We humans might be crunchy on the inside, but there are a bunch
of other vertebrates out there rocking bones inside their skin,
completely detached from the rest of their skeletons.
These structures are called osteoderms, and if you look at a few examples,
you might think their sole purpose is to turn their owner into a tank.
That’s definitely one reason an animal might evolve them.
But it’s not the only reason.
From keeping their blood from getting too acidic,
to helping them slip away from predators, here are five animals
that have found some ingenious ways to use their skin bones.
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Osteoderms have evolved a bunch of separate times,
in all sorts of four limbed vertebrates known as tetrapods.
I’m talking everything from reptiles to amphibians to mammals.
Yeah, mammals.
You might not think that an animal known for having fur or hair all over its body
would also grow bony skin.
But the armadillo’s out there anyway, living its osterodermed life.
And until 2023, armadillos were the only living mammals
that scientists had definitively proved grew dermal bones.
But as it turns out, we just needed to take a closer look.
The researchers behind this discovery were scanning rodents
from museum collections using microtomography, a technique that uses x-rays
to produce a high-resolution, 3D view of a specimen’s insides.
And while checking out their scans, they noticed that the tails of one species,
the Cairo spiny mouse, looked strangely dark.
The tails were covered in tiny osteoderms,
set together like cobbles on a medieval street.
The team then widened their investigation to other species
in the same taxonomic sub-family.
And what did they find?
A bunch of different rodents with osteoderms in their tails.
Sure, it’s not as much coverage as an armadillo’s got, but it counts.
It counts!
But why would a spiny mouse have a bone-encrusted tail?
Well, previous research has found that out in the wild,
many of them have lost their tails.
So the researchers proposed that, like an armadillo,
spiny mice use osteoderms for protection.
But while an armadillo uses them to emulate a military tank, these osteoderms
allow for a one-shot defensive maneuver in the form of a removable skin sheath.
Not only do these bones shield the inner tail's tissue and bone,
they seem to help the rodent self-amputate to avoid getting eaten.
If a spiny mouse has been cornered in a hole, or pinned by the tail,
it can shed its tail skin like a glove…
…A glove that’s sitting on top of a layer of chainmail
that prevents a predator’s teeth or claws from sinking too deep.
Now, spiny mice can't regrow their tails, but they do display an exceptional ability
to regenerate skin, muscle, and even nerve cells.
So once it’s safe, our survivor can chew off the rest of its tail
and the wound will heal pretty quickly.
Which is great for them.
But just hearing about it makes me want to watch a video of a child
surrounded by a hoard of golden retriever puppies.
To, you know, wash it out…of my brain.
Anyway, 2023 wasn’t just the year we learned
there were several living mammal species that grow osteoderms.
It was also the year that we learned that snakes…
any snake at all…could grow them, too.
Despite osteoderms evolving independently at least twenty times
throughout all of reptile-dom, snakes were a notable holdout.
And on one hand, it makes sense.
Having bones growing in your skin would seem to make a body rigid,
and snakes need to stay fast and flexible.
However, some legless lizards are completely bedecked with osteoderms
and don't have any trouble getting around.
So a group of researchers set out to find them in snakes,
again using microtomography scanners.
And sure enough, they found them…in the tail tips of four sand boa species.
Sand boas are burrowing snakes,
and their heads and tails look a bit similar to each other.
It’s an attribute the snakes use to get a tactical advantage in combat.
They basically pretend their tails are their heads,
moving them around in a head-like fashion
to lure enemies away from the biologically important stuff.
And if that tail is also armored, well, it’s even more advantageous.
Strangely enough, the team proposed that their armored tails
might be designed to withstand attacks from not only predators, but also prey.
That’s because sand boas spend a lot of time pillaging underground rodent nests,
and the tunnels they have to crawl through are pretty narrow.
So if Mama Rodent returns home from a little foraging trip
at just the wrong moment, she’ll come face to, well, not face
with an armored snake tail that looks a lot like a snake head.
And like I imagine my own mother would do,
rather than run away to save herself, Mama Rodent will attack.
While she works on the osteoderm end of the snake, the sand boa has time to plan
and execute its grand exit, with a belly full of rodent babies if it's lucky.
So we’ve got osteoderms on some animal tails that are so small
humans need special technology to see them.
That’s very much not the case with our next entry on this list, the order Crocodilia.
This group of reptiles includes actual crocodiles, alligators, and caimans,
plus a couple other toothy critters that all share an ancestor
that lived around 95 million years ago.
Croc osteoderms can help protect their internal organs and tissues,
but they also stiffen their backs,
to allow them to move more easily while on a nice terrestrial stroll.
Have you ever seen a croc scamper on land?
They do look kinda funny…and terrifying at the same time.
But that’s not all.
Crocodilians may have bone skin as a side effect of evolving away from their
landlubber ancestors and into a semi-aquatic lifestyle.
Crocs spend a lot of time holding their breath underwater.
And when they’re not breathing, carbon dioxide builds up in the bloodstream.
If they’re under for too long,
there will be so much CO2 it’ll make the blood too acidic.
It’s a condition called acidosis, and it can prevent tissues from taking up oxygen,
which can result in death.
But here’s the cool thing about crocodilian osteoderms.
They’ve got a lot of blood vessels flowing through all that bone.
So scientists think their bodies use osteoderm minerals to act as a sort of buffer.
Basically, the bone releases calcium and magnesium ions into the bloodstream,
where they help neutralize some of the excess acid.
But that’s not the only way acidosis can happen, or how osteoderms can prevent it.
During bouts of intense physical activity, a crocodilian’s tissues need more oxygen
than its cardiovascular system can supply.
So instead, these animals often have to rely on an oxygen-free, or anaerobic,
method to make the energy their bodies need.
But energy isn’t the only thing that ends up being produced.
There’s also a compound called lactate.
Which is okay in small doses, but can turn nasty in large ones.
When a croc spends a lot of time submerged underwater,
or needs a big burst of energy to chase some prey for a few minutes,
all the lactate it ends up making can lead to acidosis, too.
But one study of caimans found that their osteoderms can temporarily absorb
lactate, instead of letting it float around in the bloodstream
where there isn’t enough oxygen to clear it out.
So this bone skin offers crocs a one-two punch against acidosis,
and it’s still not the end of the story.
Because all those blood vessels may indicate that osteoderms
can also help regulate an animal’s temperature, too.
But to explore that further, we’re going to jump over
to one of the most iconic animals to ever wield them.
Stomping around what’s now Colorado, Utah, and Wyoming
roughly 150 million years ago, Stegosaurus had a double row of osteoderms
running down either side of its spine, from the neck all the way to its spiky tail tip.
Scientists still aren't sure what purpose those bony back plates served,
but thermoregulation is a hypothesis that’s popped up
more than once over the past several decades.
Considering that most animals with sticky-outy parts like large ears or horns
use them to either absorb or dump excess heat,
the thermoregulatory idea makes a lot of sense.
And these days, many paleontologists seem to agree that Stegosaurus
osteoderms had the potential to do at least a little thermoregulating.
That’s due to a few factors, like the plates’ alternating position along the back,
as well as their overall size and extreme thinness.
Scientists are also pretty sure that these skin bones
probably weren't robust enough to protect them from predators.
But if not thermoregulation, or anti-predation, then what was their primary purpose?
Well, Stegosaurus might have used its osteoderms for
showing off in one way or another.
The size, shape and placement of these bony plates
varied widely between Stegosaur species.
So they could have been ornaments that helped a given individual
recognize members of its own species.
Or maybe they were used as props in mating displays, or to intimidate rivals.
I guess it turns out that osteoderms aren’t always there to help save your life.
Sometimes, they might just zhuzh up your look.
And personally, I’m here for it.
You do you, Stegosaurus.
But why have one dinosaur example when you can have two?
Our final entry is one of the largest dinosaurs known to have osteoderms…
a species of titanosaur called Rapetosaurus krausei.
Also known as R. krausei if you’ve got somewhere else to be in a jiffy.
And some paleontologists think their fancy skin bones
served a purpose kinda similar to camel humps.
See, R. krausei lived in arid environments,
where nutrients weren’t always easy to come by.
And back in 2011, one team of researchers reported
that one of the two osteoderms they studied was hollow.
In fact, it had a cavity that measured 9.6 liters in volume!
That’s…this much.
This is really heavy.
Meanwhile, the other osteoderm belonged to a juvenile, instead of an adult.
So even though the bone wasn’t hollow, it had enough smaller empty spaces
inside it that the team proposed it could hollow out as the animal grew.
And the team also proposed that R. krausei
used at least some of its osteoderms as mineral storage reservoirs.
That might sound a bit strange, but it isn't totally coming out of left field.
Female alligators appear to extract calcium from their osteoderms
to help build their eggshells.
Researchers have x-rayed female alligators who were just about to
or had just laid eggs, and have found that their osteoderms are up to 10%
less dense than those in alligators who weren’t currently ovulating.
Of course, this hypothesis is based off a single dinosaur fossil,
so the reason why two of its osteoderms are hollow
may have nothing to do with modern alligator anatomy.
The two species merely share a common ancestor.
But that opens up a very important line of questioning.
Osteoderms are found in everything from extinct sauropods to extant spiny mice.
Is this all a case of convergent evolution, where different lineages
just happen to evolve structures that look the same and serve a similar purpose?
Or is there a fundamental piece of biology at the base of our tetrapod family tree?
Lots of scientists think it's the latter,
but they don’t have enough evidence to definitely answer that question.
At least not yet.
But for now, we can all admire the skin bones of the world,
and imagine what any of us would do if we had a set.
I’d use them as cool elbow pads to skateboard.
Now, if all our talk about croc and dino osteoderms has got you hankering
to learn more about the crocs of yesteryear…or yester-era, we’ve got you covered.
Because bone skin’s certainly cool, but ancient croc species rocked a bunch of
fun biology, from battering ram snouts to shark tail-shaped flukes.
And you can learn about some of them in our video
about the weird world of prehistoric crocodiles.
Go check it out. And thanks for watching SciShow!
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