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It is typical of fantasy stories that there will be a dragon somewhere down the line. They are typically the strongest boss right before or after the main antagonist.

Now the question is what color would the dragon's blood be, given that it can breath fire, it can produce thick scales, it has very sturdy bones, and most of all, its diet consists of meat, specifically human meat.

  1. Dragons primarily eat human flesh - Will this have any effect on the color of its blood?
  2. Dragons use oil made from human fats to breath fire - Are human lipids flammable?
  3. Dragon's blood is highly flammable - Now that's amazing.
  4. Dragon's blood can heal mortal injuries near instantly if applied directly to the injury - Cell regeneration?
  5. Rumors says that Dragons can also get nourishment from the sun - Photosynthesis?
  6. Dragon's Saliva is acidic to human and animal flesh but not to plants. - Is blood involved in its acidity?

Given the characteristics above of our dragon in this story, what would be the color of their blood if we are basing it on what we know about blood?

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    $\begingroup$ Almost all vertebrates (and a decent number of invertebrates) use hemoglobin in their blood, which gives it a reddish color. If you're looking for a scientific choice, pick that. But your six descriptions seem to require something magic, at best. $\endgroup$ – HDE 226868 Dec 10 '16 at 14:59
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    $\begingroup$ @HDE226868 so their human diet has no effect on the color of their blood? $\endgroup$ – mico villena Dec 10 '16 at 15:03
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    $\begingroup$ Assuming that a person is healthy, there should be no reason for changes in hemoglobin or other related proteins. Diet will not change this. $\endgroup$ – HDE 226868 Dec 10 '16 at 15:06
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    $\begingroup$ As far as I know, myths don't describe dragons as primarily feeding on humans. Virgin sacrifice is more of a political statement that nutritional demand. That being said, colour of blood is mostly decided by colour of oxygen transporting molecules. In most Terrestrial animals, it's red, because oxidated iron gives haemoglobin red colour, but it's not the only possible colour. For example: en.wikipedia.org/wiki/Hemocyanin $\endgroup$ – M i ech Dec 10 '16 at 15:13
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    $\begingroup$ "Are human lipids flammable?" - Worldbuilding has some of the best out-of-context quotes. $\endgroup$ – Ghotir Oct 2 '17 at 17:41
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Dragon's primarily eats human flesh - I don't know if it will have any effect to the color of its blood?

This choice of menu item makes the dragon vulnerable to extinction, for a number of reasons, but that's not really the question here, so I won't go into that (even if they are invulnerable to most things, by golly humans are clever). Menu choice doesn't determine blood color in most animals. You have to decide if the blood is iron-based or not. On this planet there are a few animals who, when their blood is exposed to air, it is blue. That's because it's copper-based.

Dragons uses oil made from human fats to breath fire - I don't know if human lipids are flammable.

All fat is flammable, but not enough to do what you are looking to do. What you'd actually be looking for is a system that breaks down food into vaporizable components. Like a biologic version of a lighter. This question is an excellent place to look at how that might work.

Dragons blood is highly flammable - Now thats amazing

You might want to look into the science of flammable liquids. This also could render them strangely vulnerable to fire, even if they aren't cut open. As far as color is concerned, look at all the substances which are flammable and how they work. It could be a vulnerability to make the blood flammable. They can have sacs or storage for fire breathing that do feature flammable substances deep within them, but to have the blood be flammable could be problematic to their continued survival. I would look to oils and alcohols as the inspiration for color if you are set on having this be so.

Dragons blood can heal mortal injuries near instantly if applied directly to the injury - Cell regeneration?

For it to be both flammable and regenerative would be touch scientifically impossible, but hey, magic can handwave anything! I'd make the blood golden based on this, just because that's pretty and magical.

Rumors says that Dragons can also get nourishment from the sun - Photosynthesis?

In this case, they would not eat people very much. And that's good. For people. And for the dragons, because if they are highly invulnerable and don't kill people often, most folks would just accept it as a force of nature that just happens once in a while.

Let's say that most dragons would die because they got so old or sick that the photosynthesis wasn't working correctly. Once that happens they have to prey on humans more often, and then they become a "problem dragon," and humans cooperate to hunt them.

It's possible that the color of the blood on these dragons who can't photosynthesize as well are actually different than healthy dragons and have different characteristics entirely. (Maybe their blood is flammable, but a healthy dragon has the healing blood). You could make it black, like motor oil that hasn't been changed in a while, whereas a healthy dragon might have golden blood. With the old/unhealthy dragons, it's a little easier to kill them, (maybe they can't regenerate as well or other problems) but still quite a task. And they require killing because they are eating more people.

Dragons Saliva is acidic to human and animal flesh but not to plants. - Is blood involve in its acidity?

Acid is acid because it has a particular pH. The acidity of something that would dissolve flesh would also be determinantal to plants. But that's science! So, let's say that the cells in their saliva have an "unreleased" acidity, contained in microscopic sacs. These sacs "sense" the characteristics of flesh (that isn't theirs), and releases the acid, only then. I don't know that the blood is at all related to this.

If this has a mystical basis, their blood can be any color you desire. This is an opinion.

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  • $\begingroup$ I am picturing a Venusaur who eats humans. $\endgroup$ – Marshall Tigerus Dec 12 '16 at 20:52
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It's settled (by talk in comments). Your dragons are different. They are not calm, wise creatures that keep to themselves and hoard gold. They are not nuisance that steals cattle and burns occasional field. They are not a strict overlord who demands a virgin a year as a pledge of subservience. And they certainly are not noble defenders.

Your dragons are a nightmarish ravenous creatures right out of a mind of HR Giger. And you want their blood to be appropriately uncanny.

So what can we do with it?

Colour

Colour of blood (or blood equivalent) of animals we know is based on colour of oxygen carrying molecules. We (as in we the living creatures, not just humans) use a lot of oxygen and our blood carries a lot of it. Haemoglobin is red, because oxygenated iron is red. Rust is nothing more than Iron Oxide - oxygenated iron, however iron is bound strongly to oxygen in rust, that's what rest of haemoglobin is about - to make iron attach and detach oxygen more easily. Oxygenated blood is bright red, unoxygenated blood (bound to CO2 instead of O2) is dark red.

However, Haemoglobin is not the only known oxygen carrying molecule. I present you the Hemocyanin: https://en.wikipedia.org/wiki/Hemocyanin Hemocyanin uses copper instead of Iron. Copper gives it blue colour in oxygenated state, while unoxygenated state is colourless. For comparison Copper (II) Oxide is dark blue (at least it looks dark blue to me: https://upload.wikimedia.org/wikipedia/commons/f/fa/CopperIIoxide.jpg), however Copper (I) Oxide is red.

What does that mean? It suggests that colour of blood is primarily based on colour of oxides of metals used to carry them. Iron is used in 2/3 oxidation states, Copper in 1/2 oxidation states, and those are most common oxidation states for those elements.

You can, with high level of plausibility, replace Iron or Copper with any metal that has 2/3 or 1/2 as most common oxidation states and blood created from those should plausibly have colour based on colour of respective oxides (2-oxide for unoxygenated Iron analog, 3-oxide for oxygenated Iron analog, 1-oxide for unoxygenated Copper analog, 2-oxide for oxygenated Copper analog). Here are some examples I found using those lists: https://en.wikipedia.org/wiki/List_of_oxidation_states_of_the_elements http://www.thecatalyst.org/oxnotabl.html and wiki articles on respective oxides.

  • Cobalt, Iron analog, deoxygenated: green/red/gray, oxygenated: black
  • Mercury, Copper analog, deoxygenated: brown, oxygenated: orange
  • Zinc, Copper analog (it has 2 as most common state, but it has only -2/+1/+2 stats total so it might still be plausible) deoxygenated: sorry, I could not find, oxygenated: white
  • Europium, Iron analog, unoxygenated: sorry, I again couldn't find, oxygenated: white but highly fluorescent (!). It used to be used in CRT screens, so how about dragon blood that glows in full technicolour under ultraviolet? Maybe Dragons can emit UV themselves and thus are afraid of certain colours of UV fluorescence? After all anything that can spill their blood is dangerous, same mechanism is why red means danger to humans.

I honestly don't know if you can use other pair of oxidation states (like 3/4 of Iridium, 2/4 of Palladium etc.), perhaps someone with better knowledge of biochemistry can answer that, or you can just ahead anyway claiming artistic license.

Whichever element you pick, you need to keep in mind that it needs to be common in your world. Dragons will have to be able to ingest enough of it, or somehow produce it themselves. Most of terrestrial live uses iron, so it's easy for terrestrial predators to acquire enough iron - it just happens naturally as they eat. Naturally, if your Dragons somehow synthesize rare element, their metabolism will slowly enrich the world with that element, so that's something you need to keep in mind too.

Food

What you eat doesn't have huge impact on how your body works. Food is broken down to base components anyway, and a lot of amino acids, vitamins as well as fats and sugars can be synthesized if there's abundance of another compounds. For example, humans synthesize Vitamin D, and turning sugars into fats for storage and fats back into sugars for actual use is standard practice for most animals.

However, not everything can be synthesized. Terrestrial life can not synthesize elements, only compounds out of existing elements, if we need calcium we need to ingest it. Same with iron, and thus unless your Dragons can somehow synthesize other elements (perhaps with magic? Philosopher's stone anyone?) they are better off having same base blood composition and thus blood colour as other things.

But hey, who said your humans have to have red blood like we do?

Flame

Fats are flamable, be them animal, human or plant. Plant oils are easiest to use, but chemical difference between solid fat and liquid oil isn't big. Mostly, oils have some double chemical bonds while solid fats don't, that's how margarine is made: you break double bonds, attach hydrogen in thus made free spots, and presto: liquid oil turns into solid fat. You can have dragons somehow reverse the margarine process to create liquid oils, add some magic to increase flammability and you should be set with plausible flaming breath.

However, I can't help you with flammable blood. Sorry, I don't have any good ideas.

Toxicity, regeneration, and sunlight

This isn't related to colour. In terrestrial animals, regeneration is handled by blood cells which don't contribute significantly to colour. I should note that by mammal standard, humans have fairly aggressive regeneration abilities, it may not seem like that but human body is good at staying alive despite injuries, though results may not be pretty because of permanent scars, but bleeding is stopped fast by platelet clogs, and damaged tissue is replaced aggressively by surrounding cells multiplying to fill the scar. We lack ability to regrow select limbs, but that's different from stopping bleeding and keeping what's left, alive.

In case of your dragons, most likely best pick is to have free stem cells in their blood in addition to mechanisms we posses. That can massively increase rate of regeneration - stem cells in platelet clog could differentiate into correct cell type to hasten regeneration. One huge drawback of that would be susceptibility to tumors, cancers and erroneous differentiation. If stem cells differentiate incorrectly, they could start growing wrong organs: random talons sticking out of scars, perhaps tiny disfigured appendages. Something like that could even accidentally happen in otherwise healthy tissue casing teeth inside muscles, talons in brain, scales inside arteries or even actual cancerous tumors. You could use that to limit their lifespan if you want - they could be very hard to kill, but at some point their overagressive regeneration will accumulate enough errors to for Dragon body to effectively kill itself. Kind of like human scars do negatively impact repaired tissue (lower flexibility, sometimes the can't actually do what organ needs to do and they only stop bleeding/other fluids escaping, etc.), but they sure beat being dead.

Further, stem cells could differentiate into white blood cells - those which handle immune reaction. Dragon blood isn't actually toxic to humans - it's a pathogen that actively fights and tries to kill non-dragon cells. Those chemical burns? They are not chemical. It's a trail left by dragon white cells pretending they are flesh-eating bacteria. Destroying entire limbs in matter of hours and killing a person in less than a day with only treatment being amputation before condition spreads too much is perfectly plausible, or you could dial it way back to only cause some nasty scaring.

And this leads us to photosynthesis. If dragons have chlorophyll in some of their skin cells, maybe dragon cells responsible for immune reaction misrecognise plants as dragon-cells and signal stem cells to assist in regeneration?

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Like every living being on this planet: Red?

Because their body works primarily like any other animal, breathing air, eating flesh.

Or if you want to give it an acid look (why should it be acid?) you can take a look at the blood of an alien from the Ridley Scott movie Alien. It's a green slime that eats its way through everything.

But I'd stay with good ol' fashioned red blood.

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    $\begingroup$ every? Hemolymph begs to differ. many animals have blue blood becasue they use copper instead of iron. $\endgroup$ – John Dec 11 '16 at 15:30
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I'll add one thing to the answers above: You are what you eat. Since your dragons eat humans, they need to use an oxygen-carrier which they can get in sufficient quantity from human flesh. @M-i-ech pointed out other metals such as mercury and cobalt but the dragon would not have these available (or would need some mineral source; a special kind of salt lick). I would stick with red, haemoglobin-based blood for these dragons.

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