8 million years ago, off the coast of Peru, a large mammal used its powerful claws to
pull itself along the ocean floor, holding fast against the waves as it foraged for seagrass.
5 million years ago, a similar creature was burrowing underground in Argentina, digging
burrows so massive that you could walk right into one without having to duck your head.
And just 20,000 years ago, another member of this same group of animals climbed the cliffs
as high as skyscrapers, leaving behind the beautifully preserved sheaths of their claws
in caves far above the ground.
But today, the only living relatives of these animals hang upside down from trees.
They poop only once a week, and move so slowly that algae grows on their fur.
But over time, sloths have seen a lot of territory, and their ancestors evolved to fill a really
surprising and diverse array of niches.
The story of sloths is one of astounding ecological variability, with some foraging in the seas,
others living underground, and others still hiding from predators in towering cliffs.
So how did an entire family of burrow-digging, sea-faring, cliff-hopping sloths go extinct,
and their only living relatives end up in the trees?
Well, forgive me for this, but:
Sloths are members of the order Xenarthra, which also includes armadillos and anteaters.
And it's not clear what the last common ancestor of all Xenarthrans was, but genetic studies
of living species suggests that it probably lived more than 65 million years ago.
Which means this early xenarthran lived among the feet of the last non-avian dinosaurs.
But it probably resembled an anteater or an armadillo more than a modern sloth, in that
it was really built for digging.
We know this, because all Xenarthrans share features that are perfect for burrowing.
For example, they all have long, curved claws; and extra contact points between their vertebrae
and their pelvis, which gives them a stiffer lower back to free up their front limbs for
But since the days of that common ancestor back in the Cretaceous Period, Xenarthrans
have crawled beyond the forest floor and into many other ecosystems.
Sloths have been preserved in the fossil record at least since the early Oligocene, and their
fossil record is huge in South America – representing about 80 to 90 different genera over the last
30 million years.
And many even lived in the same area.
One 17-million-year-old site in Patagonia was home to 11 different genera of sloths
from 3 different subfamilies!
And I mention this because … that’s just really strange.
Usually, herbivores of the same size compete with each other for food.
So how did so many sloths live together without competing each other into extinction?
The answers to that question can help us understand why sloths have been been able to occupy so
many niches over time, and why they’ve been so successful, even to this day.
For one thing, ancient sloths likely had a very low basal metabolism, much like modern
That means that when they weren’t doing much, they weren’t using as much energy
as other mammals of the same size.
This is typical of all xenarthrans, and there’s some evidence that it was the case for ancient
For example, some research has revealed a relationship between an animal’s body temperature
and the amino acids found in its bone collagen.
And a study of fossils of the Shasta Ground Sloth, a giant sloth from the Pleistocene,
found that it probably had a body temp of about 35 degrees celsius.
That’s colder than it sounds: If your body fell below 35 degrees, you’d be considered
Another thing that helped ancient sloths thrive in weird places, and in such large numbers,
is that they could eat all kinds of stuff that no other animals were interested in.
We don’t have any fossils of their stomachs, obviously.
But they probably had digestive systems that were just as complex and efficient as modern
sloths’, with four chambers, which allowed them to digest tough, fibrous leaves that
other herbivores didn’t bother with.
And the last thing that gave these ancient animals an unexpected advantage was in their
In both extinct and living sloths, their teeth don’t have any enamel, so they aren’t
But, they’re always growing, so sloths can eat pretty much anything without running out
of tooth surface.
So, by having really low metabolism, an efficient gut, and teeth that don’t get worn away,
sloths were able to make a good living by specializing in their own favorite hard-to-digest
foods that no one else wanted, so they didn’t have to compete with other herbivores, or
And that knack for picking out resources that nobody else wanted, resulted in some pretty
Take the sea sloth Thalassocnus.
Which was very hard to say
8 million years ago, in the Late Miocene Epoch, the Peruvian coast was a hot, dry desert without
much food for a large herbivore.
So to make ends meet, an early species of Thalassocnus turned to the sea and the grasses
that grew in the shallow water.
Over the course of about 4 million years, it became more specialized for an aquatic
life, developing denser ribs and limb bones to help weigh it down as it began foraging
in deeper waters.
It also grew in size, eventually becoming as big as a modern sea lion.
Still, Thalassocnus probably wasn’t a great swimmer.
Instead of having, say, flippers, it used its big claws to pull itself along the ocean
floor against the force of the crashing waves.
And this, too, was an important adaptation, because those waves were dangerous: Several
fossils of Thalassocnus have been found with broken and re-healed leg bones, signs of trauma
from being swept away and bashed up against the rocks.
But sloths’ massive claws allowed them to thrive in other habitats, too.
In Brazil and Argentina, massive burrows reaching up to almost 80 meters long have been found
in Pliocene and Pleistocene rocks.
That’s right – rocks, not just dirt.
Some of these burrows are so big that they’re practically caves, and giant ground sloths
were the only things around that could dig a hole that big.
We don’t know for sure which species managed to engineer these amazing burrows, but it
was probably something like Lestodon
Weighing as much as a small elephant and standing almost 2 meters tall on all fours, Lestodon
was certainly big enough to dig even the biggest of these burrows.
OK, but: How?
Well, even though these sloths were big and slow, they probably didn’t have to work
too hard to dig.
These caves are actually found in rock with lots of fissures and faults, so Lestodon and
other sloths likely pulled loose rocks out of the wall, instead of, like, furiously digging.
And once they were done, they had managed to make a useful refuge for themselves.
In addition to providing a safe place to live, the stable climate of the caves probably helped
keep the sloths’ body temperatures steady, a big plus when you’re, ya know, metabolically-challenged.
But, other sloths lived in caves that they didn’t have to dig for themselves.
Take the wee Peruvian sloth with the awesome name of Diabolotherium, or “devil beast,”
from the Late Pleistocene.
Unlike other ground sloths, Diabolotherium had a much greater range of motion in its
elbows and forearms, making it an excellent climber – of rocks.
Its fossils are found almost exclusively in caves, including ones 300 meters up a cliff
And while Diabolotherium was climbing cliffs, there were also other sloths doing what we know
them best for: Living in trees.
As early as the Miocene Epoch, semi-arboreal sloths can be found in the fossil record - like
Hapalops or Nematherium.
Both of them had flexible elbow joints suggest that they could climb trees, and they’re
probably ancestral cousins of the modern 2-toed sloth.
But they weren’t small like the sloths we know today.
Weighing between 40 and 90 kilograms -- say, from the size of a deer to a kangaroo -- they
could only have been supported by the very biggest branches.
And they definitely didn’t hang upside-down.
So sloths have inhabited caves, dived in the seas, lived on the ground, and climbed up
in the trees.
And with their super low metabolism, they were able to find nutrients where others couldn’t.
But this brings us to perhaps the weirdest part of the whole story of sloth evolution:
the sloths that survived to the modern day.
Both of the two modern genera of living sloths live in Central and South America.
There are the three-toed sloths, which are members of the genus Bradypus.
And there’s the two-toed kind, known as Choloepus.
And they’re both are suspensorial.
Which means they spend their time not just in trees, but suspended from trees.
Being suspensorial is basically next-level tree-dwelling, and both tree sloths are so
well adapted to it that, today, that they can barely walk on the ground at all.
But the confusing thing is … there aren’t any fossils of suspensorial sloths -- at all!
-- including those of Bradypus or Choelopus.
Which makes it really hard for us to understand their evolutionary story.
And research into their genomes hasn’t helped much either, because -- to make things even
stranger -- these two surviving tree sloths don’t seem to be very closely related, at
In fact, Choloepus is probably a member of Megalonychidae – a family that included
some pretty big, bad ground sloths - like Megalonyx, which lived in North
America as much as 10 million years ago.
But Bradypus may be the oldest and most primitive member of all sloths, its lineage having split
off from other fossil and modern sloths as much as 30 million years ago.
It’s so strange that some paleontologists put it into its own family, all by itself.
So, they come from different ancestral groups, but today’s modern sloths both hang from
Which means that being suspensorial must have evolved twice -- independently -- in each
But when you think about it, this tree-hanging lifestyle totally fits with the adaptations
that all sloth ancestors had, just as it did for those that foraged on the sea floor or
lived in cliff caves.
Living the suspensorial life uses less energy, which is great if you have low metabolism,
and those claws that used to be so good for climbing also work perfectly for hanging!
But why are these two small, tree-dwelling sloths the only ones we have left?
Well, after enjoying a good, 20-million year run during the balmy climate of the Miocene
and Pliocene, the giant sloths came face to face with the Ice Age.
Their low metabolism made it harder to be active in the cold.
And for some, their body plans adapted to the cold by becoming larger.
Because big animals are much more resilient to cold temperatures.
This is what happened to the North American ground sloth Megalonyx.
It went from being about 185 kilograms 10 million years ago, to being about a thousand
kilograms just a half a million years ago!
But being big wasn’t an advantage when the climate started to warm again.
Their low metabolism meant that sloths couldn’t control their internal temperatures very well,
and when the Ice Age ended, the bigger sloths risked overheating if they became too active.
This made it harder for them to survive and reproduce, while also making them easy targets
That was especially true in South America, where predators like wolves and jaguars wreaked
havoc on many species, driving many sloths to extinction in the Pliocene and Pleistocene.
Meanwhile in North America, almost all sloths had become really large, and therefore, harder
to cool down.
And at the end of the Pleistocene, hunting by humans may have been an issue on both continents.
But for small, tropical sloths, their combination of a low-energy diet and a suspensorial lifestyle
helped protect them from all of these threats.
So, they may have never been the fastest or most nimble animals.
But over millions of years, sloths have shown an enviable ability to gradually adapt to
environments as diverse as the ocean, hard-rock burrows, and tropical forests.
If evolution is indeed a contest among the fittest, sloths are here to remind us that
slow and steady can still win the race
Thanks for joining me today here on Eons.
And BIG thanks to our Eontologists: Jake Hart, Jon Ivy, John Davison Ng and of course, everybody’s pal, STEVE!
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