How Sloths Went From the Seas to the Trees


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: Pretty slowly. 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 digging. 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 ones do. 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 sloths. 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 hypothermic. 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 mouths. In both extinct and living sloths, their teeth
don’t have any enamel, so they aren’t as hard. 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 each other. And that knack for picking out resources that
nobody else wanted, resulted in some pretty strange sloths. 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 face. 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 all. 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 trees! Which means that being suspensorial must have
evolved twice — independently — in each lineage! 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 for predators. 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! If you want to join them in supporting this channel, head over to patreon.com/eons and make your pledge for some neat n nerdy benefits. Now, what do you want to learn about? I want to learn about how dogs turned into seals and stuff Leave me a comment, and don’t forget to go to youtube.com/eons and subscribe.

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