The Mystery Behind the Biggest Bears of All Time

In the cloud forests of the Andes lives a secretive little animal known as the spectacled bear, the only native bear in South America.

With its distinctive markings, small size, and stout muzzle, it’s pretty unique-looking as bears go.

And so are its habits.

It spends much of its time in the trees, and unlike most other modern bears, the spectacled bear feeds almost entirely on plants.

It turns out that, while this bear has some unusual traits, it comes by them honestly.

It’s the last surviving member of a subfamily called Tremarctinae, which includes the group of extinct bears commonly known as the “short-faced bears.” The name comes from early researchers who thought that the bears had short-looking muzzles, although today it’s considered a misnomer.

In any case, their snouts weren’t their most distinctive quality.

A couple of the little spectacled bear’s ancestors were downright enormous.

For example, in North America during the early Pleistocene Epoch, there was Arctodus simus.

Standing on all fours, a large member of this species would’ve been tall enough to look a full-grown person right in the eye.

And in South America, there was the amazing Arctotherium angustidens.

One specimen, discovered in Buenos Aires in 1935, had upper arm bones that were twice as long as an adult human’s and would have stood more than 3 meters tall on its hind legs!

Paws down, it was the biggest bear the world has ever known.

The story of how these bears came to be -- and how they’re related to the little, plant-eating spectacled bear -- is an amazing tale about new frontiers, rival creatures, and … continental drift.

In the face of each of these challenges, the short-faced bears turned out to be remarkably adaptable, undergoing radical changes in their diets -- and their body sizes -- to meet the demands of two changing continents.

And yet, for reasons we don’t quite understand, their adaptability wasn’t enough to keep them from going extinct.

The first short-faced bears weren’t big, scary giants.

In fact, the earliest known genus--a bear named Plionarctos--was about the size of a small spectacled bear.

Plionarctos first appears in the fossil record of North America about 7 million years ago in the Miocene Epoch, and its fossils have been found from coast to coast.

And some researchers think that it was the ancestor of all of the Tremarctines that lived in the Americas.

It descendants included the so-called “lesser short-faced bear,” which first appeared around 2.5 million years ago and was about as big as the largest American black bears alive today.

And this supposedly “lesser” bear is thought to be the direct ancestor of the biggest bear that North America has ever seen -- the enormous Arctodus simus.

It first appears in the fossil record roughly 1.6 million years ago.

It stood about 1.5 meters at the shoulder and the very largest individuals may have weighed more than a ton.

And not only was this animal big, but it was widespread.

Bones of Arctodus simus have been unearthed at more than a hundred sites across North America, from Ozark Caves to the La Brea Tar Pits--and from the banks of an Alaskan river to the wilds of central Mexico.

So, it seems that Arctodus simus did pretty well for itself.

And some researchers have proposed that it might have been the ancestor of the other giant bear found to the south -- South America’s huge Arctotherium angustidens.

This seems like it would make sense.

Because, for one thing, they’re just both unusually large.

But also, while bears of some kind had been living in North America since the late Eocene Epoch some 38 million years ago, the fossil record shows that bears didn’t arrive in South America until much later.

That’s because South America used to be basically an island, cut off from the rest of the world.

But around five or six million years ago, a land bridge began to form between South America and North America.

And by about 2.6 million years ago, in the late Pliocene Epoch, the connection was complete.

And the joining of these continents triggered a massive exchange of life, called the Great American Biotic Interchange.

Suddenly, species that were native to North America could travel south, and vice versa.

So it’s been argued that some of the big Arctodus bears of North America made the trek south, and their descendants eventually gave rise to even bigger bears in South America, with the earliest evidence of Arctotherium showing up in what’s now Argentina about 1.75 million years ago.

But, some recent genetic evidence suggests that South America’s big bears actually came about in a totally different way.

In 2016, a team of researchers took DNA from a fossilized femur of Arctotherium and compared it to the genomes of both living and extinct bears.

And the results showed that Arctotherium was actually more closely related to the cute, little, and modern spectacled bear than it was to the giant Arctodus of North America.

So now it 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 the mega-bears of North America and those in South America must have acquired their huge body sizes independently and at different times.

So what evolutionary pressures would have driven each of these bears to start living large?

Well, for Arctotherium angustidens, it might have been a response to the strange environment its ancestors found in South America.

Before South America met up with North America, the continent was full of large herbivores like giant ground sloths and armored glyptodonts.

And yet, there were hardly any big predators.

Sure, there were the terror birds, and the marsupial-like hunter Thylacosmilus.

And in the early Pleistocene, the huge saber-toothed cat Smilodon moved in from the north.

But overall, the first bears to colonize South America didn’t face much competition.

So bigger bodies might have been better for the ancestors of Arctotherium angustidens, allowing them to hunt -- or maybe scavenge -- some of the area’s giant herbivores.

Without much competition, the giant bear could’ve easily established itself as a top predator or fearsome scavenger simply by virtue of its size.

But that doesn’t mean that this bear was strictly a carnivore.

In 2009, researchers in Argentina compared skulls of Arctotherium to those of living bears, in search of clues about the ancient bear’s diet.

That’s because modern bears that eat a lot of meat --like polar bears-- tend to have smaller cheek teeth and longer jaws.

But in mostly herbivorous species, you’ll see the opposite -- shorter jaws and bigger cheek teeth.

And the skull of Arctotherium angustidens fell right in the middle.

It didn’t look like a meat or plant specialist, so the researchers figured that it was probably an omnivore, much like many modern bears.

But in the same study, other Arctotherium species landed in a different part of the spectrum.

Their skulls looked a lot more like those of mostly herbivorous bears, like the giant panda and the spectacled bear.

For reasons we don’t fully understand, the mighty Arctotherium angustidens died out around 800,000 years ago.

And it seems that its lineage had gotten the memo that bigger was no longer better.

As time wore on, Arctotherium bears got smaller, and their food habits became more herbivorous.

These descendants that lived between 800,000 and 12,000 years ago all weighed about 500 kilograms or less.

And by their skulls and teeth, we can tell that they ate less meat.

The last --and maybe smallest-- species was Arctotherium wingei, a bear that survived into the present Holocene Epoch and may have been every bit as herbivorous as the spectacled bear.

So, why did this lineage of bears change course, in diet as well as body size?

It may be linked to a spike in competition.

Remember, when Arctotherium first appeared in South America, it didn’t have a lot of rival predators to deal with.

But by the late Pleistocene, the jaguar, the cougar, and the dire wolf had all crossed into South America.

So it could be that the descendants of the giant short-faced bear responded by downsizing and eating more greenery in order to avoid competing with these new predators.

Now, back up north, and a bit farther back in time, the evolutionary pressure to sidestep competition was also felt by North America’s own giant bear: Arctodus simus.

One of this bear’s more unusual traits was its long, slender limbs, which seemed to some paleontologists to be sort of cat-like.

So in the twentieth century, Arctodus simus was typically seen as a fast-moving predator that pursued horses, bison and other big game for long distances.

According to this view, the bear wasn’t 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 massive animal on long-distance runs.

So, an alternative model suggests that Arctodus was a scavenger that used its huge size to defend the carcasses it found--and to frighten other carnivores away from their own kills.

This is a strategy known as kleptoparasitism, and it’s also one of my favorite words!

But did this giant bear have an appetite for carrion?

It’s certainly possible.

In Virginia, Arctodus fossils have been found alongside the remains of mammoths.

And on one mammoth ankle bone, there’s a bite wound that paleontologists think was made by a short-faced bear.

But again, this doesn’t mean that Arctodus simus was a hypercarnivore, or even a very strict one.

According to that skull study I mentioned before, the morphology of this species resembles modern bears that eat a broad mixture of plants and meat.

In fact, many specimens from California even have dental cavities, which suggests that they ate a lot of sugary plant foods like berries and honey.

So, their diet may have varied by region, with some populations eating more vegetation than others, depending on factors like resource availability and inter-species competition.

But if the giant short-faced bears were so adaptable, both in body size and in diet, then why did they disappear?

Arctodus simus disappears from the fossil record about 10,000 years ago.

And, as with Arctotherium in the south, no one knows why.

If this animal really was a hypercarnivore, then its demise would be easier to understand: as the big herbivores of the Ice Age died out, the bears would’ve been left with nothing to eat.

And the disappearance of Ice Age megafauna may still have played some role.

But so might the competition from modern bears.

Brown, black, and polar bears -- all members of the genus Ursus -- had each evolved and were roaming 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 something of a mystery.

Today, only the spectacled bear is remains, foraging away in the Andes.

On its furry shoulders