I am an aspiring fantasy writer, and I wanted to include rideable Wyverns in my world. There will be a magic system in my world, but I still want to make them as scientifically possible as I can. Now, believe me, I did my research (or at least as much as somebody without a degree in physics and biology could do) and I know that creating anything that is supposed to fly and be mount is borderline impossible. That's simply too much weight. But maybe that's the part in which readers would suspense their disbelief.

Now onto my notes. So my idea of a Wyvern:

  • They are tetrapods/quadrupeds.

  • They walk on all-fours

  • They are carnivorous

  • They are covered with colorful feathers.

  • They have swan-like/snake-like necks.

  • They have short, normal tails (Without barbs or stingers).

  • Their legs are short but strong.

  • They have pterosaurs-like wings, with membrane exceeding to the ankles AND FLIGHT FEATHERS (This and the next point are the ones I'm the most worried about, as I'm not entirely sure whether or not this is even biologically possible)

  • They are only about 4,5m long (14ft 9in) from the tail tip to the mouth, just as tall as a horse, maybe slightly taller, with a wingspan of 6,5m (~21ft)

  • They weigh around 60-70kgs (132-154 lb.)

  • They have hollow bones

  • They have a special sack in the belly which stores hydrogen to help them fly (they gather it naturally in small amounts by the degradation of inhaled air)

  • Flight like condors (Mostly glide, with rare big swings for gaining altitude)

  • Start flying by throwing themselves off the cliffs to gain momentum

  • Cold-blooded so they bask in sun often

  • Sexual dimorphism: Females are slightly bigger than males and sport bone crest on the back of the head.

  • For a few centuries, bred by humans

Operate mostly by the reptilian brain, live by instincts, stupider than dogs, rather unfriendly but can be thought respect to their riders. (smaller brain=less weight)

Fantasy genre touches:

  • Special wing muscles: denser but lighter than muscles found in humans for example.
  • They can support the weight of one unarmored human

Do you think this concept can work? Do you have any advice or ideas? Would bat wings be better? (I heard that they offer greater agility and maneuverability, but are not really suited for long-lasting travel.) Would two sets of wings, generate more lift?

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    $\begingroup$ Having a hydrogen sack doesn't make any sense unless you want to use it for fire-breathing purposes, it simply wouldn't produce appreciable lift (notice there are no gas-bag flying creatures in nature). For reference, a 2.2 meter diameter weather balloon made of ultra thin plastic can only lift around 2kg when inflated with hydrogen. I don't think you could put a hydrogen system in your wyvern without the peripheral organs and the containment membrane weighing more than the lift produced. Even if you could, total lift would probably be on the order of single-digit Newtons... (<500grams) $\endgroup$
    – Dragongeek
    Commented Oct 17, 2020 at 23:50
  • $\begingroup$ May interest you there is a group of bat winged dinosaurs. smithsonianmag.com/science-nature/… $\endgroup$
    – John
    Commented Oct 18, 2020 at 5:06
  • $\begingroup$ You say one unarmored human. What about a bodybuilder 6+ ft tall with no armor vs a jockey with some armor? When you're making specifications, this matters. $\endgroup$
    – NomadMaker
    Commented Oct 18, 2020 at 6:59
  • $\begingroup$ Noting stating the are quadrupeds and walk on all-fours is redundant, they mean the same thing. $\endgroup$
    – John
    Commented Oct 18, 2020 at 18:07
  • $\begingroup$ I am not very familiar with the conventions and nomenclature of modern fantasy, but in the English language, in heraldry a dragon usually has four legs and two wings, and a wyvern is type of dragon like being that has two legs and two wings. Are you talking about dragons, or wyverns? And why do you capitalize Wyvern? $\endgroup$ Commented Oct 18, 2020 at 19:18

4 Answers 4


Maximum size for flying vertebrates on Earth

Quetzalcoatlus reached 250kg with a wingspan of 10.4m. This is your best basis to work from. This paper provides a Relative Failure Force for Quetzalcoatlus' humerus of 1.85 (that is 1.85x it's body mass). This should equate to around 450kg (without accounting for any scaling) as a theoretical biomechanical limit, which allows you a potential payload of around 200kg. Relative failure force of humerus of different species

With a total mass of 450kg, the wyvern would have a max aerobic performance of 3N/kg, or about a third of what it needs to sustain flight (9.8N/kg) aerobically. In comparison, an albatross' maximum aerobic performance is ~6.5N/kg although they often use cliffs to launch. Your wyvern would also require more than twice as much energy to launch per kg (at 322 Watts/kg compared to 151 Watts/kg for an albatross) which works out at double the total flight energy requirements of Quetzalcoatlus.

At 450kg, the wing load would equate to 34kg/m2, the maximum for birds is taken to be 25kg/m2. With a payload of 70kg, the wing load becomes a more reliable 24.25kg/m2 which might be plausible. Plugging this back into the original equation gives you a max aerobic performance of 4.3N/kg, which is at the lower end of what is measured for Kori Bustards and is probably pushing the limits of what is achievable on earth. Similarly to what someone else mentioned, if your world is a clone of earth, then like horse-racing, you may need smaller, lighter jockeys to be your dragon riders.

Increases payload capacity

Lift (and drag) increases linearly with air density so doubling the air density would halve the amount of power required to fly, although I'm not sure whether this has an effect on wing loading. This is also small enough to not have a neglibile impact on anything else. This would double the partial pressure of oxygen (as well as other gasses) so respiration will me slightly easier (and potentially humans would exhaust slightly less easily) but it shouldn't produce any health effects.

Alternatively, Decreasing surface gravity by just 20% would give you a launchable total mass of 400kg, or a wing load equivelent to 238N on earth, if you needed heavier mounts. I'm not sure whether the maximum wing load of pterosaurs would be the same as birds since launch using their hindlegs, whereas pterosaurs, launch using their forelimbs. It may be that pterosaurs could launch with higher wing loads than birds although. The wing load of an albatross, Bustard, and Azhdarchid is around 12, 8 and 18 kg/m2 so it seems that pterosaurs were at the upper end of bird capabilities already.

Adressing your flight adaptations:

Some of the adaptations you listed would actually impede your creatures flying capabilities. Hydrogen sacks or other gas equivelents would increase drag by a significant amount for a negligible increase in lift. It is also very difficult to store hydrogen due to its reactivity. Flight feathers are also a bad idea as they are a large contributor to bird size limits. Flight feathers must be shed periodically but in doing so, flight capabilities are compromised. Soaring birds often extend their molting periods over 3 years or so instead of the typical annual cycle. Some birds may also shed all their flight feathers at once and be rendered temporarily flightless while they grow back. A membrane wing with pterosaur anatomy negates this limitation. It doesn't mean your dragon can't have feathers, just that they should not have flight feathers like birds. Additionally, a slower metabolism would be a disadvantage. Faster bird metabolisms are linked to their superior flight capabilities over mammals. Hollow (pneumaticised) bones, long necks, carnivory, short tails and quadrupedalism are all already pterosaur adaptations so no changes there. The one exception is that azhdarchid necks are long but very very stiff. In your wyverns, you could just give their necks more flexibility but reducing the length of the vertebrae and increasing the amount of them may help.

Even though these wyverns are larger than you expected, at 2.5m at the shoulder they're a little taller than a horse but if you want them to be shorter whilst still obeying physics, you're going to have to reduce gravity even further or create a super-dense atmosphere that generates enough bouyancy to offset the wing loading. The win is that the azhdarchic base actually works out around 4.5m long. The width shouldn't make too much of a difference since they walk with their wings folded - we also know from pterosaur tracks that they most likely walked with their limbs under their body. The wings are very important so I would not recomend reducing them (or the shoulder height unless you give them a sprawling posture. Azhdarchids had huge but very pneumaticised heads so a more stereotypical dragon head should be fine without effecting the weight distribution too much. The only important factor here is that quetzalcoatlus had very weak jaw muscles so if your dragons are feeding on more robust or volitile prey, the front end-mass will increase with musculature. The flexible neck will likely require more musculature already but looking at bird necks, I don't think this would make much difference. Since your dragons are semi-domesticated, it probably works best (And safer for your riders) if their head musculature is kept to a minimum so no issues there. Fun fact: Snakes actually have next to no neck! They're all torso. If the giraffe-like posture is a no-go, then you could increase the length of the rear legs to bring the spine into a more horizontal position. I assume you may shorten the neck anyway so their posture would adjust to suite.

As for more efficient muscles, lightweight bones, etc. I think it would cause more problems than it solves. I think your creature would do fine already. The problem with flight isn't so much the energy requirements, it seems to be more to do with wing load and the limit to how much weight the actual wing bones can support (and how long they can grow). As well as the biological factors such as molting flight feathers. One last link that may be of interest is this paper on the pteroid bone.


In summary, your wyverns should be fine, if a little bigger than you were expecting - weighing 250kg with a carrying capacity of 70kg on earth, 105kg at 90% gravity and 150kg at 80% gravity. Some of your unique adaptations would be detrimental to its flight capabilities. Whilst your wyvern is feathered, the feathers are not primary flight feathers and would function more like pycnofibres of pterosaurs. It may have some streamlining feathers over the wing membrane to increase the airfoil effect and some feathers may be used to add rounding to the wing tip although the lift would be generated almost exclusively by a wing membrane.

Your basic wyvern body plan

  • 2
    $\begingroup$ If you want some mathematical help, this paper plots bodymass and rough wingspan for different flyers. birds and pterosaurs come out at : wingspan(m) = mass in kg raised to 0.4 and for bats raised to 0.32 witpress.com/Secure/elibrary/papers/1845640012/… $\endgroup$
    – John
    Commented Oct 18, 2020 at 17:50
  • $\begingroup$ Thanks for the link, I made a spreadsheet for my own flight calcs so I'll add a new row for winspan estimates. If I get chance, I'll format it for sharing so I can link it on my answer. $\endgroup$
    – Zac Walton
    Commented Oct 18, 2020 at 18:33
  • 1
    $\begingroup$ Wow, thanks for your answer. It was really informative! So correct me if I'm wrong but the build you propose would be: Feathers, but no Flight Feathers No Hydrogen gathering bladder. Around 2,5m tall, 4,5m long with the wingspan of 10m and weighing around 250kg? I can also give up on the swan neck. Someone even pointed out, that it's evolutionarily improbable since if they are supposed to hunt by diving from the sky, they won't need long necks, because there is a risk of hitting the head with ground. $\endgroup$ Commented Oct 18, 2020 at 18:43
  • 1
    $\begingroup$ Thx Zac. You've helped me a lot. Good luck with your own projects! :D $\endgroup$ Commented Oct 18, 2020 at 19:22
  • 1
    $\begingroup$ I agree aerial hunting favors short necks, long necked birds are all swimming or wading water feeders. Although if you really want the long neck you can get a longish neck from ground feeders. $\endgroup$
    – John
    Commented Oct 20, 2020 at 1:36

Your wyverns are giant hoatzins.



Using a neotenous hoatzin gets past the hardest part of your requirements: a flying quadruped. Hoatzin chicks have claws on their wings and use them with their legs to walk like a lizard.


In a tank of water, the chicks swam by moving both wings together in a pattern resembling the breast stroke. Such simultaneous wing motions are the norm for birds, although most employ them for flying rather than swimming...

The most interesting findings came when the researchers placed chicks on a slope covered with a towel. The bird's progress up the slope was slow and halting, and it sometimes took a couple of tries before the wing-claws succeeded in grabbing the fabric. But the basic pattern was clear: right wing, left foot, left wing, right foot. Hoatzin chicks walk like four-legged animals, using their wings as front legs.

Wyverns are giant hoatzins that have kept their claws.

Another tough one is the hydrogen gas bag. But hoatzins are ruminants and hydrogen production via fermentation is totally legit.

Yeah that's right. Ruminants, like cows. But you want them to eat meat. That is fine. They do, when they can get it. They eat anything. They are bloated colorful flying goats, these wyverns.

Awesome colors - that is hoatzins. Long neck; hoatzins got that.

Sunbathing. sure. But not because they are cold blooded. Because they are covered with parasites and getting good and hot cuts down their numbers. -

  • $\begingroup$ flying quadrupeds are easy, bats and pterosaurs are flying quadrupeds. Also hoatzin are not quadrupeds, they are bipeds. they do not walk with their front limbs. $\endgroup$
    – John
    Commented Oct 18, 2020 at 13:40
  • $\begingroup$ @John take another look at the excerpt. The point of that excerpt in my post was to demonstrate how hoatzin chicks walk as quadrupeds over rough surfaces. $\endgroup$
    – Willk
    Commented Oct 18, 2020 at 14:41
  • $\begingroup$ But they don't, they use all four limbs to climb trees not to walk. By your logic WE are quadrupedal. $\endgroup$
    – John
    Commented Oct 18, 2020 at 14:42

There are problems

  1. You can't be a cold blooded vertebrate flyer, you can't maintain enough warmth in flight to get anywhere without a high metabolism. wings are too effective at cooling. Also flight is high energy demand, so that s another reason you will not see it in a cold blooded animal (well not one that weights more than a few ounces). Also feathers would be a big disadvantage to a an animals that basks, or to a an ectotherm in general. Feathers are insulation that evolved after endothermy as a way to help maintain high body temperature from internal heating. Note plenty of endotherms bask, especially one that undergo torpor, so you don't need to justify basking with cold bloodedness.

  2. The buoyancy you get from hydrogen gas is not worth the space it takes up. You need a HUGE volume of hydrogen gas to lift any appreciable mass. A 6ft diameter sphere of hydrogen gas only offsets about 8 lbs.

  3. No horse sized flyer is carrying a human, Quetzalcoatlus the largest creature to every fly might be able to carry a human. You will need to give them a significant sprawl (ala game of thrones dragons) to get horse height. A long distance flyer needs a wingspan comparable to a fixed wing aircraft.

Don't bother with 4 wings there are no vertebrate fliers with 4 wings.

It is worth noting pterosaurs has fur like feathers, and bat winged dinosaurs did exist so most of this is possible. Both groups are extremely close to what you want (all the features below), fulfilling most of your requirements.

They are tetrapods/quadrupeds.*

They are carnivorous

They are covered with colorful feathers.(well protofeathers)

They have swan-like/snake-like necks.

They have hollow bones

Sexual dimorphism: Females are slightly bigger than males and sport

bone crest on the back of the head.

*Yi are bipedal but they are also small, I could see a a return to quadrupedal with extreme sizes.

Note bat like wings give you your smallest possible wingspan, so you may want to stick to that.

enter image description here

  • $\begingroup$ Yeah that's why I added the part about stronger,denser wing muscles, which don't weigh as much as they should. I don't want it to be so gigantic, because it creates many problems: you would need a gigantic barn to keep one locked, and special landing zones in cities. I thought that with being only as tall as horse (about 2m) but being 4m long and with wingspan 8,5m it would suffice. But maybe I am wrong. Could you propose measurements which wouldn't be gigantic, but would be more probable? I don't care if it's heavier, as long as it's not 11m tall, and 10m wide. xd $\endgroup$ Commented Oct 18, 2020 at 10:58
  • $\begingroup$ @Kubsterrb17 it is not about the strength of the muscles it is about how big the wings need to be to move enough air. Worse you want a long distance flyer which means the wings need to be long and thin. You are not getting anything that takes up less space than a Cessna while landing. 8.5 meters is how big the wings would have to be on a human that just had to lift itself. $\endgroup$
    – John
    Commented Oct 18, 2020 at 13:38
  • $\begingroup$ Actually, average human male would only need a wingspan of about 6m. I've checked it. $\endgroup$ Commented Oct 18, 2020 at 15:00
  • $\begingroup$ @Kubsterrb17 only if the wings have no mass. The largest bird to ever fly weighed about ~70kg and had a wingspan between 7.5 and 8m. and soaring wings have a larger wingspan for the same mass on top of that. $\endgroup$
    – John
    Commented Oct 18, 2020 at 16:46

Having flight feathers and wing membranes in the same place would likely make the wing work like a wedge, which doesn't seem aerodynamic. However, if the membrane and flight feathers were in different positions along the wing, there shouldn't be many problems

  • $\begingroup$ Could you kindly elaborate, how it should exactly look like? Or maybe with that body proportions, I could skip the membrane completely, and make normal bird wings? $\endgroup$ Commented Oct 17, 2020 at 21:27

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