A little over 10,000 years ago, North America
was home to the largest beaver that ever existed. And, yeah, I know I talk to you all the time
about giant extinct animals, like the biggest frog, the longest snake, the hugest dinosaurs. But it’s important to me personally that
you understand how big this beaver was. It was more than twice as heavy and many,
many times taller and longer than a modern beaver. It rivaled the size of a small black bear! Its name was Castoroides and, of course, it’s
extinct now. Because, you’d definitely know if it was
still around. But, in the not-too-distant past, this giant
rodent lived as far north as Alaska and the Yukon Territory, and as far south as Florida. And back then, beavers used to be a lot more
diverse. For example, not all beavers were swimmers. There were also terrestrial beavers that dug
burrows and twisty tunnels. So, why are we left with just two species
today -- the American and the Eurasian beavers? And why isn’t one of them the giant beaver?! Well, the answer to both questions might involve
behavior. Castoroides stuck around for about a million
years. And just like modern beavers, it was semiaquatic
-- it lived both on the land and in the water. The difference is that today’s beavers do
a pretty special thing - one that the giant beaver probably didn’t, or couldn’t, do. And that one change in its behavior may have
been its undoing. Beavers make up a family of rodents called
the Castoridae. And so far, about 30 genera have been identified
in the fossil record. Scientists think beavers originally evolved
in North America toward the end of the Eocene Epoch, with the earliest known species scampering
around Wyoming 35 million years ago. There’s been some debate about what its
niche was, but it might’ve been semiaquatic. And by the Early Oligocene Epoch, fossils
from China tell us that beavers were quick to establish themselves in Eurasia. Between about 35 and 24 million years ago,
different lineages of specialized beavers started to appear. For example, one group was mostly fossorial
- they were digging animals. We know this in part because they had classic
burrowing traits like big forelimbs and front claws, as well as a short tail and neck. And many would’ve even used their teeth
for digging too. Take Palaeocastor, a small beaver from what’s
now Nebraska. During the Oligocene and Miocene Epochs, it
made huge, spiraled dens in the earth. Later, those burrows filled in with sediment,
which solidified, leaving trace fossils called “Devil’s Corkscrews,” which we’ve
actually done a whole episode on! Now, the digging beavers stayed pretty small,
only measuring about 20 to 30 centimeters long. And they tended to live in open habitats,
places where trees and woody plants weren’t really common. Which wouldn’t have worked out so well for
modern beavers, whose diets include a lot of wood. They are part of another important beaver
subgroup, one that evolved by the end of the Oligocene. Unlike Palaeocastor and its burrowing cousins,
these animals were semiaquatic. Paleontologists think that the common ancestor
of Castoroides and living beavers was a semiaquatic woodcutter that lived in the early Miocene
20 to 24 million years ago. And some of the “semiaquatic” beavers
were probably good at burrowing as well as swimming, just like today’s beavers are. And while experts aren’t sure if Castoroides
dug burrows, they know it was a swimmer. The oldest fossils of this giant beaver that
we know of come from Florida, between 1.6 million and 1 million years old. Back then, Florida was a combination of mixed-tree
woodlands and forested swamps, but wetlands weren’t limited to the American south. In the giant beaver’s heyday, the ice sheets
of the Northern Hemisphere expanded and retreated multiple times. And it looks like the giant beaver came and
went with those warm periods. So, when the ice sheets advanced, places like
Yukon Territory became cold, dry grasslands. But during the warmer interglacial periods,
forests bounced back. And the water from melting ice sheets produced
wetlands up north. The fossil record shows Castoroides repeatedly
invaded the Arctic during those warmer, wetter periods. Which makes sense, because it was clearly
built for a semiaquatic lifestyle. It had relatively short legs and big back
feet that were probably webbed. But on land, it would’ve had a pretty awkward
walk. And by awkward, I mean probably adorable Because I mean, the thing was big, up to 2.5 meters
long, and weighing about 100 kilograms. Luckily, Castoroides could monitor its surroundings
without leaving the water. Like hippos, crocodiles and modern beavers,
its eyes were near the top of its skull. This allowed it to look out for predators
on land while staying safe in the water. So, Castoroides sounds a lot like modern beavers. And fossils show that it lived alongside the
modern American beaver. Remains of both species sometimes appear at
the same dig sites. So why aren’t the giant beavers still around? Like all rodents, Castoroides had big incisors
on its upper and lower jaws that never stopped growing. Rodents constantly gnaw on things to keep
these from getting too long. And Castoroides had incisors that were bigger
and curvier than those of living beavers. The giant beaver’s incisors could grow to be over 20 centimeters long! But they didn’t have the chisel-like tips
that modern beavers have. With their shorter, sharper teeth, today’s
beavers have no trouble chopping down trees - which makes them excellent landscapers,
as well as builders. With mud and logs, they build sophisticated
lodges. The ones they use in the winter are especially
good at retaining heat, even though they have air vents. And their dams are also, just, really impressive. A beaver dam can be anywhere from one meter
to 850 meters long. And the tallest are 5 meters high! And it’s hard to overstate just how much
these structures deeply affect the environments around them. That’s what makes American and Eurasian
beavers textbook examples of Ecosystem Engineers. An ecosystem engineer is any organism that
physically changes its habitat in a way that affects other species, by altering the availability
of certain resources. And beaver dams do just that. They can change the flow of streams and rivers,
and often cause ponds to form, creating habitats for fish, amphibians, and other creatures. Plus beavers also leave their mark on forests. Some beavers cut down trees as far as 200
meters away from their home pond. This has a huge influence on which plants
grow where. Now, the two modern beavers belong to the
genus Castor which likely evolved sometime in the past 10 million years. And their dam- and lodge-building behaviors
may have evolved as a way for them to survive hard winters at higher latitudes. So, because both species are builders, some
experts think the very first beaver dams and lodges might’ve been built around this time. But it looks like woodworking wasn’t really
Castoroides’ thing. Based on a study of stable isotope signatures
found in its teeth and bones, which can help tell us what an animal ate, it looks like
Castoroides mostly ate freshwater plants, like water lilies, pondweeds, and sedges. So, diet-wise, it had more in common with
today’s muskrat than it did with either living beaver. And if Castoroides didn’t chow down on logs,
then it probably wasn’t using those things to make lodges or dams. And that would’ve left the giant beaver
at the mercy of Earth’s changing climate. When the most recent glacial period ended
around 11,700 years ago, things got warmer and drier, and the swamps and lakes that Castoroides
depended on became more scarce. Slowly but surely, its range got smaller and smaller It was eventually driven out of the Great
Plains, as wetlands and forests gave way to arid grasslands. Meanwhile, in Florida, there was a temporary
rise in scrubland and prairie habitats, which coincided with the giant beaver’s disappearance
there. The last known population of Castoroides died
out in the Great Lakes region about 10,000 years ago. But the Eurasian and American beavers are
still with us, maybe because, unlike Castoroides, they can customize their habitats by building
lodges and dams. So when the climate changes or when resources
grow scarce, they’re less vulnerable than some other species. Like its giant ice age colleagues, like the
Columbian mammoth, the short-faced bear, and other mega-mammals, Castoroides had some
impressive proportions. But today’s beavers have engineering on
their side. And whether you’re big or small, if you
can shape the world around you, it could mean the difference between survival and extinction. We’d like to thank these three researchers
for their help with this episode - Thank you so much! And if you want to know more about the beavers
that are still around, check out Animal Wonders hosted by Jessi Knudsen Castañeda Every week on the Animal Wonders YouTube channel,
Jessi features different animals and shares what it’s like to keep them happy and healthy. One of these fantastic animals is Huckleberry
the beaver! If you’d like to learn all about Huckleberry’s
story and how he’s grown from a tiny fluff to a great big member of the Animal Wonders
team, there’s a link in the description to a playlist all about him. Also, it’s been nice gnawing this month’s
Eontologists: Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick
Seifert! You can become an Eonite by supporting us
at patreon.com/eons. Plus, Eonites get sweet perks like submitting
a joke for us to read! Like this one is from Julia M. Here we go. Sight unseen. To attract new visitors,
a museum was giving away dinosaur vertebrae: Everyone was taken aback! I just. I'm speechless. And as always thanks for joining me in the
Konstantin Haase studio. Subscribe at youtube.com/eons for more adventures
in deep time.
