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Dragons are a grand classic of fantasy. For this question, let's assume we're talking about the following stereotype:

  • hatches from an ostrich-sized egg and can grow to mountain size if nothing limits its growth (enough food and space)
  • lizard-like appearance, bat like wings, able to fly
  • breathes fire
  • carnivorous

With an earth-like biology (or with believable variations), how close can we get of this dragon?

Explanations are especially needed for:

  • fire breathing: how it works? how it happened through evolution? how it impacts the nutrition requirements (if gas is produced in high quantity, surely there is an impact there)? how is it not deadly for the dragon itself?
  • flying: is the energy requirement believable (even with heavy fire-resistant scales)? how much can the body look like a komodo dragon without it being an aerodynamics issue?

And any other issue you can come up with ;-)

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    $\begingroup$ I don't know. What makes the most sense? I believe dinosaurs were neither (sciencemag.org/content/344/6189/1216.short), nowadays "reptiles" are cold blooded, and these dragons we're describing breathe out and resist fire... $\endgroup$ – Sheraff Sep 19 '14 at 9:05
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    $\begingroup$ To address the dinosaur comments, the Earth was a very different climate at the time, so comparisons would be hard. There are hypotheses about the factors which allowed dinosaurs to be so large, and the same factors could have led to the Pleistocene megafauna (such as increased atmospheric oxygen). The good news is that humans did live during the Pleistocene. It might be possible to suggest humans could coexist with dinosaur-like dragons in the same climate. $\endgroup$ – Bryan Sep 21 '14 at 6:56
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    $\begingroup$ How about legs? One of the things that bothers me slightly - almost all animal life is quadruped, and more legs is the realm of insects and spiders. Even insects - mostly 6 legged with wings, not many (any?) examples of 4 legged fliers. So 4 legs and 2 wings doesn't fit. Would your dragon by wyvern (bird) style? $\endgroup$ – Sobrique Nov 25 '14 at 17:41
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    $\begingroup$ Consider Anne McCaffrey's Pern books for a good in-depth exploration of this and Terry Pratchett's Strata for a brief passing version - in which dragons also travel exceedingly fast by tucking their heads under their bodies and using their breath as an afterburner. $\endgroup$ – glenatron Dec 4 '14 at 10:34
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    $\begingroup$ This made me think of The Flight of Dragons by Peter Dickinson. Might be of great use to you. en.wikipedia.org/wiki/The_Flight_of_Dragons_(book) They also made it into an animated film. Check it out. $\endgroup$ – ZanderXL Nov 3 '17 at 11:35

18 Answers 18

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Growth / size

The main argument against mountain-sized dragons is the square/cube law. As an animal increases in size in its linear dimensions, but retains roughly the same proportions, its surface area and strength increases proportional to the square of its length, but its mass increases proportional to the cube of its length.

To give an example of this, compare a mouse with an elephant. A mouse has relatively slender legs, while an elephant has thick legs necessary to support its weight. An insect is an even more extreme example of low size - they can have very thin legs compared to their bodies. Taking the example of increased size to the logical extreme, if an animal was to continue growing, it would eventually become so heavy that it would no longer be strong enough to move itself, and/or its bones would break under its own weight. The point at which this occurs depends on the strength and mass of the creature's body. Obviously, if stronger, lighter materials than those found in terrestrial animals were in use, the animal could grow larger.

As the question specifies, using earth-like biology and conditions, the upper limit of size for a land animal would be on the order of tens of tons, certainly not more than a hundred tons.

Flight

The ability of a creature to fly is also dependent on the square/cube law. Larger creatures just don't fly as well as smaller ones. In an earthlike environment, the best fliers are no more than about a kilogram in weight, and flying creatures become progressively more clumsy as they get bigger.

There is a school of thought that a non-magical dragon could lighten its body with hydrogen gas trapped in large pockets within its body to the point that it could fly despite its apparent large size, however this would result in a dragon as fragile as any bird capable of flight.

Another school of thought is that young dragons fly in order to disperse themselves from their parents, since a carnivore weighing several tons would eat a whole lot, and lose their ability to fly as they grow. An adult dragon may retain wings, but use them for display, not flight.

Breathing fire

As to breathing fire, this could be explained with the example of the bombardier beetle. These beetles can produce a chemical reaction in their abdomens that is sufficiently energetic that the reaction byproducts are literally boiling, and it directs this from a nozzle at the end of its abdomen toward attackers. Breathing fire is a logical extension of this sort of ability.

A dragon could produce and store a volatile high-energy fuel, or a liquid fuel with even more energy, and ignite it in many different ways, such as chemically-produced heat, or an electric arc. By squirting the fuel out fast enough from a duct venting into the mouth as the dragon exhales, it need not ever come into contact with the burning fuel. (Think spraying the flammable gas from a spray can over a cigarette lighter - it doesn't melt the plastic nozzle.) With a change in biology so that a dragon could precipitate metals such as aluminium or magnesium, a dragon might even be able to spit a liquid mixture akin to thermite that would spontaneously combust due to the presence of other reactants.

Or, as Anne McCafferey proposed in the Pern series, exposing certain rocks to acids produces spontaneously-flammable phosphine gas.

The main problem with fire-breathing is the amount of energy required to produce the reactants. Whether the dragon itself or some symbiotic organism produces the reactant, it takes an input of at least as much energy to produce the fuel as the release of energy when the fuel combusts. The best solution is if the energy comes from outside, as is the case with Anne McCafferey's Pernese dragons.

Fireproof

The point of "breathing" fire in the manner I have suggested means that for the most part, a dragon need not actually be too fireproof, since like a human fire-eater, the fire is not inside the dragon. On the other hand, a certain degree of heat resistance would be useful for a creature that could make a mistake with its own fire - or get into a fight with another creature that also produces fire.

There are a number of adaptations that could help a creature that must maintain a relatively low body temperature deal with extremes of heat.

  1. Some organisms, known as extremophiles, are known to live and grow in temperatures below freezing and above boiling point, achieving this through specialized proteins. Some desert living mammals, whose normal body temperature is 37°C, can survive a body temperature up to 50°C, whereas humans start having problems with hyperthermia at 42°C or less. It is not beyond the bounds of possibility that a dragon could have evolved proteins that allow it to survive much higher than normal body temperatures, perhaps well beyond 50°C, though most likely not over boiling point.

  2. Where parts of animals bodies experience extremes of temperature, counter-current heat exchange mechanisms in their blood vessels greatly reduces the flow of heat to or from the extremities exposed to those temperatures. By selectively passing blood through countercurrent heat exchangers or not, peripherals can be maintained at a higher or lower temperature than the body core.

  3. Carbon (which is in relatively abundant supply in carbon-based organisms) in the form of nanotubes or sheets has very high thermal conductivity. By strategic placement of such materials, the heating effect of point heat sources could be spread over a wider area and even transmitted rapidly deeper into the body, preventing burns by allowing an overall, but lower, increase in body temperature. Such materials also have the added bonus of being very strong. It is not beyond the bounds of possibility that a dragon may have evolved to be able to produce such substances.

  4. By maintaining a high body mass, an organism which uses water as a solute for its metabolism would require a great amount of heat to raise its body temperature significantly - water has amongst the highest heat capacities of any substance per unit mass.

  5. Creatures whose body temperatures rise higher than normal must eliminate heat, by radiation and/or evaporation. Having a large pair of wings gives a dragon a large surface area through which it can radiate and evaporate excess heat, and its large mass gives it plenty of water that it can evaporate to shed the large amounts of heat that may be involved.

The combination of all these factors may result in a creature that, at its adult size, could be trapped in a burning wooden building for quite some time before it was particularly inconvenienced by excess heat, possibly on the order of some minutes to a quarter hour or so before its body temperature rose to a point that threatened its life. If it escaped such a situation alive, it could then shed the excess heat quite quickly.

Carnivorous

A carnivorous lifestyle is the most likely thing about a dragon. Carnivores obtain their energy far faster than herbivores, though they do have to work harder to catch their food. Herbivores can't afford to stop chewing for long, but carnivores (such as cats) can have a relatively quick meal and then sleep the rest of the day.

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    $\begingroup$ @DonyorM, the weight advantage with filling hollow bones with hydrogen would be quite low unless the bones were very large and mostly gas filled, and this would make them quite fragile. $\endgroup$ – Monty Wild Sep 21 '14 at 9:20
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    $\begingroup$ Could you add fireproofing to your answer, please? I want to validate your answer as I believe it's the most detailed, believable and structured so far. I think this question will be seen by a lot of people throughout the lifespan of this website and it'd be worth it to have a good validated answer. $\endgroup$ – Sheraff Sep 22 '14 at 9:18
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    $\begingroup$ As for the fire-breathing, the term to look for is "hypergolic". $\endgroup$ – a CVn Sep 28 '14 at 17:37
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    $\begingroup$ @trysis, I didn't write "hypothermia" (abnormally low temperature), I wrote "hyperthermia" (abnormally high temperature). 42°C is not typically fatal, but it does cause some mental impairment and other problems, which is why I also used the word "start". $\endgroup$ – Monty Wild Dec 18 '16 at 22:09
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    $\begingroup$ @n00dles when you want to explain something weird and expensive in biology the answer is usually sex, maybe fire battles is how dragons fight for mates, or maybe the females only pick males that can produce the biggest flame. $\endgroup$ – John Nov 3 '17 at 13:13
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Anything is possible.

EDIT INFO: Ok, after a bit of research I'm changing some of my answer. I'll leave the original parts underneath the new parts, if some part was majorly edited. Most of the new information came from this Discovery Magazine article.

Fireproofness

Scales would be made out of bone. They would be made out of practically solid compact bone, unlike the ones in our body. This would give them the strength needed, without adding an unreasonable amount of weight. These scales also would not burn, unlike normal reptile scales. I think a dragon that could withstand very high temperatures (Ex. swimming in lava), isn't possible under earth-like biology. Most standard internal organs need a pretty constant internal temperature. But with a well insulated skin (lots of fat), and fire-proof scales, a dragon could probably withstand what fire he causes.

Growth

Your dragons size is very interesting, growing all the way from a small creature to as big as mountain. At first this may seem fairly fantastical, but it isn't too bad. The dinosaurs did practically the same thing. So I think we can say that this is possible, and has even be seen in real earth biology.

Flight

Wings are easy, it is not hard to imagine a reptile with wings. They even existed in the form of Pterosaur. They would be controlled with muscles within the breast of the beast, likely any other winged creature.

The main problem of dragon flight is that they are just too big. To get enough lift for the massive creatures would make the wings too unwieldy. But it is possible that they could still fly. The dragon's bones would be hollow, like both birds and dinosaurs were. but instead of being filled with air, their bones would be filled with lighter than air gases such as hydrogen, helium, or, most likely, methane, which is a byproduct of flatulence. The dragon's body would also be filled with large sacs filed with these gases as well. This would also make dragons more agile, because they would be easily light on their feet. This would also mean they could look pretty much like anything, because aerodynamics is no longer in effect.

Firebreathing

This idea specifically comes from the discovery magazine article listed above. Dragons would have two sacs with holes that enter to their mouths. Inside one sack live organisms (like yeast) that produce ethanol. Inside the other lives bacteria that produce sulfuric acid. These two gases are allowed to mix inside the dragon's mouth, just before it breathes fire. The chemical that is produced when these two mix is Diethyl ether, which is highly flammable. The tiniest amount of friction can light it (spilling a beaker on a table is capable of lighting it). The dragon would have to not inhale while this was going on (this would of course be an involuntary reflex).

As the scientist who was interviewed in the article above points out, we don't always know how something evolved. But similarly complex defense systems have evolved on earth (the bombardier beetle is an example). A guess as how the sacs are formed is given below.


The original purpose of the two "sacs" was a very simple, an ineffective, breathing mechanism. These sacs were criss-crossed with blood vessels and bones, leftover from gills, and used for the first mechanisms of breathing. As a better breathing system (lungs) developed, the sacs fell out of use, and bacteria and yeast took over them. These then began producing the above substances to create fire.


EDIT: The next too sections are outdated but kinda fun, so I left them here. This is another technical possibility of fire breathing

This idea comes from the How to Train Your Dragon books. All dragons could have two holes in there mouth, these holes would excrete a highly flammable gas during exhaling, this gas is then breathed out of the mouth. At the edge of the mouth, there would be an instrument (likely two special teeth) that when rubbed together would produce a spark. When the spark hit the gas, it would light, creating a plume of fire. This gas would be hydrogen, made from splitting the water molecules made from splitting glucose. This would not have a significant impact on the dragons energy levels, because it would use the energy made from the glucose to split the water. The carbon-dioxide made from the split would be exhaled as normal, and the oxygen would be released with the hydrogen to increase the flammability of it. (One hole releases hydrogen, the other hole releases oxygen). Both gases are stored in special sacks behind the holes.

To protect against the mouth being burned, the front part of the inside of the mouth would also be covered in scales. No essential organs (not even the nose) would be housed near the front of the mouth. However, if the gas was somehow blown back down the dragons throat, it could be killed that way.


Next up, evolution of fire breathing. Michael Kjörling kindly asked how this would be possible. The original purpose of the two "sacs" was a very simple, an ineffective, breathing mechanism. These sacs were criss-crossed with blood vessels and bones, leftover from gills, and used for the first mechanisms of breathing. As a better breathing system (lungs) developed, the sacs fell out of use, but occasionally unused hydrogen, oxygen, and carbon-dioxide still ended up in them. These two gases would leak into the mouth of the dragon as the sacs opened and closed. The sacs would open when the mouth of the dragon opened, and close when it closed.

These dragons ate a high iron content. As dragons developed some teeth began to have a higher and higher content of iron in them. These teeth were stronger, and could break more things. At one point, a chance mutation caused two teeth to be angled in such a way that they would cause a spark. This would then cause the mixture of hydrogen and oxygen in the mouth to ignite, and the process described above would happen.

Energy

A dragon would use a massive amount of energy. But so did dinosaurs. A dragon would have to eat a large amount of food, but that would be possible. Especially if dragons laid and wait and did nothing for much of the time. Then they would eat only occasionally, and then in large amounts as there prey would be unprepared. The cost of flying wouldn't be much higher than that of a bird because of the lighter than air bones/sacs. Fireproof scales are just standard bone, and would not be that energy consumptive to create. Breathing fire costs little energy, because it is done by using the work symbiotic creatures.


Finally, don't argue. Dragons don't like it. If that didn't convince, read the ending.

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    $\begingroup$ You'll have to hope your dragon doesn't get the sniffles! $\endgroup$ – Liath Sep 19 '14 at 7:35
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    $\begingroup$ Good questions, I'm not as good in the scientific stuff, but I'll look up some answers. @Micheal Real quick note: I actually don't really believe in evolution (this not a place for a debate though), which part of why I didn't include it in the original answer, but I still see what I can think up. $\endgroup$ – DonyorM Sep 19 '14 at 7:40
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    $\begingroup$ No, this is not the place to discuss evolution vs e.g. intelligent design. For the moment, let's just go with that the OP seeks answers "without magic". In my opinion, some sort of "god", and I use that term widely here, can be considered a form of magic. $\endgroup$ – a CVn Sep 19 '14 at 7:48
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    $\begingroup$ Remember that for evolution to work you don't need an advantage gained at every step...you just need for it not to be too large of a disadvantage. Variation then spreads through the species until eventually something kicks in that does provide an advantage which then grows to dominate. $\endgroup$ – Tim B Sep 19 '14 at 10:03
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    $\begingroup$ There's actually a phenomenon in earth biology where creatures get bigger as they get older without stopping. I had some trouble finding the proper name of this phenomenon but it basically happens because the creatures' DNA lacks the proper termination markers to halt growth. This usually means the creature has a shorter lifespan, since repeated cell division leads to telomere shortening which means at some point the creature won't be able to heal itself or replenish blood vessels. $\endgroup$ – Wesley Obenshain Sep 20 '14 at 14:46
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I could hardly beat the great level of detail in the other answers, but I like dragons a lot and I think I can share my relevant knowledge with you.

Hatching and Growing

I would assume a dragon would be quite similar to the dinosaurs or the creatures that represent the evolutionary phase between reptiles and birds (like the Archeopterix), based on the popular representations in folklore tales and fantasy stories. Egg hatching then would be the way of reproduction. The growing can depend on many factors, but in most cases it is determined primarily by the DNA. An interesting case (aside from the topic) is a Liger, that is a hybrid produced of a male lion and a female tiger. It is the biggest cat in the world and for a reason - the genes for growth reduction are carried by the male tiger for the tiger specie and by the female lion for the lions specie respectively. Thus a liger has no genes for resticting his growth and so it was believed1 to grow all his life, becomming the largest cat in the world. If dragons were not a regular specie, but produced as a hybrid betweeen species so that it happens to lack growth restriction genes, or if their DNA did not have the growth restriction for other reasons, then a dragon could grow in size troughout his life.

Flight

Flight is a very energy intensive task and has a lot of requirements for the body structure, mass and size in general. Thus, it would eventually contradict with the first point of near-limitless growth. We could still assume a dragon is similar to a bird-like reptile, or a pterodactyle, it would grow large enough and still be able to fly. If growing continues to a certain point, it is possible the dragon reaches size and mass that would not allow him to fly. Since then, a dragon could adapt and lead a life similar to the non-winged dinosaurs and the other ground predators.

Breathing Fire

Breathing fire, literally, is something that I personally would not believe. However, since this characteristic is taken from foklore tales, we could assume these are either exaggerated descriptions of a real capability, or otherwise twisted by the story retelling. Still, I would associate this ability with the description of the Komodo Dragon, which is a large lizzard of the Varanidae family. These lizzards are known both for their large sizes and "poisonous" saliva. Their saliva is not really poisonous, but it has special bacteria which causes the decay of any exposed flesh. The exposure wounds similar to burns of acid, and these could have been attributed to a "breath of fire" by our uneducated ancestors. I watched a BBC show a long time ago (I am unable to recall the name), which also supported this thesis, and added some ancient cultures drawings as a proof - the drawings were of large Varanidae-like lizzards breathing fire. (It is still uncertain if these would rather contribute to the "fire lizzard" folklore specie, insted of supporting the dragon existence, however)

Carnivous

If the creature is indeed derived from a dinosaurus, or is a reptile-bird specimen, then it could share the same need for energy we already know these species have (dinos were large and ate a lot, burds need great amounts of energy to fly and lead quite a dynamic way of life). Definitely, a dragon would be a mobile creature with active lifestyle (able to fly), which requires a lot of energy. It makes sense that the dragon would more than often predate on large and potent prey in order to support its existence. In addition, it could also have evolved the ability to go into a lethargic state, like some mammals and reptiles do if the time of the year is not suitable for frequent feeding. In some folklore tales, and modern fantasy stories, dragons are often being "awakened" from years of sleep by the story's main character.

I hope I was helpful.


1 The Liger was believed to grow trougout his life, due to the explained above genetics. This statement was based on outdated information I had on the topic. However, newer information regarding this hybrid specie does not support the lifetime growth ability (refer to wikipedia article and Tim B's comment below). The misbelief was further supported by the characteristics of the other lion/tiger hybrid - the Tigon, that is created by a male tiger and female lion. The tigon retains smaller sizes in comparison to both his parent species, due to having both lion and tiger growth restriction genes.

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    $\begingroup$ From the wikipedia article : It is wrongly believed that ligers continue to grow throughout their lives due to hormonal issues. It may be that they simply grow far more during their growing years and take longer to reach their full adult size. Further growth in shoulder height and body length is not seen in ligers over 6 years old, same as both lions and tigers. $\endgroup$ – Tim B Sep 22 '14 at 8:19
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    $\begingroup$ @TimB, indeed, you have a point there. The first time I've read about Ligers and Tigons (the other lion/tiger hybrid), the growth difference was explained by the genetics of the species, and the assumption of the Liger's lifetime growth was established. I've fixed the post now, as this information is outdated. Too bad I did not read the Liger's article again to prevent the inacurracy in the first place. So, thanks for pointing it out. $\endgroup$ – Ivaylo Slavov Sep 22 '14 at 17:02
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FIRE BREATHING

Well, fire-breathing without burning yourself is obviously possible. In addition to various systems already proposed with binary compounds, I'd like to point out that you could have self-igniting saliva that includes a stabilizing compounds. This IIRC is the method many poisonous snakes use with their poison. They simply secrete the anti-dote together with the poison and it will protect them from incidental exposure, but when the venom is "used properly" the protection will fall short and the target will be poisoned.

As the saliva glands -> venom glands path is the logical pathway for the evolution of fire-breathing reptile this would be the proper starting point IMHO. Poison could be initially an irritant, evolve to being corrosive (ants have acid for example), and if the corrosive is oxidative that is short path to making things burn. It would likely be a mixture of several oxidizers and matching stabilizers. Oxidizers would probably mostly be based on oxygen from the air. Chlorine might be easy enough to get near the ocean. Fluorine would be kind of neat to add, but I am not sure if there is a convenient source.

As a practical matter the stabilizers would probably be something that is rapidly vaporized. Inside the dragon it would be continuously replenished, but as soon as the "fire-breathing" started the chemical balance would break almost instantly.

Some fuel beyond the methane from breathing would probably be also mixed in. Some mixture of alcohols and fat, I'd guess. To give pleasant clinging effect when breathing on someone. And give a higher temperature to get fires started. And cause serious burns even if the target doesn't ignite. Although a strong oxidizer would make most organic things burn well enough.

The probable evolution path would be from a spitting venom, initially targeted at eyes. Then mating rituals. So it would be flashy and mostly cause damage to exposed parts. On targets of reasonable size. If the dragon is much larger than the target, as implied by the question, it would be a lethal weapon.

In the real world the above would kind of imply evolution from snakes as most likely option. I'd actually go with that as snakes re-evolving legs would neatly sidestep issues with six being a wrong number of limbs and allow 2 wings plus 4 legs or 6 legs configurations.

FLYING

The big issue with dragons, especially when combined with flying, is obviously the size. Realistically the issue has been solved by engineers with either high-energy power generation (airplanes and helicopters) or by making the aircraft float in the air with lifting gas. Since the physical problem to solve is the same these would be the solutions available to draconic (non-magical) evolution as well.

I find the concept of fire-breathing animals, probably commonly fighting each other, being filled with hydrogen less than convincing. Helium would work, but the only sources for it would be ingesting large amounts of methane (methanovore?) or radioactive generation. Tapping underground methane would actually be kind of interesting, as it would explain why dragons spend centuries in underground caverns doing nothing visible to humans. They are breathing in the methane, living from the energy of it and harvesting mixed in helium. Actually eating methane might be a draconic attribute even if you skip dragon-blimps. Apart from matching the iconic behaviour of dracons, a source of methane is useful to fire-breathing.

Third lifting gas option would be hot gas, either plain air or steam. This might actually work for a VERY large animal. And needing to generate high temperatures would match with evolving other pyrotechnics.

Problem is that dragon-blimps don't really look very dragonic. So I think I'd skip this option and focus on increasing power density.

I think the easiest way to up power density would be to increase the number of hearts. Basically biological power density is limited by the ability to remove heat and metabolites from tissue. You also need to bring energy in, but that is not generally the issue when talking about power density. This kind of implies that dragons would be able to boost their metabolic rate to level insane. Which would imply very high blood pressure at muscular system, while other parts, such as brain, maintain normal pressures. And one heart maintaining high pressure at a very large animal wouldn't really work that well anyway. Separate secondary circulation systems that boost pressure selectively might work.

By itself this wouldn't be enough, but if you can increase blood pressure, and maybe even have an entirely separate circulation of "something else" with higher power density, you can also replace muscle cells with something else. You can't really make cells work with power beyond certain level, but cells could generate structures capable of higher power in a manner similar to how hair is created. I have no idea how high this could get the power density, but high enough that power density wouldn't be the biggest issue...

So you end up with the structural strength being the big issue. Hollow or otherwise low density bones are pretty much compulsory. Even then you'd probably need "biologically generated but not living" structures such as I used to dodge the power density issue. Nature has some remarkably strong protein based fibers and glues, so composite structure of fibers combined with strong adhesive seems likely. Something similar to what trees use with cellulose and lignin? Trees can grow to large sizes and protein based solution would plausibly be stronger. Supplemented with similarly reinforced "semi-exoskeleton" of powerful natural armor (very dragonic) and the high density muscles discussed before, this might be enough. Certainly you could get something larger than any living dinosaurs.

Breathing enough oxygen would still be an issue. Dinosaurs and birds have more efficient lungs than mammals do but still. If we assume that the fire comes from a powerful oxidant, the dragon might be capable of storing large amounts of the oxidant and use that oxygen to power up. Or possibly the "new muscles" required anyway, don't consume oxygen directly but work more like a plug-in hybrid does. Dragon sleeps and charges the batteries for the muscles and then the muscles can do a specific amount work without extra metabolism needed beyond the secondary circulation for cooling and energy.

I can see how the exotic bones and hide would evolve naturally as sizes go up. Reinforced structure is useful in intermediate forms and based on natural proteins. Same with extra hearts. Extra control of blood pressure in extremities is useful in intermediate forms for large animals. Evolution for exotic muscles is harder... Then again if we assume a snake re-evolved limbs and already assume an unusual configuration, the exotic muscles might not be evolved from normal muscle to begin with. Which doesn't really suggest how they evolved, but opens up the possibilities enough for the lack of explanation to be less bothersome.

Sorry, for rambling, but this is actually something I have though occasionally about...

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    $\begingroup$ Very cool answer ;-) Though it is structured in content, it is not structured in form. Could you add a few titles so it's easier to read? $\endgroup$ – Sheraff Dec 3 '14 at 10:23
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Growing from a very small size to an extremely large size is no problem, as dinosaurs show (and remember, ultimately even the largest dinosaur started as a single cell; the size of the egg is basically about how much initial nutrients are stored for the organism until it is ready to get out of the egg).

I don't think you could get literally to mountain size, but if taking that as hyperbole, I'd say that 12 meters of length for T. Rex is already a very impressive size (and at least in the fantasy with dragons I've read, which admittedly is not very much, the dragons weren't much larger).

Now flying could be a problem with large sizes. However a Quetzalcoatlus was still of impressive size. Bat-like wings should probably not be a problem. Whether a lizard-like appearance would be realistic probably depends on how exactly you define "lizard-like". There would probably be some shape requirements for aerodynamic reasons. But otherwise I see no reason why this should not be possible.

The most problematic point, of course, is breathing fire. As far as I know, there's no known animal that does or did that. However, we know that inflammable substances (alcohol!) can be produced by organisms, and we know that in principle it should be possible to safely emit fire (every fire-eater shows this). The main problem would be how to ignite the breath (in a way not to harm the dragon itself). However, I guess that should be a solvable problem; there could be for example some substances which the dragon could emit in small amount together with the inflammable gas or aerosol, which has a strong exothermic reaction in air, thus igniting the gas. Maybe there's a chemist here to tell whether that would be possible/feasible.

As Michael Kjörling mentions in the comments, a substance that inflames in air would be hard to contain, and thus very unlikely to evolve, therefore a better solution would be a two-component ignition, where each substance in isolation is harmless, but when they meet they ignite. If the substances have a use also in isolation, this is more likely to evolve, and it is not unrealistic that at some time a mutation caused them to be emitted at the same time, giving the ability to breathe fire.

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    $\begingroup$ It would probably be pretty hard to evolve keeping the igniter away from air. For a start, any time any of it is expelled, it must be replaced by something. Airtightness is difficult enough to design, let alone evolve. It could however be a pair of gases which do not combust on their own even in contact with air, but which have a strong chemical reaction when coming in contact with each other. I don't know exactly what that might be, but it makes more sense than using something that produces a strong reaction in contact with air. $\endgroup$ – a CVn Sep 19 '14 at 7:57
  • $\begingroup$ @MichaelKjörling: Good point. I'll work that into my answer. $\endgroup$ – celtschk Sep 19 '14 at 8:00
  • $\begingroup$ aerodynamics is a good point: can you fly well with jaws that ressemble more a komodo dragon or an alligator than a bird (like the Quetzalcoatlus)? $\endgroup$ – Sheraff Sep 19 '14 at 8:18
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    $\begingroup$ I believe I saw an explanation where dragons ate a certain kind of rock and kept it in a kind of second stomach, when they wanted to breath fire they released some of it and ignited it with a chemical mixture. I can't for the life of me remember the name of the book. $\endgroup$ – Mourdos Sep 19 '14 at 11:47
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    $\begingroup$ @Mourdos, was it Anne McCafferey's Pern books? They ate phosphine-beraring rock and burped pyrophoric phosphine gas. $\endgroup$ – Monty Wild Sep 20 '14 at 12:53
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I think most of the answers focus on bringing dragons to humans. I argue the opposite would be easier to do scientifically. Modify the world to support dragons, then bring humans in as otherworldly visitors in advanced habitat modules and pressure suits. I'm not sure how to make the humans appear medieval in that circumstance, but the question didn't request the usual fantasy backdrop. Nor was any implication made that humans must be comfortably habitable.

Perhaps the dragons exist in a thick atmosphere which is highly combustible with the addition of some biologically inert catalyst.

The dragons could fly with their large masses due to the thick atmosphere, although it would lead to higher atmospheric pressures.

The chemistry of the air and the biologically inert catalyst are beyond me.

A small spark could come from high concentrations of iron in the teeth and tongue. Calcium is pretty reactive in its elemental form; maybe the reaction of the dragon's saliva with its teeth creates a small flash. The saliva would be like snake venom, contained in sacs. That'd cause a lot of tooth decay. Add shark teeth growth so the teeth are replaced.

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Hmm, seems to me that it's pretty obvious that dragon-like creatures are biologically possible, because we know that they used to exist. We call them dinosaurs.

"Grow to the size of a mountain" is pushing it, but some dinosaurs certainly got very big. Big enough that a person might describe them as "the size of a mountain".

"Lizard-like appearance." Yes. Reptilian, anyway.

"Able to fly." Well, you run into a square/cube problem if you want a very large creature to fly: The mass of a creature increases with the cube of it's size, but the surface area of its wings only increases with the square. As lift depends on wing size, in practice it appears that living creatures, at least those using the sort of biological processes that we are familiar with, run into a limit here.

Among real dinosaurs and dinosaur-like creatures, there were some that were very large, like diplodocus, and others that were able to fly, like pterodactyls. But none that were both very large and able to fly.

"Breathes fire." First we should ask exactly what that means. Do we mean specifically something like the fantasy picture of a dragon, spitting flame out of it's mouth like a huge flamethrower? There's no creature alive today that does anything like that. Though of course that doesn't prove it's impossible. There are creatures like the bombardier beetle, that sprays a combination of chemicals out its rear that makes a little explosion. Beetles are tiny, but if you scaled up the quantities of chemical, it might make a nice little explosion that could be called "breathing fire". In any case, it demonstrates that the principle is not impossible, but in fact exists in a creature that we can observe today. The bombardier beetle has a complex collection of chemicals that enable it to spit out this little explosion without blowing itself up or setting itself on fire. It doesn't seem implausible that a different combination of chemicals on a larger scale could make a more "impressive" fire.

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  • $\begingroup$ A big pterodactyl at ~200 kg is about the size of a year-old baby cow. Dunno if it's quite up to "the size of mountains." $\endgroup$ – Serban Tanasa Jan 20 '15 at 1:20
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    $\begingroup$ @SerbanTanasa Years ago I read that pterodactyls are likely the limit of how big a creature can be and fly using body parts and biological processes like those we are familiar with, and the writer had some calculations to that effect. I recall reading elsewhere calculations that pterodactyls should not have been able to fly, and the writer speculated that either the atmosphere was thicker back then to make it possible, or that they only flew by first climbing up tall trees or cliffs and jumping off. 'Afraid I don't have either reference handy. $\endgroup$ – Jay Jan 20 '15 at 1:22
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If you increase the oxygen content of the atmosphere and make it more dense (as some scientists believe the earth once had), then you can allow for both larger land creatures as well as greater buoyancy for flight.

It would also make it much easier for any flammable gases produced by the dragon's gut (say, methane) to catch fire.

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Just a note on fire. I was reading through the other answers and I was really impressed with the idea of fire-breathing being biologically possible with chemicals.

Has anyone considered something like a biological binary compound to create fire?

For example: Gland A creates Agent A. Gland B creates Agent B. Dragon shoots a compressed mixture of Agent A, then Quickly shoots a second puff of Agent B. When Agent A and Agent B mix, they instantly react with oxygen.

POOF!~

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    $\begingroup$ That's basically how bombardier beetles work, so yes. They have a third chemical in their bodies that kills the reaction so stray mixing of the two chemicals does not blow up the beetle. $\endgroup$ – Jay Feb 9 '17 at 16:04
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A fairly rational science fiction dragon model has actually been done already, Anne McCaffrey's Dragonriders of Pern series of books. Here's the Wikipedia link: Dragonriders of Pern

It's a good model to start from, and I think it's the best 'realistic' model I've seen.

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    $\begingroup$ NOT! IMHO, anyway. As I recall them, Pern's larger dragons might be OK biologically, the problem is having humans ride on them. Sitting on the shoulders, as is commonly pictured, is going to completely screw up the weight & balance, and mess with the aerodynamics. Not to mention that it gets pretty darn cold hanging out in the wind at any altitude. The sensible alternative here, I think, is to assume that the giant dragons are fish stories, and they really don't get much larger than say a swan or similar. $\endgroup$ – jamesqf Jan 6 '15 at 4:40
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    $\begingroup$ Could you edit in some information about how Pern dragons satisfy the constraints in the question? Links are important for further information, but we're looking for answers that contain the actual answer, not just pointers to possible answers. (Since Pern is set on another planet with different properties, it's not automatically clear that those dragons would work on earth -- but you could explain that in an edit.) $\endgroup$ – Monica Cellio May 28 '15 at 14:50
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    $\begingroup$ Pernese dragons were specifically stated in some of the books to use telekinetic abilities to assist their flight, as well as being able to teleport and communicate telepathically. They don't meet the requirements of the [science-based] tag, since these abilities would be considered [magic]. $\endgroup$ – Monty Wild Jan 19 '16 at 22:48
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In the future, when time machines are ubiquitous, send someone clueless (intern?) to late Triassic period to capture a sample of pterodactyls for research. Let intern set up wrong target date for transfer in the time machine so they get sent to Middle Ages instead. Ooops!

Of course these "dragons" do not breathe flames. I do not think that there would be an evolutionary pressure for a flying predator to evolve fire breath - what it will be the use for it? One hint about it uselessness for predators is that it did not evolved in the nature. Flying predator's main advantage is speed and ambush, fire-breathing is for no use for such hunter. Some poisonous/smelly excretions are more often used as defense mechanism.

My solution has 3 out of four requirements, and is 100% plausible (after invention of time machine of course).

Pterodactyls and other flying dinosaurs do not resemble komodo dragons (which are reptiles, way too slow and inefficient to fly). Science suspect these were warm-bloodied animals. But they do resemble dragons of lore :-) - even more than komodo dragon does, so...

I read a story like that, but some people believe that it is a valid answer only if I pretend to imagined it myself, so forget about that story.

Sorry I did not added links first, I thought that interested parties would know pterodactyl. Also, I do not know how to build a time machine, sorry it that is a problem for you.

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    $\begingroup$ This doesn't answer the question, and should be a comment. "How can dragons be explained without magic" is not answered by "there's a book on a related subject". $\endgroup$ – ArtOfCode May 28 '15 at 9:30
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    $\begingroup$ Let's agree to disagree. My answer is about fetching pterodactyls using time machine (which is not magic, for some definition of "magic", as requested by OP), as I read in some story. Of course your answers are all original, nobody ever wrote story like that. Good for you. Would you like my answer more if I did not mention the story and pretended I invented it myself? $\endgroup$ – Peter M. - stands for Monica May 28 '15 at 13:04
  • $\begingroup$ No, this answer is a comment about a related bit of literature. Not an answer. Answers in this style frequently get deleted. $\endgroup$ – ArtOfCode May 28 '15 at 13:09
  • $\begingroup$ Do pterodactyls resemble komodo dragons (with fire-resistant scales)? How would you add fire-breathing into this, which the question asked about? Please edit to more-directly address the question. Thanks. $\endgroup$ – Monica Cellio May 29 '15 at 3:14
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Focusing on the fire breathing, I'll quote a bit of Heinlein:

" (...) Eight feet high at the shoulders, a few tons each, and teeth as long as any forearm—all they need is to breathe flame"
"Oh, but they do! Didn't I say?"
I sighed. "No, you did not."
"They don't exactly breathe fire. That would kill them. They hold their breaths while flaming. It's swamp gas—methane—from the digestive tract. It's a controlled belch, with a hypergolic effect from an enzyme secreted between the first and second rows of teeth. The gas bursts into flame on the way out."

Glory Road by R. A. Heinlein

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Averting a little bit the classical representation of a european dragon: what if it's not a giant "Smaug" type creature that breathes actual fire but an actual living creature stories about which are greatly exaggerated.

  1. Wyvern type body structure akin to the flying reptiles from our world or maybe more on the fantasy side given right planet surface conditions. This eliminates the problem with an extra set of paired limbs.

  2. The size is not so giant, more like the biggest flying birds or reptiles that are or had been in our world in given environtmental conditions. Think of the ~2.5-3.5m wing span.

  3. It lives in grass planes or savannahs and hunts mostly on grazing animals of small to medium size and on small mamals that live in borrows(rodents, rabbits and meerkats) while it's young and small.

  4. The "fire breath" is not actual fire as it's difficult to explain the evolutionary need for it or any of the intermediate steps that would lead to it but a liquid toxin that evaporates rapidly and may be acidic based thus leaving "burns" on the affected skin. The toxin itself is more of a paralyzing/anesthetic substance like Chloroform as inhaling it cause the neural system to malefunction or even shutdown.

To summ up: it's a big flying reptile that lives in planes and savannahs, hunts by suprisingly attacking its' prey from airbourne spitting the liquid toxin over it. The toxin starts to evaporate and inhalation of vapors leads to paralysis/coma/death through suffocation at which point the pursuing dragon comes down and easily finishes off the prey since it can't ressist anymore. The toxic breath may be also used to ward off scavengers(lions, hyenas, wolves etc) while the dragon consumes his prey or maybe those won't even be attracted to it due to the smell.

While the dragon is juvenile it may hunt on smaller mammalians by exhaling its' toxin inside their burrows and tunnels forcing them to abandon it or die(since for their smaller bodies a smaller dosage of vapor would be enough) at which point it catches them or digs the corpses out. "smoking out" thing.

Now if we introduce humans into its habitat it's likely that the dragon may attack a single individual or even a small group since volume wise humans are more or less the same as its usual prey and prior human arrival the dragon was the apex-predator of its biome. Those who survive will have burnings from it's breath which they may take for fire-inflicted burns due to the low levels of science.

And of course the stories will be greatly exaggerated growing the creature from 3 meters to 15 and claiming that it can breathe fire so hot the metal melts(actually rapdidly rusts or acidizes losing the required properties in the process) and capable of buring entire un-exisiting villages - "true story, bro, I never lie: ask my mom."

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The flying would be hard, because it is just not feasible to expect a dragon that is tons of pounds to be able to fly. UNLESS it's weight is less that it would seem. Extra gas, hydrogen would be most likely, would potentially be able to decrease the weight, but its debatable whether the minimum amount of organs needed to keep an animal that size alive would themselves be too much weight to fly.

Fire breathing is easy. You could look at the Bombardier beetle and do something like that. This beetle creates an explosive gas in a chamber inside his body then expels in through holes in his abdomen. With only a slight few routing changes, that could be quite feasible.

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First I'll address fire-breathing

The dragon could produce a very flammable liquid in a gland, and could spit it out, past the teeth. The dragon could click his teeth in a special way, producing a spark, lighting the gas on fire.

The dragon could fly but the wingspan would be colossal. As for the fireproof scales, it's possible to, if they're thick enough and the skin underneath is totally protected.

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https://en.wikipedia.org/wiki/Chrysopelea

A Flying Snake

While you did say a "lizard-like" appearance I believe a more oriental eastern dragon would also fit your criteria. However, the flaring of the ribs could occur even in western dragons. Combine this with the natural balloon sacs and much larger wings mentioned by previous posters and, you have flight capable of dive bombing onto prey. Furthermore, a dragon does not have to be skilled at more graceful movements in the air as it:

  • will most likely be the biggest baddest thing in the air
  • will most likely eat large landed prey like deer.
  • could not be graceful like most birds at it's size to begin with (no barrel rolls for you today Mr. Dragon).
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Other people have already mostly covered this. I think flying is the biggest problem. The bones need to be elephant-strong but not elephant-heavy.

If it's big and active it probably needs to eat a lot. The easiest stuff to eat a lot of is grass etc. It ferment carbs and cellulose in it's big fermenting stomachs, it must carry a lot of it for some time to get the fermentation, and that's heavy. So far it's a lot like a lizard-elephant. But it can produce methane and it can burp the methane, so all it needs is a way to ignite that, which other posters have considered solutions to.

Possibly it could fart methane and light that too. Fire at both ends.

Maybe it only flies when its stomachs are empty. If it really wants to fly it empties them? An empty elephant is probably a lot lighter than a full elephant....

Also, methane has a little lift but not much.

density of various gases

Ammonia would be just about as good. Ammonia has a different flame and various other dangers. A dragon wouldn't necessarily evolve to use the gas with the best lift, it would use whatever the easiest evolutionary path led it to.

video with burning ammonia

As others said, hydrogen would be better. And the dragon doesn't have to look like a dragon after it empties its bowels of everything but hydrogen and flies.

what a flying dragon might look like

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Historic references to dragons may be pertaining to dinosaurs that have survived the ages. Some people subscribe to this theory. It may sound outlandish in contrast to conventional education, but then technically, what is a crocodile? A crocodile is a dinosaur that survived through the ages. Ask any biologist. Ergo, it is not even outlandish to say that there may have been dinosaurs in the middle ages since there are still dinosaurs now.

In fact, although unlikely, it's possible that dinosaurs still exist today, limited to some remote and uncharted area. Until humans emu-walk every square meter of land surface, and video tape every cubic meter of sea water, there is no definitive proof that every last dinosaur has died out.

Now, how do you prove that there was never a kind of dinosaur that looks exactly like the imaginary dragons depicted in Hollywood movies? Failing to find fossils of it is not a proof of it never existing. My only concern would be whether dragon flight is bio-mechanically possible. I'm not qualified to comment on that.

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    $\begingroup$ This is significantly incorrect. Crocodiles are not dinosaurs, nor are they descended from dinosaurs. We do however have surviving dinosaurs even today, they are called birds. Also an argument that "we don't know of everything that ever existing, therefore it is possible dragons did exist" can be applied to literally anything you can think up, which isn't very good evidence for their existence, and definitely doesn't answer the question $\endgroup$ – Kevin Wells Sep 21 '16 at 19:23
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    $\begingroup$ Saying that dragons could be a variation of dinosaur that has survived until now does answer the question. Also: "Crocodilians are more closely related to birds and dinosaurs than to most animals classified as reptiles, the three families being included in the group Archosauria ('ruling reptiles')" en.wikipedia.org/wiki/Crocodile $\endgroup$ – Lorry Laurence mcLarry Oct 11 '16 at 9:52
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    $\begingroup$ It is true that crocodilians are more closely related to dinosaurs than most reptiles, but that doesn't make them dinosaurs (which is what you claimed in your answer). Also, being closely related does not make them descendants at all, just that their lineages split off more recently than the split between reptiles and dinosaurs. If you really think you are right then follow your own advice and "ask any biologist" if a "crocodile is a dinosaur that survived through the ages". Really, please do $\endgroup$ – Kevin Wells Oct 11 '16 at 16:45
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    $\begingroup$ More importantly, even if you remove the biological inaccuracies, it is still a bad answer because you contradict yourself. You say "there are still dinosaurs now" and then in the very next sentence you say "In fact, although unlikely, it's possible that dinosaurs still exist today". $\endgroup$ – Kevin Wells Oct 11 '16 at 16:47
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    $\begingroup$ Not exactly a contradiction; or certainly not "more importantly"... I'm saying that the distinction between crocodile and dinosaur seems fairly arbitrary. It does not mean I am incapable of constructing a sentence that accepts the arbitrary distinction if I choose to. That part of my statement certainly should not have been confusing. FINE!!! crocodiles aren't dinosaurs, they are "archosaurs" which dinosaurs also are. I should have said that dragons could be surviving archosaurs. (a word that hardly any reader or even myself was familiar with) Most pointless nit-pick ever... $\endgroup$ – Lorry Laurence mcLarry Oct 12 '16 at 9:26

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