Hey everyone, Kallie here with our new shirt that YOU can get at DFTBA.com, link in the
description. It’s comfy, nice, and this is a real pocket! So, if you’d like one,
go get it and help us out!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, 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
rests the legacy of those goliaths of the recent past -- the tremarctines.
Thanks for joining me today in the Konstantin Haase studio! And extra big thanks to our
current Eontologists, Jake Hart, Jon Ivy, John Davison Ng and STEVE!
If you’d like to join them and our other patrons in supporting what we do here, then
go to patreon.com/eons and make your pledge!
If you enjoyed this tale of the short-faced bear, then you should check out our episode
on another epic animal: the bone-crushing dogs!