In the evolutionary arms race  between predator and prey,   mammals have mostly gotten the short end  of the stick when it comes to body armor.

There are, of course, a few  exceptions to this rule. Like, a couple of different groups  of mammals have spines or quills,   like porcupines, hedgehogs, and tenrecs.

And then you’ve got the pangolins  with their keratin scales, which,   like quills and spines, are made of the  same material as hair and fingernails. But these defenses are really only surface-level. If you want more heavy-duty body  armor, you need to get bones involved.

And that makes these super-healing  spiny mice super weird mammals. [♪♪ INTRO ♪♪] If you want to support the show,  the Bizarre Beasts pin club is open   for subscriptions again from  today through December 11th! Sign up now and the first pin you get  will be one of these Bizarre Beasts! Spiny mice and their closest relatives   belong to four different genera of  rodents in the subfamily Deomyinae.

Sometimes they’re all called ‘spiny mice’ and  sometimes each genus gets its own common name,   so for the sake of simplicity, we’re just  going to call them all spiny mice here. Spiny mice are found throughout Africa, in some  parts of the Middle East and into Pakistan,   and on a couple of Mediterranean  islands, including Cyprus and Crete… though they might be extinct on  Cyprus, no one’s entirely sure. They mostly eat insects, but they’ll  also consume small vertebrates and   plant material, and sometimes even each other.

And their behavior is all over the map. Some of them are nocturnal, some are diurnal,   and some are crepuscular,  which means active at twilight. Some of them build nests and simple burrows,   and others just hang out in rock  crevices and termite mounds.

And they can be social or solitary  or bonded in monogamous pairs. Spiny mice really just do it all. And, looking at them, you might be thinking: that is just a regular mouse, where is the armor?

Well, they got the name ‘spiny mice’ because  many species in the group have stiff hairs   in the outer layer of the fur on their  backs, similar to the spines of hedgehogs. But their real, secret armor is  actually hidden in their tails,   which researchers only fully described  in a paper published in May of 2023. It’s made of bony plates called osteoderms that   form within the middle layer of  the skin, known as the dermis.

The dermis is sandwiched between  the outer layer of the skin,   the epidermis, and the deepest inner  layer of the skin, the hypodermis. The spines, quills, and scales  that make up the visible armor   of other mammals all come from  modifications of the epidermis. But osteoderms are different, because  they’re made of bone, not keratin.

And they’re kind of a weird type  of bone for mammals to have. Most of the skeleton of mammals and other bony  vertebrates, like reptiles and birds, forms   when a cartilage template of a bone is slowly  replaced by actual bone tissue during development. But there are exceptions to this rule.

They’re called dermal bones, and  they’re built within the dermis itself, they’re literally bones that  grow within that skin layer. Osteoderms form in this less-common  way, and along with the collarbones,   they’re the only dermal bones found  outside of the skull in mammals. And spiny mice aren’t the only  mammals to have osteoderms.

Armadillos have them, and some giant  ground sloths, and one strange,   ancient hedgehog-like animal from  the early Eocene also had them. But they are a very rare  evolutionary innovation in mammals. Outside mammals though,  osteoderms are kinda everywhere.

They’ve evolved independently  at least 19 times in amniotes,   the group that includes all  reptiles, mammals, and birds. Lots of different kinds of lizards  have them, some dinosaurs had them,   and crocodilians and turtles have them. And they’re pretty common in amphibians, too.

Like, there are even frogs that  have them and they seem to have   evolved independently a number  of times in that group, as well. But, why are osteoderms so common in  reptiles and amphibians, but not in mammals? Well, it might just be that mammals have hair,   which basically gives evolution a  different pathway for building armor.

But then what’s going on with  spiny mice, who have both? Why do they have osteoderms in their tails? Well, it may actually be connected to another  weird evolutionary adaptation of theirs: super-healing.

See, spiny mice have incredibly fragile skin. And this is going to sound  kind of gross and painful,   but it tears much more easily  than the skin of other mice. The good news is that they  can regrow their tissues – skin, nerves, muscles, even their spinal cord – completely without scarring and they do it  twice as fast as their rodent relatives.

And their tear-away skin really comes into play   when a predator tries to chomp  on the tail of a spiny mouse. Basically, spiny mice can do this  thing called ‘degloving’ their tails,   where the outer skin of the tail peels away from  the muscle and bone in the grasp of a predator. Apparently this does not  cause much bleeding, which is… good?

I guess? And this is also going to sound gross  and painful, but the mice then chew   off their tails because, while they can  heal a lot of different kinds of tissue,   they don’t seem to be able to  grow back their entire tail skin. One study even found that, in  some populations of spiny mice,   63% of males and 44% of females were  missing all or part of their tails.

And one possible reason de-gloving seems  to work as a defensive strategy is that   the osteoderms might provide enough of a barrier   to keep a predator’s teeth out of the  part of the tail that doesn’t detach. This would give the mice a chance to escape,  by trading their tails for their lives. And it looks like this ability, along with  their osteoderms, probably evolved only once – in the shared ancestor of the  whole subfamily of spiny mice,   rather than independently in each genus.

One of the authors of the study that  described the osteoderms thinks this   is because the closest living relatives  of the spiny mice subfamily, the gerbils, don’t have tail osteoderms. And in evolutionary biology, we tend to prefer  the evolutionary tree with the fewest steps – the simplest explanation is the most likely one. But explaining these mice with their  tear-away tail skin and secret armor   reminds us that even simple-looking  animals can be more than meets the eye.

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