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. [♪ INTRO] 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! [♪ OUTRO]