Assuming dragons exist today on earth, and are four-legged, with wings, and cold blooded reptiles, what evolutionary purpose would there be in being able to breathe fire?

This question is ignoring how it would be possible, how breathing fire could be scientifically accurate ect. and just about why dragons would evolve to breathe fire in the first place. What purpose does it serve for their survival?

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    $\begingroup$ Proscribed burning is recommended in many environments so as to increase the number and availability of critters to eat. $\endgroup$
    – David R
    Commented Apr 11 at 15:11
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    $\begingroup$ If they are four-legged with wings they can't be reptiles. Reptiles have four limbs, so if they have wings they can have only two legs, like birds and bats (effectively making them wyverns). $\endgroup$ Commented Apr 12 at 9:11
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    $\begingroup$ I'll add my usual note to this. When one species has evolved to breathe fire, assume everything else in the environment is somehow adapted to that level of fire use. There will be other creatures that use fire (perhaps the phoenix does set its nest on fire), there will be other creatures that take advantage of the dragons use of fire. There will be birds that follow the dragons to eat the bugs that escape their fires. Nature is a system so this will not be isolated. $\endgroup$
    – Separatrix
    Commented Apr 12 at 9:56
  • $\begingroup$ Cold-blooded creatures wouldn't have the energy to spare to breathe fire, unless they're getting the energy from an external source. $\endgroup$
    – Monty Wild
    Commented Apr 12 at 11:41
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    $\begingroup$ @MontyWild How does that follow? Cold-blooded creatures have MORE chemical energy to burn (one way or another), given the same diet as a warm-blooded equivalent. Unless your "external source" means "an environment warm enough for cold blooded creatures to survive", which is true but also an odd tangent to take. Not to mention that fire-breathing can be decoupled from metabolic energy-- ruminants produce methane that contains a lot of potential energy utterly unavailable to the animals in question, and it seems to be working alright for them. $\endgroup$
    – Jay McEh
    Commented Apr 12 at 15:54

13 Answers 13


It was a happy accident of sorts.

The dragons, being a very large and airborne reptiles, needed two things:

  • ability to store a lot of calories for flight, in some more efficient way than fat, because fat makes them too heavy to fly

  • ability to get rid of the extra weight if they needed to suddenly flee, quickly (it is better to get rid of the fuel tank and fly away on fumes for a short distance, than to keep it and die anyway).

Generation after generation, dragons evolved to convert eaten food not into fat, but extremely energy-dense liquid that they could instantly vomit out when they needed become lightweight to flee from a predator (like a bigger dragon or some ground based predator that could not follow them).

After some critical point, the "dragon-oil" became so energetically rich, that it essentially became rocket fuel, that would just spontaneously ignite when vomited out a a high enough pressure - the air friction alone would make it burn with atmospheric oxygen it smacked against.

And thus, the dragon oil became a weapon, more powerful than any other creature had (until humans).

Dragons would not hunt with fire (as this would be insanely wasteful) but would instantly kill any predator that attacked them, including other dragons, unless the other dragon was bigger, and could belch fire at a greater distance to hit them first. Dragons quickly evolved to have an "arms race" between themselves, not just for survival from cannibalism, but for sexual selection.

This had yet another benefit: a dragon mother could carefully vomit dragon-oil into the mouths of their offspring, (the way pigeons or seagulls feed their young). This allowed the young to be optimally fed and grow very, very fast, and very very big, which increased their chances of survival greatly.

These two forces (dragons evolving to be better fire breathers, and evolving to be bigger to store more fuel) resulted in giant monstrosities that became as big as possible without losing the ability to fly. Then they speciated: some dragons actually lost the ability to fly, but could now grow even larger - these became wyrms. Others optimized for flight, and more efficient, more precise use of fire, for both combat and sexual display - those are the graceful wyverns and the high-soaring drakes.

Another side effect was also that "dragon oil" is so energy efficient, that a dragon might sleep for years, in deep hibernation, living off of the stored fuel. This led to vastly increased lifespans: not only dragons were already very long lived, as all big reptiles are, but they now could spend decades in "death-like" hibernation during which they barely aged. Where active dragon could live for 100-200 years, a hibernating one could live for a 1000, spending most of it asleep, with short, brutal periods of ravenous hunting, violent mating and then creating offspring, before going back to sleep.

When humans evolved sapience, it did not take long for them to realize that killing a dragon not only gives them a ton of meat, but also easily a hundred liters of super-fuel that they could use to heat their homes, light their lamps with, even use in forges or as weaponry.

Thus, dragon hunting soon became a profession not different from whale-hunting. Oil is called black gold for a reason!

  • $\begingroup$ This is a very interesting backstory for dragons! Thanks for writing this. $\endgroup$
    – justhalf
    Commented Apr 14 at 13:31
  • $\begingroup$ So if they shoot out too much fire they fall out of the sky? $\endgroup$ Commented Apr 14 at 13:54
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    $\begingroup$ @CaptainCodeman No, it would actually makes them lighter and faster as they spit fire, but the get hungrier FAST. Which would also result on hunger rampages on easy prey afterwards. $\endgroup$
    – lilHar
    Commented Apr 14 at 19:25

Sexual selection is usual the reason why species show expensive behaviors or traits.

In the same way a peacock uses its tail to impress the female and be chosen for mating, the tail which is surely a hindrance for the movement of a bird which mostly roams the floor of forested areas, a male dragon uses its fire breath to impress the female and settle the dispute about which of the males is going to be mating with her.

No matter how it is made, the fire breath is terribly expensive in energetic terms, and whoever manages to do it bigger is also the one who has the better set up for thriving in the environment and thus worth the energetic investment of reproducing.

  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Commented Apr 13 at 7:17

It started out as a way for a cold-blooded creature to exploit warm-blooded niches

Being cold-blooded limits where you can live quite a bit. You can't live in the polar regions. You can't live at high altitude, or deep in caves where the sun can't warm you up in the morning (outside the hottest environments). That's a lot of real estate that's open to exploitation.

Natural selection can't really search for optimal solutions when it comes to new traits; it can optimize a novel trait once it develops, but it can't pick and choose the optimal means of solving a problem. Turns out, an early dragon ancestor found a really inefficient means of producing heat when needed via a fairly energetic exothermic reaction (be that separating and later recombining volatile metals, consuming reactive chemicals, whatever). And conveniently, they lived near an area where there were relatively few birds and mammals, or even none at all, occupying warm-blooded niches (e.g. New Zealand had no native mammal life, and birds don't truly dominate their niches), so even an inefficient solution still provided access to plenty of resources found in places that couldn't be exploited by the competition. Initially, it might not have even been for the purposes of finding food, but merely having places to retreat where the fully cold-blooded predators couldn't pursue.

Since competition for those niches was light, even inefficient heat production provided a major benefit; they wasted more energy producing heat, but gained access to resources unavailable to their competition. And once that small advantage developed, natural selection took over, optimizing the biology supporting this trait to require less resources (allowing it to be used for longer to delve more deeply into caves, climb higher up mountains, expand towards the poles, etc.), modifying the sense of taste and smell to prefer foods providing the nutrients needed to fuel this trait, and eventually, move it from a utility function (heating the body in the cold) to an offensive weapon (expelling the flame in large quantities, rather than producing it in smaller quantities internally).

Evolution has turned "utility" traits into weapons like this before, in the real world. The knifefish family all developed the ability to produce electric fields to sense their surroundings; they're largely nocturnal, and live largely in rivers with high sediment levels, so vision isn't particularly helpful, but they can extend an electric field into the surrounding water and sense perturbations in the field to find prey, avoid predators, and navigate in general, without relying on sight. And for most knifefish, that's all they do. But electric "eels" (not actually eels) take it to an extreme, dedicating roughly 80% of their whole body to generating electricity (seriously; all their vital organs are located in the front 20% of the body, with the rest being electric organs sequestered in the back 80% of the body), making it an effective means of both offense and defense.

Your dragons followed a similar evolutionary path, where the baseline version of a trait provided one significant benefit, even when not highly optimized, and a highly optimized version of the trait could be used for an entirely different purpose. There are probably many related species that produce flame in smaller amounts solely for its original purpose, dragons were just the first ones to develop it into a weapon, and they've dominated that niche ever since. By the time warm-blooded animals arrived, the dragons had already reached near-parity in their ability to withstand the cold, despite their comparatively inefficient means of warming themselves, and were more than capable of surviving, and (once they developed flight or long-distance swimming abilities, depending on the type of dragon we're talking about) spreading to new environments outside the limited region they evolved in.

It needn't be a standalone part of their natural biology

To be clear, the trait may not be something the dragons evolved directly. An organ that begins hosting a symbiotic, or even parasitic, organism might evolve to contain and encourage the growth of the organism (e.g. a pouch in the digestive tract that provides an ideal environment for yeast to ferment carbohydrates into alcohol, initially evolved as part of normal digestion, to increase caloric yield from food it couldn't digest naturally for whatever reason). As another answer has mentioned, they may consume flint to aid with digestion. The combination of a store of flammable liquid and a source of sparks obviously has risks, and the storage organ becoming stronger, more flame and pressure resistant, venting out of the body (starting as burps/vomiting, possibly developing or repurposing dedicated path over millions of years, much like cetaceans moved their nose to the top of their head), and increased control over the striking of sparks are all possible adaptations along the way. The end result is a creature that remains fully cold-blooded in their natural biology but produces fire, with aid from symbiotes and the environment, to artificially heat themselves as the next best option.

  • $\begingroup$ A cold-blooded creature doesn't have the energy to spare for generating fire. The laws of thermodynamics are such that fire is an even less efficient way of warming up than internal heat generation, and cold-blooded creatures are lower-energy creatures to start with. $\endgroup$
    – Monty Wild
    Commented Apr 12 at 11:37
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    $\begingroup$ @MontyWild Fire doesn't have to be efficient, just sufficient. Real life is full of creatures so spectacularly inefficient that entire food chains subsist on their waste products. And yet. $\endgroup$
    – Jay McEh
    Commented Apr 12 at 17:42
  • $\begingroup$ @MontyWild: I 100% agree it's inefficient. There's no way the trait would survive and evolve if the existing niches for warm-blooded animals were mostly filled, thus the need for an isolated environment, a la New Zealand, Komodo Island, etc., with no meaningful warm-blooded competition. The initial trait might not even be truly evolved by the creature in question; it could have an organ that hosts a symbiotic (possibly initially parasitic) organism that produces the fuel in the first place (e.g. like yeast producing alcohol). It's implausible, not impossible, and that's all we need. $\endgroup$ Commented Apr 12 at 18:34

bombardier beetle produces heated up acid that explodes and burns the victim as it is boiling hot....it doesn't cause flames but...

but it's chemistry stuff, mix it with something else and it may just spark flame.

heck mixing straight up water with pure salt or potassium or other salt like metals results in flaming explosions.

potatoes have plenty of potassium! fish have plenty of salt!

acids can dissolve metals in their purer forms!

animals produce acids!

reality ain't that far from fantasy.

we had freaking flying giraffes roaming the sky 65 million years ago..

so go ahead make your dragons.

bombardier beetle+ Quetzalcoatl = easy peasy dragon

also how does an animal store explosive metals without exploding itself?


and fat is just oil stuck inside small ball shaped cells.

store the explosive stuff in oil.

defecte or spit the explosive stuff, and at the same time expell some water then boom

if the two substances meet in air they will create a fire wip-like stream...if they meet on the victim then the prey victim will be turned into a living candle.


If you eat me, you explode cause I store explosives in my body.

also real life :if you eat me, you die cause I store extremely deadly poison in my body.

see? fantasy is real

  • $\begingroup$ This answer has worthy ideas, but... $\endgroup$ Commented Apr 12 at 17:38

As with most evolutionary traits, it likely didn't go directly from non-fire-breathing dragons to fire-breathing dragons; there were probably steps in between where they had noxious but non-flammable breath, then flammable breath but no way to self-ignite, then self-ignition capability.

As for possible reasons why fire-breathing might be evolutionarily beneficial: perhaps their prey developed an illness or parasites or poisonous glands that were highly toxic in their natural state, but cooking the meat before consumption denatured it and allowed the dragons to survive -- or even if not fatal, it would allow them to consume more calories than dragons that lacked the ability, and hence live longer and compete better for mates and offspring.

This would even help the older generation with flammable but not self-igniting breath -- just as the self-igniters have an advantage over them, they have an advantage over the merely noxious dragons, provided they could find some other method (e.g. striking sparks from rocks with their claws) to ignite their breath.

But it's important to note that evolution does not deliberately develop traits after a challenge has presented itself -- it's rather that the traits develop randomly on their own, and then once faced with a challenge, those that have certain traits do better than others that lack them.

  • $\begingroup$ If you can't turn your dinosaur-sized pile of literal crap into something that won't foul the environment in a way that makes it unsuitable for humans, then they will be exterminated. Dogs bury their crap. Cats are also willing to live with us because we don't crap where we eat. Dogs will also eat it, if that's what happened, to keep the litter clean. As with most evolutionary traits, those whom have achieved symbiosis with their environment are usually better off. ['they developed an illness or highly toxic parasites'] if they didn't burn their crap on the cave floor that they slept on. $\endgroup$
    – Mazura
    Commented Apr 13 at 20:06

They ignite their farts!

Or rather what would be their farts if they hadn't evolved a means of storing methane gas in an organ a bit like a cows rumen. Just as with a cows and other ruminants methane gas accumulates in this organ due to the presence of abundant methane producing bacteria. When they want to produce fire dragons contract muscles lining the organ forcefully. This is usually done when they feel threatened or for displays during mating/territorial disputes. (Although if they haven't 'belched' for some time they will also do it at random to avoid discomfort.)

Finally there are also 'dry' iron rich follicle like organelles lining their upper throat which the dragons reflexively scratch together during the processes of expelling methane in order to create a spark.


Flammable gas (hydrogen and methane) is produced even by humans, just goes out mostly through the other end of the food tract. But is there any a difference from which end the dragon in breaching fire from? We just would need larger amounts of gas and some kind of igniter. Diphosphane looks like a good igniter, it spontaneously ignites in the air but is otherwise not very toxic and looks like there is a biological pathway to synthesize it.

This looks achievable in terms of evolution and could serve as a defense against small predators hunting in packs. Sexual selection for sure would also be a factor, maybe adding good color to the flame.


They spit a chemical that ignites shortly after exposure to air

This is how Moorcock portrayed them in the Elric series.

We can surmise that the chemical originally wasn't flammable in this way, and was used as some kind of defense mechanism (like skunks or bombardier beetles, but from the front end instead of the hind end). At some point flammability entered the gene pool.


Well this question was answered by a french Youtuber (who's also a biologist) with this video : https://youtu.be/vpmrR7HBNh8?t=368 I skipped the part where he explains how the dragon could produce a flame because you didn't asked for it, but I suggest you take a look. To summarize his point :

Cooked food is better than raw food

With his flames, the dragon can cook the food, which is easier to chew and digest. Also, the heat removes toxins, bacteria, and parasites. Researcher also found that great apes prefer cooked food.

Then, as L.Dutch said, sexual selection get in the mix

Because breathing fire makes the reptile more able to survive in its environment, the female choose to mate with the male who produce the biggest (or at least the most efficient) flames. And in a few generations, those big lizard have the power to burn entire villages.


Fire-breathing as a deterrent

Making fire costs a lot of energy. Like an absurd amount. There are a lot of ways to alleviate this problem, but regardless you're blowing out stuff that might otherwise be used as energy for the body. So why do it?

The trick is that they use it as a deterrent. Much like bees. Many species of bees die after a sting, in contrast to wasps that can sting many times. The reason is that leaving the stinger is often much worse for the stung creature. Any predator thus runs a high risk of being damaged. For many predators any damage can be fatal, as they need to be in peak condition to hunt. That is why many predators are justifiably cowards. Most often they'll stop a hunt if they know something is dangerous to them.

A dragon uses flame as a deterrent. They can warn with one or a few small flames, making sure the opponent knows the dragon is not to be trifled with. If it does attack, the dragon has just a single blast in the tank, but this is enough against most predators to damage or scare them enough. Predators learn, and not knowing if a dragon has flame in them they'll most often leave it alone.

It can be used on the side in hunting, but probably mostly in making a tiny fire to control where their prey is running away to. Unless it's something like a flamethrower, flames aren't all that damaging in a single belch. Only if something catches fire, or the flames are merely a byproduct of something hot (like flamethrowers!), you'll be able to actually use it. Again, expending so much energy to get energy for eating is a bad trade off, so it is best avoided.


As a side effect

As others have mentioned, you don't have to go in a direct line to fire breathing. If you want to a creature to fly, it needs some way to reduce body density. Birds did this by evolving feathers and hollow bones, but those adaptations come with trade offs. As long as you don't get too picky about the real-world physics, perhaps 'dragons' went a different route and evolved to store a low density gas like Hydrogen inside their body. The primary evolutionary advantage was to facilitate flight, but somewhere along the line a clever dragon figured out that burping in just the right way would cause a fireball. This system could actually lead to some interesting plot devices; breathing fire is using up the thing that keeps them airborne, a potential weakness to exploit for a smart opponent.

Alternatively, another common trait among reptiles is venom. The original adaptation was an ability to spray a stream of venom to take down smaller, more agile prey. Different prey types require different types of venom so they evolved the ability to store multiple types. Somewhere along the line, a mutation occurred where the two types of venom would auto-ignite if mixed together. This would be a handy adaptation since it is more difficult to evolve fire resistance than a venom immunity. As a result, the mutation spread and now you have fire-breathing dragons. As a bonus, this would probably be more realistic than 'flaming gas'. A gas cloud is going to be dispersed by the wind. A flaming liquid would stick to the target and do a lot more damage.


Rather than evolve entirely new organs for spark-creation, dragons have evolved a trait shared by other large, carnivorous animals like crocodiles and alligators: gastroliths. It also just so happens that dragons prefer flint as their gastrolith of choice. Somewhat fitting, since they are cave-dwelling animals, often frequenting the same limestone caves where flint is so easily found.

Swallowing fire-starting rocks makes for rather fiery eructation when combined with the massive amounts of methane released after each epic-sized meal.

So that's the mechanism. How can this trait be beneficial? To make a fantastical trait like fire-breathing believable, I would choose to bend another, more mundane trait (even if it's equally unlikely) -- let's say that dragons have evolved the ability to respirate carbon dioxide (CO2), instead of oxygen like other animals.

As mentioned before, dragons are cave-dwelling animals. They are subject to environments with extremely limited airflow, exacerbated by all that methane gas they release. What better remedy can there be for a CO2-breathing animal than to burn off the methane and create a CO2-rich environment?


Hunting preys out of their burrows

They are big and well visible. Often they can't manage to take their preys by surprise, all they can do is see how they take shelter in their burrows.

Hungry and tired dragons realised that their big bodies with fast movement generated a lot of heat and tried to use their hot breath to push the preys out of the burrows. It worked, but only when the burrows were not too deep. With time their breath became more powerful, but it required a lot of energy, moving frantically their body to generate heat. Evolution and chemistry came to their aid and the breath became ignitable.


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