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This is a question I've been thinking about for a while now.

Dragons are, by all observances related to the reptile family, (or maybe lizards) either way these are cold blooded animals. Common places to see dragons are on mountain tops, flying high in the sky, "from the North" etc. Reptiles are cold blooded, and these are large reptiles. So I wondered: How would one explain the 'evolution' of fire breathing?

I've also come up with an answer, which I will post, but I wanted to see how it would stack up with answers given by the imaginative bunch here.

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    $\begingroup$ Possible duplicate of How Could Dragons Be Explained Without Magic? $\endgroup$ – JDSweetBeat Mar 10 '15 at 12:58
  • $\begingroup$ @DustinJackson I thought there was one like that, I couldn't find it! This seems to be a subset of that one, dealing only with fire-breathing. Go ahead and mark it as a dup if everyone agrees. $\endgroup$ – bowlturner Mar 10 '15 at 13:05
  • $\begingroup$ Where would fire breathing be more useful than in the cold!?! $\endgroup$ – James Mar 10 '15 at 13:56
  • $\begingroup$ I think this is a more specific question than that in the 'How could dragons be explained without magic' question. Leaving it open. $\endgroup$ – James Mar 10 '15 at 13:58
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    $\begingroup$ ...to burn Threads? (/me ducks) $\endgroup$ – Rmano Mar 11 '15 at 9:54
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As discussed in this article, which references an article from Discover Magazine and one from Scientific American on the same topic, a reasonably plausible way for an animal to breathe fire would be by mixing some combination of body chemicals that produce a flammable chemical, ignited with a spark created in their mouth. The article quotes some scientists imagining how it might work:

According to paleontologist Henry Gee, dragons might biologically synthesize diethyl ether. Here’s his quote from the Discover Magazine piece:

“Yeasts and other organisms produce ethanol as a waste product, and there are bacteria that excrete sulfuric acid (they’re responsible for corroding concrete). I could imagine a microbial community in which diethyl ether is made as a waste product and exploited by dragons to breathe fire.”

As the dragon spews this chemical cocktail, all it has to do is generate a spark to light the flame. As Kyle Hill suggests in his Scientific American piece, this might be achieved by mineral coatings on the teeth or ingested rocks and stones in the beast’s gizzard.

The article notes a somewhat similar case, the Bombadier beetle which has "evolved to squirt an explosive stream of heated venom from their abdomen". They do this by combining hydrogen peroxide from one part of their body with hydroquinones from another part, which when mixed in a special "reaction chamber" creates a strongly exothermic chemical reaction which "generates enough heat to bring the entire mixture to a boiling point". Looking at the Bombardier article on wikipedia, there's a discussion of how it evolved:

The full evolutionary history of the beetle's unique defense mechanism is unknown, but biologists have shown that the system could have theoretically evolved from defenses found in other beetles in incremental steps by natural selection.7 Specifically, quinone chemicals are a precursor to sclerotin, a brownish substance produced by beetles and other insects to harden their exoskeleton.[9] Some beetles additionally store excess foul-smelling quinones, including hydroquinone, in small sacs below their skin as a natural deterrent against predators—all carabid beetles have this sort of arrangement. Some beetles additionally mix hydrogen peroxide, a common by-product of the metabolism of cells, in with the hydroquinone; some of the catalases that exist in most cells make the process more efficient. The chemical reaction produces heat and pressure, and some beetles exploit the latter to push out the chemicals onto the skin; this is the case in the beetle Metrius contractus, which produces a foamy discharge when attacked.[10] In the bombardier beetle, the muscles that prevent leakage from the reservoir additionally developed a valve permitting more controlled discharge of the poison and an elongated abdomen to permit better control over the direction of discharge.7

So you could postulate a basically similar evolutionary sequence for dragons, imagining they originally evolved from smaller ancestors which faced dangers from predators, and these ancestors originally started upping the levels of some of these chemicals (ethanol and sulfuric acid, in Gee's scenario) in their body as a natural defense. Perhaps the greatest concentrations could exist in their stomach since Henry Gee suggests in the Discover article that they could be generated by bacteria, and the stomachs contain lots of those. If they were originally herbivores who had evolved a multichambered stomach like cows and other ruminants, one chemical might be more concentrated in one chamber while another chemical might be concentrated in a different chamber (alternately they might have evolved to cultivate microorganisms which produce toxic chemicals in their salivary glands, since this is thought to be the evolutionary origin of venom glands in snakes).

If they had already evolved higher concentrations just so predators would be deterred from eating them, then this might later evolve into a strategy where they vomit up the chemicals of their stomachs when threatened, as a further deterrant. If there was continually selection on this, eventually they might evolve to produce very explosive reactions like in the bombardier beetle. Then, as the Scientific American piece suggests, they could evolve to strike sparks and ignite the chemicals in their mouth by striking together mineral coatings on their teeth, or spitting out gizzard stones to knock against each other and create a spark (gizzard stones are ingested by chickens to help with digestion, and sauropod dinosaurs are thought to have done the same thing).

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What if the 'fire-breathing' came from not from a weapon but for survival. Being a large reptile helped it survive cold climates by having more body mass, but it still wasn't enough! So it started chemical reactions to warm itself from the inside. This warmth is spread out to the rest of the body to help it act as a 'homeotherm'. But it continued to increase the heat output to help is survive colder and colder environments (people were pushing back!).

Then it started belching fire, to relive the stresses building up inside. As they grew in size it also got harder and harder to fly, so by belching the fire at the ground starting small fires, prairies, forests; they could create thermals to help lift itself into the sky. The dragons that survived were the ones with better heating systems and the ability to fly better, so the 'heating' units got larger and more powerful. Until we get the dragons we see today!

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  • $\begingroup$ "As they grew in size it also got harder and harder to fly, so by belching the fire at the ground they could create thermals to help lift itself into the sky." Not very realistic. Dragons weigh more than hot air balloons I'd imagine. $\endgroup$ – Neil Mar 10 '15 at 9:36
  • $\begingroup$ @Neil Thanks, I had a vision, but I didn't write it all down. Added more, there was something else I thought of this morning too, but can't think of it now. I'll like add it later. $\endgroup$ – bowlturner Mar 10 '15 at 12:35
  • $\begingroup$ Wouldn't that essentially make a dragon warm-blooded? (not that it would be a problem...) $\endgroup$ – Maxime Lucas Mar 10 '15 at 12:42
  • $\begingroup$ @skysurf3000 yes, and no. it would be like it's carrying around it's own hot little sunning rock. It could let the 'fire' go down to hibernate or conserve energy etc. $\endgroup$ – bowlturner Mar 10 '15 at 12:45
  • $\begingroup$ @bowlturner - The dinosaur Stegosaurus had a layer of fins that lined it's back. Originally thought of being as defensive, it's now thought these were used for temperature regulation. At a certain size, a body generates it's own heat and doesn't need the outside influences. Too hot becomes an issue and these fins cool. I like your same use of it here, though I'm not sure if a colder climate is necessary...any pressure to grow larger would work. And for what it's worth, if you classify a dragon more as a dino, warm blood/cold blood doesn't really work as a classification for it any longer. $\endgroup$ – Twelfth Mar 11 '15 at 0:16
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Fire breathing developed as a way to help raise their young.
Being reptilian, dragons lay eggs, but eggs need warmth to mature.

Because dragons are solitary creatures, and because dragons will eat other dragons eggs at every opportunity, and male dragons will consume their own eggs if given the chance, female dragons do not have anyone to share nest sitting duties with. And since dragons spend a lot of time in cold areas like caves, mountains and such, it was really hard for them to get away to hunt during breeding season.

Like other reptiles, proto dragons had chemical glands in their mouths containing poison and other chemicals, and over time these chemicals developed combinations that would react to each other to produce heat. The proto dragon would use these chemicals to warm the rocks around the eggs so they could get away to hunt, and since they didn't have to fast during egg laying season they became bigger and stronger than other dragons.

Over time these chemicals became more and more reactive until combustion would occur when they mixed.

This is the major reason why dragons from tropical areas are smaller and don't breath fire. See komodo dragon as an example of this. They have the venom glands of their northern, cold weather cousins, but because of the climate they never needed to develop fire breathing.

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  • $\begingroup$ Awesome! I like that answer! $\endgroup$ – bowlturner Mar 10 '15 at 18:23
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Fire-Breathing as a Heat Dump

You said it yourself:

Common places to see dragons are on mountain tops, flying high in the sky, "from the North" etc.

Dragons are from cold areas. What if they need to be cold - they die if they get too warm? Some sort of metabolic process that either fails or self-destructs at "high" temperatures.

Then the ability to dump heat rapidly becomes a survival mechanism - during warm periods they can take the environmental heat and breath it out to keep themselves cool.

This works better with magic - scientifically there are better ways to dump heat - but you might be able to stretch it either way. Maybe there are periods where it gets extremely warm, and environmental cooling (like sweating) is insufficient, so they need something active.

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  • $\begingroup$ Another good answer! $\endgroup$ – bowlturner Mar 10 '15 at 19:25
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One simple solution : cooked meat is better than raw meat. Cooked meat is easier to chew and to digest and cooking destroys most bacteria, parasites and toxins in the meat.

So dragons able to breathe fire would eat better and avoid some infections caused by the bacteria usually found in their meals. This gives them an advantage over dragons that can't cook their meat and so the genes allowing fire breathing become dominant in the dragon population thanks to natural selection.

You could also imagine sexual selection : dragons could favour fire breathing partners since cooked meat ensures a better survival rate for the young.

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