How Armadillos Get Their Armour

Screaming hairy armadillo

Reptiles are scaly, and mammals are furry. That's a broad general rule, and holds in most cases. But there are, of course, a number of exceptions. Many mammals, such as dolphins and hippos, are not at all furry, for example. And, while there are no hairy reptiles, there are two groups of mammals that have scales.

One of these, the pangolins, have scales that are quite unlike those of reptiles, giving them a unique appearance. The other is the armadillo family, and, in their case, the scales really are quite reptile-like. It's a form of parallel evolution, where two or more groups of animal independently evolve the same solution to a particular problem. The ancestors of armadillos would have been as hairy as any other mammal, but when they evolved protective armour, they ended up with something that is quite similar to the scaly plates of, for example, crocodiles.

It's not the only way in which armadillos are weird, although it may perhaps be the most obvious on casual inspection. They're fairly strange on the inside, as well, with, among other things, a different structure to their backbone and hips than that of most other mammals. Their closest living relatives are the sloths and anteaters, which are rather odd animals themselves. Even then they aren't all that close - about as much as humans are to rabbits, perhaps.

The armour of most species of armadillo covers their back, flanks, forehead, and tail. The underside has relatively normal skin, and in many species is quite hairy. Even the armoured parts of the body do, in fact, have hairs, although, at least in the more familiar species, these are short and sparse, poking up between the scales.

The skin of mammals (and, for that matter, vertebrates in general) consists of two layers. The inner layer, or dermis, consists of connective tissue, with blood vessels and nerves, and also containing things like sweat glands and hair roots. Above this is the epidermis, a solid layer composed of multiple sheets of cells. The cells at the base of this layer are continually dividing, pushing the new cells upwards. As this happens, the cells flatten and die, eventually forming a sheet of keratin, a tough protein that forms the outermost protective layer of the body. The keratin constantly flakes off, so that it can be replaced as it is worn away by contact with the outside world. Eczema and psoriasis are two of the things that can happen when this process fails.

In humans, and most other mammals, the keratin layer is very thin and flexible, allowing us to move about without difficulty. It varies across the body, though, being thickest on the soles of the feet, which, especially before the invention of shoes, are the parts most likely to be rubbing against rough ground. In armadillos, the scales are also formed of keratin, but here it is much more compact and solid, rather as it is when it forms human fingernails and toenails.

But this isn't the full extent of the armour. Underneath, in the dermis, are solid pieces of bone called osteoderms. In humans, the only bones present are those in the skeleton proper - a set of internal supports and protective cases, most of which are linked together by various kinds of joint. But, in many animals, pieces of bone can also form in more external parts of the body, forming an outer shell, often with no direct connection to the internal skeleton. This is what happens in crocodiles, along with many other kinds of animals, including a number of fish and even amphibians, as well as reptiles. There are no birds with osteoderms - being armoured isn't conducive to flight - and armadillos are the only living mammals to have any.

Over most of the armoured parts of the body, the osteoderms lock together to form a solid sheet of bony amour. This naturally makes armadillos relatively inflexible, and the sheets are not so solid where they are found on the legs. As is obvious from looking at any armadillo, they are also divided into segmented sheets along the middle of the body, the only reason that the animal is more flexible than a tortoise.

The structure of the osteoderms is similar to that of any other bone. They have a solid outer layer, surrounding a more spongy interior with spaces for blood vessels and so on providing nutrients for the bone's living tissue, without which it couldn't repair itself. In some armadillo species, there are even bone marrow cavities in the centres of the armour plates, although this is not universal. On the other hand, there are some differences, most notably the presence of cavities for hair follicles and sweat glands, which the armadillos still need, despite their armour.

One way to help us understand how such strange structures evolved is to see how they grow in very young armadillos. One has to be very careful when doing this, because what you aren't going to see is the evolutionary process unfolding like a time lapse film.

Back in the late nineteenth century, once evolution had been accepted, it had been thought that embryos of animals run through their evolutionary history as they grow. They start as a single cell, become more complex as they develop, and, in the case of mammals, go through a phase in which they look somewhat fish-like, before developing limbs, lungs, and so on. It turns out that this is a massive over-simplification. For one thing, the 'fish-like' stage of a mammalian embryo doesn't have any gills. In reality, the growing embryo isn't showing what its ancestors looked like as adults, it's following the path of their embryos, at least until it needs to make a change so that it can grow into something different. And, naturally, it's also constrained by the need for things like an umbilical cord, and other purely 'embryonic' structures.

Nonetheless, seeing how embryos, or newborn young, develop, can at least give us some pointers in the right direction. A new study, by Cecilia Krmpotic, examined and compared, for the first time, the armour of two different species of armadillo in newborn young. It's not the first study to be done on the development of armour in young armadillos, which date back to at least the 1920s, but it is (apparently) the first to try and compare different kinds of armadillo to put them into a clear framework.

The southern long-nosed armadillo (Dasypus hybridus), is a close relative of the nine-banded armadillo familiar to many residents of the southern US, among other places. It lives rather further south, in southern Brazil and Paraguay, in Uruguay, and in north-eastern Argentina, but, physically, it looks very much like its better known relative. The screaming hairy armadillo (Chaetophractus vellerosus) - and, yes, that's really it's name - lives further west, in northern Argentina, southern Bolivia, and western Paraguay. It looks rather different, being much hairier and slightly less heavily armoured, and, indeed, as armadillos go, it isn't a particularly close relative of the other species.

One of the findings of the study was that the structure of the epidermis was different underneath the scales, lacking a key layer that, in humans, is partially responsible for waterproofing, and also for shedding dead cells from the surface of the skin. The hairy armadillos were born entirely without scales, and their skin had much the same structure all over the body, whereas the long-nosed armadillos had more 'normal' skin only on their undersides, where there were no scales. On the other hand, the hairy armadillos did have more developed hair follicles, something that the long-nosed armadillos lacked, even though they have hair as adults.

Long-nosed armadillos, like the nine-banded armadillos that were the only species to be thoroughly studied before, also turn out to be born with some of their bony armour already in place. It's thin, incomplete, and not yet very effective, but it is there. The hairy armadillos are born without any trace of the armour, which must obviously develop some time later. There are some folds in the skin where the bands around the mid-body are going to form, but the skin itself is no tougher than anywhere else on their bodies.

This difference is likely due to the difference between the young of the two species, and shows that there isn't simply a 'one size fits all' description of armadillos as a group. Long-nosed armadillos are born reasonably active, already looking much like miniature adults, and are able to walk about within a few hours. Hairy armadillos, on the other hand, are born blind, and almost incapable of movement, with their ears flat against their skull, somewhat as young kittens or puppies are. At least in the closely related big hairy armadillo, the eyes don't open for two to four weeks after birth, and until then, they are clearly pretty helpless.

It is, however, somewhat unusual that helpless young are born with more hair than those from species that are more active. In many mammals with blind and helpless young, such as mice, those young also also hairless. While the newborn pups of hairy armadillos are indeed bald, their hair follicles are better developed than in the long-nosed species, and presumably begin to sprout much earlier. This may be because of their preferred dry habitats, which will have relatively cold nights compared with the moister environments of the long-nosed species.

As for the osteoderms themselves, there's no way to tell how they form in the hairy species from this study, as there was not even the faintest sign of their presence in the specimen examined. In the long-nosed species, however, they were already growing, and they were surrounded by a cluster of specialised cells called osteoblasts. The function of these cells is to create bone, but they're normally found either inside existing bone, or in cartilage, not in the skin.

With the internal skeleton, what usually happens is that the embryo first lays down a framework of cartilage, and, as it develops, this mineralises and transforms into bone, following a fairly predictable pattern. Even by the time that humans, for example, are born, this process has yet to fully finish, which is one reason why babies are delicate compared with adults. This process was observed, as one might expect, elsewhere in the armadillos' skeletons, but not in the osteoderms.

With the little plates of bone simply lying in the dermis, surrounded by bone-building cells, it appears that the collagen fibres of that skin layer are being woven into a sort of immature, non-mineralised, bone called osteoid. As minerals begin to deposit there, this slowly turns into true bone, and so the osteoderm is created, without the need for cartilage.

Crucially, this doesn't seem to be what happens in crocodiles, or most other scaled reptiles and amphibians. Here, the collagen itself begins to mineralise, without the need for osteoblasts, or any kind of osteoid, let alone cartilage. This shows that, despite their similar appearance, the bony scales of armadillos are quite different from those of crocs and the like.

Interestingly, though, it is the way that the bones of the skull form in vertebrates. Unlike limb bones, ribs, and so on, there is no cartilaginous framework for the skull; many skull bones form directly from osteoid. Note that these are, roughly speaking, plate-like bones - if somewhat curved and modified. We've known this for a long time, and a popular explanation has always been that the bones forming the armoured casket around the brain were, in some distant ancestral vertebrate, protective osteoderms.

[Picture by Arnaud Boucher, from Wikimedia Commons]