Why is equid dentition so extremely complex and space-consuming?
(writing in progress)
This topic of equid dentition is remarkable for two reasons. Firstly, the teeth of horses and their relatives are adaptively extreme. Secondly, they have been so poorly described that most naturalists may remain unaware of their true nature.
One might think that, with an animal as familiar as the horse (with many veterinary problems regarding the teeth), it would be the simplest thing to look up the facts. Instead, in the absence of a basic, clear description in the literature, I here attempt one of my own.
The horse is hypsodont, in contrast to mammals with ever-growing teeth, which are instead called hypselodont.
Equid teeth do not simply keep growing, as they do in rodents (https://en.wikipedia.org/wiki/Rodent), lagomorphs (https://en.wikipedia.org/wiki/Lagomorpha), and the various extinct notoungulates (https://en.wikipedia.org/wiki/Notoungulata) of South America. Instead, there is a system so indirect and so complicated that it is hard to describe.
Instead of having ever-growing teeth, adult equids have deep but fully-formed teeth, that lie hidden in deep sockets and gradually erupt. By erupt I do not mean ‘grow’ in the sense of the ever-growing teeth. I mean a kind of recoil out of a deep socket, so that the roots – which have been fully formed and ‘sealed’ at adulthood – actually well up out of the jaw towards the surface, and are finally exposed when the worn-out tooth falls out in senility.
An empty space is left behind by the eruption of the long, columnar tooth, but I have yet to find a statement in the literature on what this contains. I assume that it contains liquid rather than gas.
The sockets of equid teeth are so deep that they actually project into the jaw sinuses. This in itself is unremarkable, because even human teeth show the same pattern. However, in equids the jaws are so deep that it is noteworthy that, even with all that space in which to embed the teeth, the socket-casing still projects into the sinus.
I have found statements in the literature that, as the tooth erupts, the sinus is enlarged. This implies that the socket-casing is indeed remodelled as the animal ages. If so, it is not simply the case that the whole socket is fixed after adulthood, and simply vacated by the erupting tooth. Instead, a complex process occurs as the tooth rises, roots and all, out of its socket. A space is left behind, but the original socket-casing is also reconfigured, by a breakdown and reformation of bone. This is surprisingly complex, compared to the simple theoretical alternative of ever-growing teeth.
Can we quantify the space involved, relative to, say, the cranium?
The brain of the domestic horse is about 0.6 kg, which means about 600 cm3.
The horse has 24 cheek-teeth; the occlusal surface of each seems, at a rough guess, to be about 2.5 cm X 2.5 cm.
The horse grinds away about 10 cm of tooth matter on each of its 24 cheek-teeth, and it grinds away this depth of tooth despite its tooth being built to be extremely tough, with not a mere coating of enamel but a folding of enamel in a matrix of dentine to form a washboard surface.
If we assume that the buried part of the tooth, at the point of adulthood, is 10 cm deep, then this means that the buried part of each cheek-tooth has a volume of about 62.5 cm3. This value, multiplied by 24 teeth = ca 1500 cm3.
So, by these rough calculations, at the end of an individual’s life it has vacated a volume within its jaws of more than double the volume of its brain. Dividing 1500/600 gives a factor of 2.5.
If one allows for the fact that the domestic horse has a brain rather smaller than that of its wild ancestor, and the fact that the cranial cavity is partly filled with fluid as well as brain, then this factor might shrink to about 2 instead of 2.5. This is because the division would more resemble 1500/800 = 1.9. On the other hand, the horse also has incisors. These are smaller than the cheek-teeth, but presumably add to the total volume, perhaps boosting the figure from 1500 to 1700 cm3. In that case the calculation produces a value of 2.1.
So, although my figures are approximate, it seems safe to say that the volume within the skull that is vacated by eruption of all the teeth does indeed at least match that of its cranial cavity. It may possibly be double that volume.
All the photos below refer to the domestic horse (Equus caballus).
The top right photo in http://www.listentoyourhorse.com/wp-content/uploads/2015/06/eruption-rate-guide-krystin-dennis.jpg-e1434004544834.jpg
shows the depth of the fully-formed cheek-teeth at the point of adulthood. Each tooth is about 12 cm deep. All of these centimetres will be worn away by the time the animal dies of old age, as the columnar tooth erupts from its deep cylindrical socket by a mysterious process involving hydrostatic pressure and ligamental tension. In the bottom-left photo, it is clear that the teeth have largely erupted (i.e. most of their columnar length has been used up), but it is not particularly clear what has happened inside the vacated sockets. My guess is that the empty socket is partly remodelled to be shallower (increasing the volume of the jaw sinuses), partly filled up with spongy bone or a lattice of flimsy bone, and partly fluid-filled (which means a gap in old skulls that is now air-filled if one cuts into the bone to expose the socket).
The following http://www.treasurenet.com/forums/attachment.php?attachmentid=358731&d=1332412060
is gratifyingly clear in quantifying the rate of wear as the teeth, which are fully finite at the point of adulthood, erupt from their deep sockets.
See http://www.thinklikeahorse.org/images/H%20jaw%20xray.jpg.
Many authors are captivated enough with the extremely deep teeth of horses that one can find plenty of illustrations of this depth. Much of the great size of the head of the horse is owing to the sheer depth of jaws necessary to house such deeply buried teeth, in effect a system in which a lifetime’s worth of tooth is stored in the head at the point of adulthood. But one can clearly see the implication that, as these great batteries of teeth erupt and vacate sockets that are about 10 cm deep, something has to fill the vacated space.
The following http://static1.1.sqspcdn.com/static/f/375254/15395940/1322798522910/skull-young-horse-long-teeth.jpg?token=oZal8wATQ94n9x47pDI6w5MScQo%3D shows the jaws of a juvenile individual of the horse at the stage of growth when the teeth are approaching full formation. It shows that the full depth of these deep jaws will be filled, by the point of adulthood, with the extremely deep, fully formed teeth that amount to a great stored dentition, i.e. good for a whole two-decade lifetime of chewing.
The following X-ray https://s-media-cache-ak0.pinimg.com/736x/40/fd/ae/40fdae6747f9e3e2d1b837babacfdb46.jpg
shows again how deep the jaws are and how this depth can be explained by the sheer depth of the columnar, stored dental capacity. Each one of these teeth keeps erupting out of its socket continuously until the animal is senile and has run out of occlusal area to chew on. The total collective volume of tooth socket vacated through the animals lifetime must surely exceed the volume of its whole brain. Come to think of it, it looks possible that the collective volume of vacated sockets, all teeth considered, left and right, upper and lower, incisors and cheek-teeth, is several-fold the volume of the cranium.
But what is the sense in this arrangement compared to ever-growing teeth?
http://cache1.asset-cache.net/gc/140550122-horse-skull-side-view-x-ray-gettyimages.jpg?v=1&c=IWSAsset&k=2&d=a%2BlB7wvrigChN5spb6Uih%2BO0jP1Hh3ygUTUNoW0ikr6NkqIT1jgv0T7p5TRszVIH
The following is similar but even more graphic: http://images.cpcache.com/merchandise/514_400x400_NoPeel.jpg?region=name:FrontCenter,id:70994328,w:16.
The following article does verge on a coherent description of what is remarkable about equid dentition. But it still omits much: http://onlinelibrary.wiley.com/doi/10.1002/ar.20676/pdf.
The following photos nicely illustrate the difference between the full-size, largely ‘stored’ tooth, which is fully formed at adulthood but lies deeply buried in the jaw, and the nearly worn-out tooth of old age. The photos really do show the enormous loss of mass that occurs through the animal’s life, as its teeth wear out with chewing and the hidden column of tooth matter erupts in compensation.
The following shows the cheek-tooth when it is still hardly worn. About 90% of its length is below the gum-line, buried in a socket in the jaw. The root has closed and there is no longer any meristematic activity. This columnar tooth erupts by a process of ‘recoil’ from its socket, not by a process of continuous growth from the root.
http://www.animaldental.com.au/info-Horses_need_dentistry.html
The following, from the same source, shows how shallow the same tooth will be after a lifetime of chewing, when most of its mass has been worn away. But what fills the ca 10 cm deep socket left behind?
The following again shows how shallow the cheek-tooth of the domestic horse is in old age.
http://theequinepractice.com/pull-the-tooth/
And the following shows how deep it still is in a 7-year old individual, when most of the mass of the tooth has yet to be worn by chewing.
http://theequinepractice.com/pull-the-tooth/
The question is: why is the horse hypsodont instead of simply being hypselodont, given that hypsodonty is extremely complex whereas hypselodonty is extremely simple?
Why not just keep growing the teeth to compensate for this continual wear? That is what hares do, and it’s also the way elephant tusks grow throughout life. Instead, the horse’s teeth have a system that is far more complicated – and involves continuous generation of bone matter anyway as the bottom of the tooth socket reconfigures in adjustment to the rise of the closed roots up from their initial position, deep in the jaw, towards the surface at the gum line.
The dentition of the horse involves at least three ‘feats’:
- the whole 12 cm deep tooth must be fully formed once the animal reaches adulthood,
- the socket in which this tooth then rises must be reconfigured in reaction to the gradual vacation of the socket by the tooth, and
- the animal must ‘store’ a lifetime’s worth of teeth instead of simply producing tooth matter as needed.
To understand my answer, please understand one concept and two simple principles.
The concept is of a dental meristem. The term 'meristem' refers strictly to plants (https://en.wikipedia.org/wiki/Meristem), but it is useful in this zoological context.
The base of the tooth in equids cannot be called a root, because in zoological terminology the root of the tooth is an anchoring structure, not a meristem. Our human teeth have roots but these roots are inert, sealed, and terminal. The incisors of rodents and other hypselodont mammals are technically rootless, but they possess a meristem.
The first principle is that ever-growing (hypselodont) teeth can constitute an 'Achilles' heel', because everything depends on their meristem. If the meristematic tissue breaks (which it can because it involves mineralisation), then there is a serious setback which can upset the whole dentition given that
- teeth have to occlude with teeth on the opposite jaw, and
- teeth have to operate consistently with surrounding teeth.
The second principle is one of scaling.
Small mammals are unlikely to break their dental meristems, because of the same principles that allow certain modes of locomotion (e.g. flying or jumping) in small animals but not in large ones. A lagomorph can chew as vigorously as it likes but, simply because its jaw is so small, it is unlikely to break its meristems. But any mammal of body mass exceeding 50 kg, according to my hypothesis, risks breaking its dental meristems because of juddering and vibration in the jaw and teeth as food is milled.
This can to some degree be compensated for by having the meristem deep in the jaw, so that the tooth is buffered. However, at some body size even this can no longer work, and the sheer force of the movements involved tends to break the meristem. This only needs to happen once in the lifetime of each tooth for it to be a serious problem, because no tooth operates in isolation. If the animal breaks the meristem of one tooth, it essentially breaks the whole dentition.
It is significant that elephants and the walrus (https://en.wikipedia.org/wiki/Walrus) have canines in the form we see them. In the case of elephants, the tusks are free of enamel, and consist of dentine alone, which is more flexible and less brittle than enamel (which is precisely why ivory is so sought-after). Elephants subject their tusks to stress but
- the tusks are far more flexible than cheek-teeth, and so tend not to break at the meristem, and
- even if a tusk breaks off this is not life-threatening to the animal.
In the case of the walrus, the tusks do indeed contain enamel. However, but the whole relationship between mass and force is altered because they are used underwater. The aquatic milieu of this dentition allows hypselodonty at increased body size in the walrus for much the same reason that the largest mammals of all are whales.
At the body sizes of elephants, even hypsodonty no longer prevents breakage. This is, I hypothesise, why elephants have a different system, i.e. a conveyor-belt on each jaw in which teeth continually emerge at the posterior edge and move gradually forward to be worn away and disintegrate at the anterior edge. This is even more complicated than hypsodonty.
So, in a nutshell, my explanation is that the horse is too large-bodied (probably by an order of magnitude in terms of body mass) to use hypselodonty (= ever-growing teeth) for chewing food. It thus has no choice but to use hypsodonty, and to pay the costs involved, e.g.
- the storage of a lifetime’s worth of tooth in a head made sufficiently massive to accomplish this, and
- the complex adjustments necessary in the socket as it is vacated by the wearing tooth (which involve hydrostatic pressure, ligamental elasticity, and reconfiguration of the socket/sinus interface and the gingival interface).
The basic insight gleaned by all of this: hypselodonty may be adaptive only in relatively small-bodied mammals.
One problem with my explanation is that many extinct South American notoungulates seem to have featured hypselodonty, including the massive Toxodon (https://en.wikipedia.org/wiki/Toxodon), which was a megaherbivore (>1 tonne body mass). Giant sloths also had hypselodont molars as far as I know.
My answer to this is that the pace of life was probably particularly slow in these archaic forms. This means that they could afford to eat little, and to chew slowly and carefully at all times. Indeed, they could possibly afford to fast for weeks if they did break a meristem.
(writing in progress)
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