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How plausible are naturally-evolved, biological, fixed-wing flying creatures? Could such a creature plausibly evolve in Earth-like conditions? If so, what method would it be most likely to use for propulsion, and how large could such a creature realistically become? If such creatures are not plausible in Earth-like conditions, how much would conditions have to change for fixed-wing creatures to become plausible?

To clarify, these creatures would have completely fixed wings. Any kind of wing that has a capability for motion does not count, even if such wings are held immobile for long periods while soaring. Propulsion cannot come from any kind of flapping motion of the wing.

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    $\begingroup$ If only fixed wing birds could have "ground crews" that can service them and provide protection. Otherwise fixed wings are insurmountable liability on the ground. $\endgroup$ – Alexander Jun 25 '18 at 23:46
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    $\begingroup$ @Alexander, while the main thrust of this question is evolutionarily plausible propulsion systems for biological creatures other than flapping, I may eventually ask another question about ground protection and nesting habits. Landing may prove tricky for these creatures. $\endgroup$ – Gryphon - Reinstate Monica Jun 26 '18 at 0:04
  • $\begingroup$ @Gryphon Maybe look to the real fixed-wing flying things for landing? My answer used jet propulsion as an idea, so if they slowly decreased hydrogen combustion they could land on a "runway" of flat ground. Ventral "skis" would give them more traction. Going with this hypothesis, it seems like they'd be best suited to an ice sheet/frozen water body environment. $\endgroup$ – SealBoi Jun 26 '18 at 11:59
  • $\begingroup$ Albatrosses can lock their wings, but they do flap them occasionally . . . $\endgroup$ – FoxElemental Jun 26 '18 at 16:25
  • $\begingroup$ Even so-called "fixed-wing aircraft" would not be sufficiently "fixed-wing" to satisfy this question. They adjust their flaps, slats, and ailerons in flight. Furthermore, they have many moving limbs (propellers), either inside of shrouds, or outside the aircraft. $\endgroup$ – Jasper Feb 9 at 1:05

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Non-stop flyers

As stated in other answers, these birds would be vulnerable on land. So we are looking for birds that never land. It's not totally impossible, some real birds are capable of flying continuously for years.

You can make it seem valid from the point of view of evolution. For instance by deciding that the planet is entirely covered in water. Or maybe because it's too dangerous to land because of predators. So the longer you stay in the air, the safer you are. Until you evolve to stay in the air forever.

You can eat other things that are flying.

Propulsion

As they can't flap to regain altitude or speed, these birds need an alternative source of propulsion. Some ideas:

  • Rely on vertical air movements, like real birds. Your world must then have plenty of these.
  • Some animals/vegetables/fruits are large gas balloons, your birds could sometimes rest on them, use them as lifts to regain altitude or lay their eggs on them.

Fixed-wing justification

If your birds stay in the air perpetually and are using these propulsion means, then they might be more efficient with fixed wings:

  • Less muscle to maintain -> less weight, less energy
  • Fewer articulations -> less weight, fewer risks of breaking something

That said, they will still need some moving parts to steer in the air.

Reproduction

A comment asked about reproduction. How will the creatures give birth in the air? I propose the following mechanisms:

  • Birds drop eggs in the water, or on whatever soft enough surface. When the babies are born, the wings are not yet fixed, so they can start flying. The wings rigidify at puberty.
  • As stated above, they lay eggs on big gas balloons or similar buoyant organisms.
  • The eggs themselves are lighter than air because they have a hydrogen pocket in them. So they float around until the birth of the baby, which can directly start flying.
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    $\begingroup$ A bird that never land have to give birth in air. That's possible as some mammal give birth to (almost) fully functional babies. But can you imagine this if the baby has rigid spread wings? $\endgroup$ – Madlozoz Jun 26 '18 at 14:32
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    $\begingroup$ @Madlozoz Birds usually lay eggs ;) I added some thought about that in my answer. $\endgroup$ – Legisey Jun 26 '18 at 14:47
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    $\begingroup$ An excellent answer. Laying eggs is not necessary, so such creatures could simply rise to the highest heights they are able to before giving birth, and then have a live child. Instincts would take over, and the infant would take flight, either to be abandoned by its parents or watched over by them, depending on the breeding strategies of the species. $\endgroup$ – Gryphon - Reinstate Monica Jun 26 '18 at 15:03
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    $\begingroup$ Marsupials, marsupials can continue to mature in their mother's pouch until they are ready to fly alone. They would need to continue to grow after that, or they wouldn't fit in the pouch @Madlozoz, @ Legisey $\endgroup$ – Rafael Jun 26 '18 at 19:31
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    $\begingroup$ Child wings are folded but when they drop at birth, the wind of the fall push on the wings and shoulder bones lock themselves forever. Which mean the child don't even need any instinct, it's mechanical. For propulsion in adults, what about a feathered tail and some kicking ? $\endgroup$ – Echox Feb 7 at 9:53
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Sure. You need:

Jet Propulsion

These creatures below are called Skewers. They were envisioned by the writer and exobiologist Wayne D. Barlowe in his excellent book Expedition: Being an Account in Words and Artwork of the 2358 A.D. Voyage to Darwin IV.

enter image description here

I remember that they were jet-propelled, but I don't remember the exact mechanisms of it. So, this is the part where I get to speculate.


Many creatures today have symbioses with microbes in their gut. An example would be the ruminants, whose digestion is aided by microbes in a stomach called the rumen.

It is not difficult to imagine that a creature (Perhaps not a bird) could evolve a symbiotic relationship with a gut-living microbe that produces hydrogen as a metabolic waste product. If these animals could store and compress that hydrogen in organs like lungs, it could bring a lot of possibilities.

If the animals had a tube leading from the lung-like organs to their rear, the compressed hydrogen could be expelled in that direction where it is ignited by bioelectricity, and voila, combustion.

The skewers from Expedition were huge, at least Quetzalcoatlus size, and since it is primarily a work of science, I can assume that it is plausible that these fixed-wing birds could grow to very large sizes.

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    $\begingroup$ and they could be named Fartopteryx volitans... $\endgroup$ – L.Dutch - Reinstate Monica Jun 26 '18 at 10:59
  • $\begingroup$ @L.Dutch Brilliant! $\endgroup$ – SealBoi Jun 26 '18 at 11:02
  • $\begingroup$ Note the energy economy of the skewers makes them impossible. The skewers would need to be eating non-stop just to supply fuel. Darwin IV has much lower surface gravity and even then it is impossible. $\endgroup$ – John Jan 25 at 19:12
  • $\begingroup$ @John What if we coupled very low gravity with a really dense atmosphere? Even better, what about a methane-heavy atmosphere, with as much methane in the air as CO2 on Earth - obviously, it'd be one toasty planet, but presumably still habitable. $\endgroup$ – SealBoi Jan 25 at 19:22
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    $\begingroup$ If they're expelling their own hydrogen then they're rocket-propelled, not jet-propelled. Jet engines suck in air from the atmosphere and then expel it at high speed. $\endgroup$ – Mike Scott Mar 15 at 14:03
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I'll answer from the point of view

How plausible are naturally-evolved, biological, fixed-wing flying creatures?

I find it very difficult, to say the least.

  • Being unable to fold your wings is a huge disadvantage on land: they are necessarily frail and would be exposed to predators and crash against obstacles on the ground.

  • Landing, I think, is the most difficult task for fixed wings, although, maybe, a huge moving tail could help overcome the difficulties.

  • Being able to fold wings costs very little: wings are light and easy to move.

Could such a creature plausibly evolve in Earth-like conditions? how much would conditions have to change for fixed-wing creatures to become plausible?

Birds with fixed wings could kind of survive in deserts with no predators nor obstacles, but would they outcompete their flexible counterparts? Flying further is usually the result of flapping. Mountains (bare, to avoid making obstacles) could help them take off, allow ascending currents to keep in the air and make landing a little less dangerous.

It's difficult to find an evolutionary pressure that could evolve motionless wings. But they could be born mobile and become fixed at adulthood: otherwise, eggs should be long.

One possibility is that evolutionary pressure stops fighting some kind of osteoarthritis-like degenerative sickness of the joints to favor some other advantageous traits (like longer, wider wings)

If so, what method would it use for propulsion? [And other ideas]

Apart from flightless birds, like penguins or kiwis which could survive with fixed wings but are out of the scope of the question, there are birds adapted to glide and can actually survive with very little flapping.

The closest you can get to the kind of bird you're looking for with existing species is Andean Condors:

enter image description here I'm so majestic that all the countries in my range have chosen me as their national bird (image: Wikipedia)

They are the biggest birds on Earth by wing area (spanning about 11 ft long). A big area helps them glide effortlessly. They are adapted to mountains, very good at finding ascending currents and taking off from cliffs. If one of them had its wings suddenly fixed, it'd be more likely to have trouble landing than taking off or flying long distances without flapping (what they actually do).

Condors are apex predators (no one trying to kill them) and live in mountains, where they happen to find fewer obstacles. Their habitat favors non-flapping flight and take-off.

Perhaps what you need is a big-tailed condor (i.e., with a long, wide tail), whose wings become fixed OR are very small at the time of hatching

I don't think a bird could realistically live without any landing. Though they could hunt (or even gather) while flying, mobile wings increase maneuverability, thus increasing the chances to hunt (which would be a strong evolutionary force for wings not to become fixed).

how large could such a creature realistically become?

Apparently, birds can grow larger. Paleontologists argue that there have been birds with estimated wingspans of about $7\,m$. And the good news is that the bigger the bird, 1) the better it is at gliding, 2) the less efficient flapping becomes, 3) the more fixed their wings are

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  • $\begingroup$ look at the other answers, there are albatross that don't land for years... birds are certainly capable with not only living with out landing, but also thriving. landing happens when they need to breed typically, but something that isn't a bird wouldn't necessarily need to worry about that... $\endgroup$ – whn Jun 26 '18 at 17:34
  • $\begingroup$ @snb but the Albatross eat fish. They could still eat flying fish without having to take off again from sea level, though $\endgroup$ – Rafael Jun 26 '18 at 18:17
  • $\begingroup$ You don't need to stop flying to eat fish, and there were other birds mentioned that ate food from other birds so they didn't even need to get that close. $\endgroup$ – whn Jun 26 '18 at 18:29
  • $\begingroup$ @snb you're right, you'd also need flying eggs/portable nests $\endgroup$ – Rafael Jun 26 '18 at 18:42
  • $\begingroup$ The "Non Stop Flyers" answer goes over this, they don't need to be birds, live birth in air would also work, or some other mechanism. If you really need it to be a bird you can have eggs that will land in the ocean, and have animals that don't yet have fixed wings until they are older, or have them propel themselves via some other means into the air early on. Eggs could be carried on the parents, but then that is basically just live birth anyway. $\endgroup$ – whn Jun 26 '18 at 18:49
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Possibly there could be a bird-like creature that had four wings, one set of which never flapped but was rigid when flying though maybe it folded up when not flying.

And maybe the other pair of wings of the bat like creature would flap when flying.

So maybe the more rigid pair of wings was for lift, like the wings of an airplane, and the flapping pair of wings was for thrust, like propellers or jet engines on airplanes.

Or maybe the pterosaur-like creature would use its wings for lift and a little steering, and used a fan like tail for thrust, like many sea creatures swim through the water.

Or maybe the bird-like creature would have a siphon like squids do, and would use stiff wings for lift and air blown through the siphon for thrust.

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  • $\begingroup$ Multiple wings! This is exactly what I thought when I saw the question. $\endgroup$ – Andrew Brēza Jun 26 '18 at 17:58
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They could evolve from flying fish

These fish have fixed wings whilst flying and they are driven by their tail which hangs down into the water. Over millennia this tail fin could enlarge and evolve to propel them through the air

Videos of flying fish

Record flight: https://youtu.be/5lYnFDtFx20?t=66

Being chased by predatory birds: https://youtu.be/5lYnFDtFx20?t=280

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Well...if these creatures spent their lives on treetops, gave birth rather than laying eggs (So? Platypus is a mammalian that lays eggs!) so that the little ones clung to them like bat kits do...it could work. They'd be a strictly gliding species, their range of maneuvers would be limited, but as long as there's a good thermal they'd do just fine.

Now, eating could be a problem, since they cannot maneuver to prey. You need a world where the air itself carries an abundance of microorganism, a sort of aerial krill, these birds eat just by passing by. In a balanced ecological chain, it would be helpful because this 'windkrill' could be dangerous if it accumulated in excessive quantities on the ground or on trees.

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Jet propulsion?

Definitely not feasible, at the very least with naturally-evolved, biological and earth-like premises. Why? It's not a fuel issue, you can pretend to find it somehow, it's a temperature issue! It's something that requires special metal alloy to survive, carbon-based bones and tendons can't stand a chance.

Moreover, there are mainly two types of jet engines: turbojet and ramjet. Turbojets are the motors you usually see when you take a commercial flight, and the name itself suggest that it needs a turbine to compress the air. Since no flying animal developed any bone mechanism that's similar to a helicopter rotor it's really difficult to justify a turbine (for simplicity, a turbine is a rotor on steroids). The turbojets also won't work because of geometry complexity.

On the other hand, the ramjets are not complex at all, they use their own speed and geometry to compress air. The issue is that they can't operate at a standstill and they are efficient at supersonic speeds: you can't lift off, and supersonic flight requires a massive amount of additional "fuel" and involves even more heat issue because you may have to refrigerate also the wings and not only the engine. If something like that can be part of something "natural" we can also pretend that grasshoppers can jump so high to be used as a homing pigeon to deliver messages to the moon.


If the wings' limit is only that they can't be flapped to provide thrust, but they could still be folded (like carrier-based aircraft), it won't be a tremendous deal for a cliff based bird.

A hawk doesn't really need to flap it's wing except for lift off from the ground. If it could drop itself from a high cliff it could jump, prey another bird, and carry it to the nest only with "fixed" wing. The foldable wings are probably required because of nest dimension, cleanability, and rest (imagine trying to sleep with spread wings on a cliff that produces by default strong gust of wings).

If you really need a liftoff from the ground I can suggest you something like a
rocket engine (still difficult to justify, but way easier than jet propulsion).

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Maybe their wings would not evolve from limbs. They might evolve from back plates like those of the Stegosaurus, and later grow very large due to sexual selection but also lighter to support their size, and then they would be repurposed for gliding. Still, it would be better if they could at least make the wings stand upright so they could move better. Or maybe the wings/plates are of a rigid but thin and flexible material, so they can bend out of shape when the creature needs to get between or under obstacles and go back to their original shape without breaking.

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Well this already happens many times in nature, although the “wings” can both flap and articulate (because nature doesn’t like stupid designs), they can and do fly without flapping at all. So your question boils down to “can these animals still work if the wings are fixed?)

The Amazon flying spider turns its flat body into a wing and glides from tree to tree. It’s gravity propelled flight without flapping. If it were permanently shaped this way, it’s aerodynamic performance would not change. However nature would need to give it new legs, and I think that can be done.

The majority of web-weaving spiders fly with no wings at all. Charles Darwin was amazed to find spiders landing on his ship hundreds of miles from shore. It’s called “ballooning,” where they simply pay out a length of silk which is caught by the wind, combined with electrostatic forces, that pulls them along.

Spider ballooning

In this case your question is reduced to, “Can nature evolve spiders with fixed wings?” I think that is a viable evolutionary development.

Some answers feel the outstretched wings will become a nuisance on the ground. Nature disagrees. One extremely successful animal leaves its flight gear outstretched all the time, and can fly without flapping:

Dragonfly

So really your asking if we glued a dragonfly’s wings down (but still allowed them to pitch), would it still glide?

The answer is yes, but it’s Going to have to learn to forage for food because it won’t be the apex predator we know. It can climb a tree or cliff and glide for days on thermal currents in gravity-propelled flight.

But if you prefer propelled flight, nature has also done this. The squid uses hydraulic pressure to accumulate momentum in the water, then launches itself into the air, flattening it’s tentacles, arms, and fins into rigid wings for gliding flight.

Flying squid

In this case your question becomes, “can nature make a squid with rigid fins?” Again, I think this is viable, but not a smart design. And if nature could evolve a very lightweight squid, it could likely glide almost indefinitely on thermal currents.

Next you asked for how large such a creature can be. Honestly this is a function of how lightweight you can make your skeletal and muscular tissues. A thin exoskeleton like the dragonfly can be scaled up fairly well, but the animal will become quite delicate as it gets larger. Hollow bones of birds may also allow pteranodon-sized gliding animals. Nearly any creature which can launch itself high enough off the surface can take advantage of thermal currents and regular winds.

For the largest possible rigid wing creature you probably want to evolve a living dirigible with fixed wings and anguilliform locomotion. In this case, your question becomes, “Can a ballon-like creature create hydrogen gas?” I think this is also reasonable. If this creature exhaled H into its own balloon, and could articulate its body, it becomes a living airship.

Sorry this isn’t as cool as jet-propelled pteranodons, but I took the word “viable” seriously. I also took the liberty of making the wings articulate so it can steer (like ailerons) , but not flap. Because we don’t even have strictly “fixed wing” aircraft.

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This sounds like the kind of thing that you might find in the atmosphere of a gas giant. In this case, you don't need to worry about land (because there isn't any).

Lift could be provided by the swirling thermals that we can observe ourselves in our own gas giants.

If you want your creature to lay eggs then they could be like gas-filled 'mermaids purses' (shark eggs) which would be buoyant at a certain depth in the atmosphere. When the chick hatches, it enters a shallow dive to gain speed before being lifted up by the aforementioned thermals.

As mentioned above, even fixed-wing creatures would need moving control surfaces of some kind.

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  • $\begingroup$ i think gas giants tend to have lots of liquid, more than gas $\endgroup$ – Andrey Mar 15 at 14:04
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Your bird's wing could have a cross-section similar to that of a conventional plane, or a ground effect vehicle, and have the ability to flex its wing in a manner that it could switch between the two. It would spend most of its life airborne, only landing to produce offspring. It would primarily glide, using a conventional wings cross-section to maximize its gliding ability, and that of a ground effect vehicle when close to the ground.

A flexible tail would be used to steer the bird through the air.

Its beak would be long and pointed, similar to a spear. This will allow it to spear its next meal whilst gliding close to the ground or when up high. Having it spear its target from head-on would enable it to keep more momentum than twisting through the air.

The eggs would be laid in small recesses created by the bird's beak on steep cliffs, they could throw themselves off to become airborne again.

Such a bird would be easy prey when grounded, as such, they would benefit from living in a mountainous country so that if they ever did become grounded, they could launch themselves from the nearest cliff face.

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