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So, a common trope when it comes to dragons is that their underbellies and necks are vulnerable and covered with soft scales, whereas the back, head, and sides are very hard to damage.

But why would that be the case? Storywise, my dragons have metabolic pathways to create and use high-quality CNTs (carbon nanotube) to reinforce their tissues, which is what allows them to fly.

They have stronger tendons, bones, and scales, scales that can vary in size. CNT is flexible, thus armor, made with it, can be too. So, I just eliminated the last practical reason for why dragons have soft underbellies and neck...

at least the last one I could think of.

So, why would piercing and slashing weapons have an easier time penetrating a dragon's underbelly? For what practical reason would the underbelly and neck be so soft/weak?

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    $\begingroup$ Because vulnerable underbellies are quite common among living things? Alligators & porcupines are obvious examples, but it seems to be the general case. $\endgroup$
    – jamesqf
    Jun 25 '20 at 17:41
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    $\begingroup$ @jamesqf for land animals it's because their underbellies are never exposed... $\endgroup$
    – somebody
    Jun 26 '20 at 0:57
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    $\begingroup$ @somebody Dragons tower over most other beings though. If anything you would think their bellies would be armoured. $\endgroup$
    – DKNguyen
    Jun 26 '20 at 1:25
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    $\begingroup$ Replace underbelly with cloaca and the answer is obvious. You cannot have an armored cloaca as it needs to be big enough to shoot out a horse-sized baby dragon. In fact many mythologies describe shooting babies at you as dragons' primary form of attack/defense. $\endgroup$
    – Daron
    Jun 26 '20 at 11:32
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    $\begingroup$ An out-of-universe answer is that dragons need to be killable. A totally armoured dragon does not leave our hero a method of killing it. Just like the death star mysteriously have a penetrateable ventilation shaft. Like the door in Titanic was not big enough to carry Jack as well. (In fact it was, but had the director known that, he would just make it smaller) $\endgroup$
    – Lenne
    Jun 28 '20 at 12:39

13 Answers 13

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They have stronger tendons, bones, and scales, scales that can vary in size. CNT is flexible, thus armor, made with it, can be too. So, I just eliminated the last practical reason for why dragons have soft underbellies and neck...

No, you haven't! Just because they have the metabolic pathways to produce CNTs doesn't mean they have the metabolic pathways to produce CNTs cheaply or efficiently. Armor that you don't absolutely need is a waste of energy to produce and maintain--and furthermore, even light armor is not weightless armor, so armor you don't need is just armor that makes it harder to fly.

So, do they actually need belly armor at all? Most predators don't actually have much armor. It isn't useful for them. Tigers, lions, wolves, bears, and so on rely on overwhelming force to take down prey before it has an opportunity to hurt them, and armor would only make it more difficult to chase down their prey. Most of the time, you find armor on creatures that need it to regularly defend from attacks, not on creatures doing the attacking. And dragons are so overwhelmingly powerful and dominating that it makes you wonder, why do they even have any armor at all? And why do they have it on the top and sides, when, y'know, they can fly?

The obvious solution, to me, is that they have armor not because it protects them from gallant knights--most of whom will never get close enough to land a blow in the first place--but to protect themselves from other dragons. If they are fighting another dragon on the ground, they can more easily keep their belly protected by just keeping it near the ground; and if they are fighting in the air, attacks coming from above are much more dangerous than attacks coming from below (where gravity would be working against the assailant). Thus, back armor makes sense--but belly armor remains largely pointless, and in fact is detrimental to any dragon who would have to fight with it in the air, where it would only slow them down, lower their flight ceiling, and make them less maneuverable.

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    $\begingroup$ @John And yet, there are mammals with armor, almost exclusively herbivores. So merely "they are mammals" doesn't hold up. And I am not aware of any evidence of carnivorous dinosaurs having significant armor, either. $\endgroup$ Jun 25 '20 at 17:57
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    $\begingroup$ @John "Scales" is not the same thing as "armor". Ankylosaurus had armor. Lizards and snakes mostly do not have armor. Birds never lost their scales--they still have them on feet and legs, but flying birds won't develop armor precisely because they have to fly--just like dragons. $\endgroup$ Jun 25 '20 at 18:06
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    $\begingroup$ @John Actually, male lions do have armor: their manes help block bites to the neck from rivals. They're also (primarily?) a Zahavi handicap, but still do serve a functional purpose. Female lions, who do most of the hunting, don't have 'em $\endgroup$ Jun 25 '20 at 20:01
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    $\begingroup$ @John Scales have lots of uses. Grip, water retention, coloring... and certain types of modified scales also serve as armor. But insofar as they are generic skin protection, I'd put typical scales on the level of "clothing"--which also serves as protection--rather than "armor". And again, I know of no evidence that, say, raptors, or Tyranosaurids, ever had scutes. Herbivorous mammals don't have trouble developing armor in the form of horns, bone plates, or extra thick skin (like rhinos) or fat layers, yet carnivores mostly skip those features. So again, "they're mammals" doesn't cover it. $\endgroup$ Jun 25 '20 at 22:49
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    $\begingroup$ @john Mammals do have protection both as scales (pangolins have scaly armor) and in the form of fat and tough skin (leather armor exists). Scales≠armor. T-rex had scales, but weren't covered in armor, ankylosaurus was covered in armor (osteoderms) like modern crocodilians. Scales' primary function is protection...against water loss and the sun, just like bird feathers and our fur (some lizards also use them to swim in sand). Armor is know to be a trait favored predominantly for creatures that suffer predation (crocs and carnivorous turtles aren't the rule, neither was T-Rex ). $\endgroup$ Jun 26 '20 at 11:54
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Protection in their sleep

Dragons are at the top of the food chain and are probably rarely in need of armour as dragons aren't usually attacked by predators.

At least while awake!

Maybe your dragons hibernate (or just simply sleep) and this is the point where they are most vulnerable. But when they sleep they are usually face down or curled up in the foetal position so their bellies aren't accessible due to their incredible weight, as such they never developed armour on their bellies.

(Armies of archers and knights in shining armour are relatively new developments, (compared to evolution) or might be a rare occurrence, only attacking "problem" dragons, so dragons as a species have not adapted to them.)

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Because they evolved in the absence of technology

In evolution, each species has to balance being well defended against other considerations, such as how difficult it is to grow armor, or how armor reduces mobility and flexibility.

Many real armored creatures, such as the armadillo and the hedgehog, lack any sort of protection on their bellies. They crawl around on the ground, and it would just get in the way.

Dragons presumably evolved over millions of years, where the only predators that could threaten them were stuck on the ground. When crawling around, dragons don't usually need armor on their bellies because their bellies are facing the ground. When flying, they are out of reach of all their enemies.

Not only that, but carbon nanotube armor sounds heavy, and they need to be light enough to fly. Carbon nanotube armor is also probably difficult to grow, requiring a huge diet... If a dragon can't find enough food to grow/replenish their armor, they may starve.

In most fantasy settings, just like in real life, humanoid technology is a relatively recent development (maybe a dozen thousand years). So dragons would not have had enough time to adapt through evolution to counter longbows and skilled swordsmen.

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The scales are strong but they don't conduct heat very well.
If the dragon was fully covered in thick scales the beast would overheat and cook itself as soon as it needed to exert itself.
So it evolved with much thinner and smaller scales in the least vulnerable areas of the body (see other answers also why the belly is less vulnerable).

This is by the way also why most dragons, especially in warmer climates, like to sleep in caves lying flat out on their belly. The cold cave-floor is a good heat-sink and helps them to cool down.

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    $\begingroup$ Carbon nanotubes are good conductors, so if anything the scales would work like the plates on a stegosaurus $\endgroup$ Jun 26 '20 at 13:24
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    $\begingroup$ @PeteKirkham They can be good conductors, but it depends on how the tubes are ordered within the material. It makes a lot of difference if they form an amorph material or something with a more crystalline structure. CNT isn't automatically equal to good thermal conductivity. $\endgroup$
    – Tonny
    Jun 26 '20 at 15:36
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When dragons copulate, they do it front to front.

Dragons with softer belly were thus selected because it feels better for both parties when they rub against each other (and they don't get tangled or stuck where strong scales would clash against each others).

Dragons usually being pretty smart, looking for pleasure during sex wouldn't be such a weird concept.

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Flexibility

Like most quadrupeds, most of the moving parts of a dragon tend to be on the underside. In order to accommodate free movement of their legs, the skin on the bottom must be able to deform more, and thus it must be softer. The back of a dragon barely deforms at all when they move (except around the wings if they have them - these are another weak spot.) The chest must also be more elastic in order to allow them to breathe, just as most air-breathing animals' chests expand and contract while breathing. Soft underbellies are common in land-based quadrupeds for just this reason. Covering the chest with tough, rigid armor would make agile movement very difficult.

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Dragons have Pouches like Kangaroos.

Rather than laying eggs and abandoning them, Dragons lay their eggs inside their bodies, like some sharks or the vivivparous lizard. A heavily armored belly is a disadvantage as it makes the eggs/babies more likely to get squished when the dragon lies down.

On the other hand an unarmored belly is not much use since (other than people) it only fights prey animals or other dragons. Prey animals are too small to reach the belly once they are in the dragon's mouth. Other dragons are too tall to easily reach the belly.

Smaller dragons are somewhere in between. If they are small enough to reach the belly, they are small enough to grab in the mouth and hold far away from the belly.

Similar: Dragons give birth to live young.

The bottom half of the dragon cannot be too rigid, as it must expand as the baby grows. Especially the cloaca must be soft so the baby (horse-sized) can fit through. That means there's a horse-sized target between the back legs.

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Same reason why armadillos have soft bellies: the back armor is really though, but it also limits motion and growth.

Dragon bellies may grow when there is a lot of food available, and shrink back when they lose weight due to hibernation or poor availability of prey. If their bellies were harder, this fattening and thinning could cause the armor there to break. The scales on the bally should still be hard enough to give humans a hard time in a fight.

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I would think that, being a dragon, they would need to be light enough to fly. When evolving into what we see them as today, their wingspan must be big enough to make a certain amount of force and beat a certain amount of times per minute, to lift them, that anything else is extra. If they had too much weight, they won't be able to fly, but with their bones being hollow too, it gives them room to make more adjustments, like armour. In the long run of a dragon's life, armour on the sides and back make more sense than the stomach that will be near the ground or above an aerial enemy, using gravity against them. Some dragons in literature and movies, for instance Smaug, used gems and riches to fashion an armour light enough for him to carry without loosing power in the air. There are many legends of them fashioning and wearing jewelled breastplates, for protection and to show off their hoard. Any creatures that a dragon were to pick up and eat, provided it was over the age of 50, wouldn't be able to reach the unprotected stomach at all and any creatures large enough to oppose it, mainly other dragons, would be too large to reach it. :)

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A consideration. Carbon nanotubes would be metabolically expensive, so metabolic conservation of energy principles must apply here. The neck and head would need heavier armor, the wings and shoulders also. The tail, as a biological weapon must be reinforced. Some portions of the underbelly are vulnerable, but would be partially protected by the ribcage, which would imply that such areas could be less protected by CNT armor. Underarm areas would be less likely to be protected, as would some limbs (especially back legs). Stomach could be protected as dragons would be vulnerable in flight, but lower area near hip is protected by lower limbs and is unlikely to be armored. Metabolic expense is a primary reason that biological creatures often lack such armor, CNT is very different from most biological armor as it is more flexible, but could be considered more similar to chitin or keratin than shell or bone armors. Dragons would then have to either eat massive amounts or hibernate for years... both consistent with dragon-lore. But similar to dragon-lore, metabolic considerations also entail that dragons would have a "chink" in their armor. Considerations here suggest that it would be their armpits, some chest area, or close to the hip.

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  • $\begingroup$ It might not be that difficult though, what we're struggling with is 1. Creating CNTs in bulk and making them long and without defects 2. Putting them into composites without degrading them. A creature, designed to produce it, should have an easier time with all those steps. After all, it's in nature's nature to make up for bulk material strength with unique arrangements. $\endgroup$ Jun 26 '20 at 1:37
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The dragons breed eggs for procreation and thus need an underbelly which is soft and warm, which means no armor.

Being an advanced species both sexes care for their offspring, making a soft belly a necessity for males as well.

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Dragons must have weak bellies or necks because, since the 1500s, they have become too large and thick-skinned for a warrior to kill them otherwise. Early illustrations show that dragons were no more massive than knights, and low to the ground, like a crocodile or a kimodo dragon. Since then, they have become flying tyrannosaurs. Note that JRR Tolkein's enormous dragon Smaug was perfectly well armored everywhere, except for a single gap. Once again, that was necessary, so that he could be killed with an arrow in glorious battle. If you don't want an egregious weakness, then either don't make them so bloody enormous, or don't require their defeat in single combat.

PS: As for science, it suggests that armored giant fire-breathing flying animals find existence difficult, because these characteristics do not play well together.

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They do NOT have to have unarmored underbellies. Smaug, for example (in the book, at least; don't know about the movie) knew of that weakness and so armored himself under there.

The reason pretty much every land creature has a soft underbelly is for flexibility. Doing sit-ups, for example, while wearing scale armor on your chest is well nigh impossible; likewise, a real dragon would find it hard to flex around. Tolkien just ignored that, and so can you!

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