river to the wilds of central Mexico.

So, it seems like Arctodus simus did pretty well. And some researchers have proposed that it could have been the ancestor of the other giant bear found to the south: the enormous Arctotherium angustidens of South America. This seems like it would make sense. Because, to begin with, they are both unusually large. But also, while some type of bear had been living in North America since the late Eocene, about 38 million years ago, the fossil record shows that bears did not arrive in South America until much later. This is because South America used to be basically an island, isolated from the rest of the world.

But about five or six million years ago, a land bridge began to form between South America and North America. And about 2.6 million years ago, at the end of the Pliocene, the connection was complete. And the union of these continents triggered a massive exchange of life, called the Great American Biotic Exchange. Suddenly, species native to North America were able to travel south and vice versa. Therefore, it has been argued that some of the large Arctodus bears of North America traveled south, and their descendants eventually gave rise to even larger bears in South America; The earliest evidence of Arctotherium appeared in what is now Argentina approximately 1.75 million years ago. .

But some recent genetic evidence suggests that the great bears of South America actually emerged in an entirely different way. In 2016, a team of researchers took DNA from a fossilized Arctotherium femur and compared it to the genomes of living and extinct bears. And the results showed that Arctotherium was actually more closely related to the cute, little, modern spectacled bear than to the giant Arctodus of North America. So it now seems more likely that members of some other lineage of North American short-faced bears moved south and became the ancestors of Arctotherium and the spectacled bear. And if that's true, then it means that North American megabears and South American megabears must have acquired their enormous body sizes independently and at different

time

s.

So what evolutionary pressures would have driven each of these bears to start living large? Well, for Arctotherium angustidens, it could have been a response to the strange environment its ancestors found in South America. Before South America met North America, the continent was filled with large herbivores such as giant ground sloths and armored glyptodonts. And yet, there were hardly any large predators. Of course, there were the terror birds and the marsupial hunter Thylacosmilus. And in the early Pleistocene, the huge saber-toothed cat Smilodon arrived from the north. But overall, the first bears to colonize South America didn't face much competition.

Therefore, larger bodies might have been better for the ancestors of Arctotherium angustidens, allowing them to hunt—or perhaps scavenge—some of the giant herbivores in the area. Without much competition, the giant bear could have easily established itself as a top predator or fearsome scavenger simply by virtue of its size. But that doesn't mean this bear was strictly a carnivore. In 2009, researchers in Argentina compared Arctotherium skulls with those of living bears, looking for clues to the ancient bear's diet. This is because modern bears that eat a lot of meat, such as polar bears, tend to have smaller molars and longer jaws.

But in mostly herbivorous species, you will see the opposite: shorter jaws and larger molars. And the skull of Arctotherium angustidens fell right in the middle. It didn't seem like a meat or plant specialist, so researchers thought it was probably an omnivore, much like many modern bears. But in the same study, other Arctotherium species fell into a different part of the spectrum. Their skulls looked much more like those of mostly herbivorous bears, such as the giant panda and the spectacled bear. For reasons we don't fully understand, the mighty Arctotherium angustidens became extinct about 800,000 years ago. And it seems that his lineage had received the note that bigger was no longer better.

As time went on, Arctotherium bears became smaller and their eating habits became more herbivorous. These descendants who lived between 800,000 and 12,000 years ago all weighed around 500 kilograms or less. And from their skulls and teeth we can tell that they ate less meat. The last (and perhaps smallest) species was Arctotherium wingei, a bear that survived into the current Holocene epoch and may have been as herbivorous as the spectacled bear. So why did this lineage of bears change course, both in diet and body size? It may be related to increased competition. Remember, when Arctotherium first appeared in South America, it didn't have many rival predators to contend with.

But by the end of the Pleistocene, the jaguar, puma, and dire wolf had crossed into South America. So it could be that the descendants of the giant short-faced bear responded by reducing their size and eating more vegetation to avoid competing with these new predators. Now, back north, and a little further back in time, the evolutionary pressure to evade competition was also felt by North America's own giant bear: Arctodus simus. One of the most unusual features of this bear was its long, thin limbs, which appeared to some paleontologists to be somewhat cat-like. Thus, in the 20th century, Arctodus simus was typically seen as a fast-moving predator that pursued horses, bison, and other large game over long distances.

According to this view, the bear was not just a hunter, but a hypercarnivore, an animal that overwhelmingly ate meat. But in recent years, closer inspection has shown that the bear's leg bones were probably too thin to support such a huge animal in long-distance races. An alternative model, then, suggests that Arctodus was a scavenger that used its enormous size to defend the carcasses it found and to scare other carnivores away from its own prey. This is a strategy known as kleptoparasitism and is also one of my favorite words. But did this giant bear have an appetite for carrion?

It is certainly possible. In Virginia, Arctodus fossils have been found along with mammoth remains. And on a mammoth ankle bone is a bite wound that paleontologists believe was made by a short-faced bear. But again, this does not mean that Arctodus simus was a hypercarnivore, or even a very strict one. According to that skull study I mentioned earlier, the morphology of this species resembles that of modern bears that eat a wide mix of plants and meat. In fact, many California specimens even have tooth decay, suggesting that they ate a lot of sugary plant foods like berries and honey.

Their diet may therefore have varied by region, with some populations eating more vegetation than others, depending on factors such as resource availability and competition between species. But if giant short-faced bears were so adaptable, both in body size and diet, why did they disappear? Arctodus simus disappeared from the fossil record about 10,000 years ago. And, as with Arctotherium in the south, no one knows why. If this animal were truly a hypercarnivore, then its demise would be easier to understand: as the large herbivores of the Ice Age became extinct, the bears would have been left with nothing to eat.

And the disappearance of Ice Age megafauna may still have played some role. But so could competition from modern bears. Brown, black, and polar bears, all members of the genus Ursus, had evolved and roamed North America by the mid-Pleistocene. So the reasons behind the decline and fall of Arctodus, and the abrupt disappearance of Arctotherium angustidens and its descendants remain a

mystery

. Today, only the spectacled bear remains, searching for food in the Andes. On their hairy shoulders rests the legacy of those goliaths of the recent past: the tremarctines. Thank you for joining me in Konstantin Haase's studio today!

And many thanks to our current eontologists, Jake Hart, Jon Ivy, John Davison Ng, and STEVE! If you would like to join them and our other patrons in supporting what we do here, please visit patreon.com/eons and make your contribution. If you enjoyed this short-faced bear story, then you should check out our episode about another epic animal: bone-crushing dogs!