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I'm working on a humanoid vertebrate species that I'd like to be capable of true, flapping flight, but still have arms and legs completely separate from the wings. I also wish to avoid a "six-limbed vertebrate" scenario, since it has a number of fairly closely related species that only have four limbs. So, I've locked onto the idea of rib-supported skin membranes like the ones that lizards of the genus Draco use to glide.

I know one question here already addressed the possibility of a similar set-up, but I'm working on the assumption of possibility and wanting to know what the changes required would look like. In particular, I want to know where the flight muscles for such wings would be located and whether, once flapping flight was evolved, a horizontal stabilizer like the Draco lizards' gular flag would still be necessary, but help visualizing any other prerequisite changes would also be appreciated.

If it matters, the species in question is a sort of warm-blooded pseudo-mammal, and the range of motion for its spine is more mammalian than reptilian.

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    $\begingroup$ Wouldn't it be easier to make their wings bat-like? Bats can use their forelimbs for grabbing and whatnot. Maybe they should have 3 elbow joints on their arms so that they don't fold as awkwardly and they can actually use them as arms. $\endgroup$ – Real Subtle Mar 2 '18 at 12:58
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Lizards breathe using their intercostal muscles - the muscles between the ribs. This (in essence) moves the ribs back and forth, changing the body volume and producing inspiration and expiration.

Linked is a movie showing slow motion fluoroscopy of a lizard breathing.

http://www.xromm.org/projects/varanus-breathing

fluoroscopic movie of lizard  breathing

It does not take much imagination to turn this motion into flapping. One could unmoor the muscles from the sternum anteriorly and use the two sets of intercostals to power the rib wings up and back. Or one might have an elastic connection to the sternum - the lizards power the wings up (dorsally) with the intercostals against this elastic resistance, and then get a powerful downstroke when the muscles relax and the elastic connection to the sternum pulls them back ventrally. Vice versa would work too - elastically drawn up then powered down.

The linked site lays out the various motions of the ribs during breathing. They actually rotate some as well. It would be a good thing for a flapping flyer to rotate the wings on the upstroke to offer less resistance then rotate back for the downstroke to maximize resistance. I take away from the site that the intercostals are responsible for rotation as well.

Lizards already have their intercostals doing double duty - both breathing and locomotion. I think this use would actually augment breathing while flapping - which seems to me a good idea giving the energy / oxygen demands of flapping flight.

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    $\begingroup$ While this is definitely helpful, one thing that troubles me about this setup is that while lizards do use intercostals for both breathing and rapid locomotion, they do so in a manner that prohibits breathing while running, because the muscles are used in different ways for either activity. You ever see how most lizards will sprint for a few seconds, then pause, then sprint again? It's stopping to breathe. However, seeing as the flapping would be derived from the breathing motion, it may be reasonable to assume it could be done while breathing. I'll probably use this, thanks. $\endgroup$ – Cowrie Mar 9 '18 at 20:46
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First, that's not flight, that's gliding.

Without some mechanism to generate lift, i.e. by flapping, flight by the use of rib extensions is basically impossible. They can be extended, but in Draco the wings are fixed.

However, you can possibly develop some form of "false" ribs, by employing a novel joint above the ribcage, perhaps attached by cartilage and ligaments to lengthened neural spines. This would allow you to anchor "pull-up" muscles to the longer neural spines. To get around needing deep breastbones for the chest to anchor muscles to pull the wings down (take a look at a pigeon skeleton, their keels are huge relatively speaking), they could have anatomy that automatically forces the rib-wing, once extended to want to fold forwards (have the tips meet in front of the creature) so that the major force would be from the spinal process muscles pulling the wings back and open and anatomy pulling them closed. Sort of like a clap, where the effort is in you drawing your hands back, not in bringing them together.

Also, unless the creature is very short-legged, I don't think you're going to have enough body length, even if you took the rib wings from behind the shoulders to the hip to give you an adequate wing to mass ratio for flight or gliding, a human torso is too short to fit a wing that can support the heavy head and legs, the legs make the whole thing too back-heavy.

If you are okay with "rib_like_" structures, you might be better off attaching the extensions to be above the spine themselves. They can still use the neural spines to anchor muscle, but the down-stroke force would have to be generated by muscles pulling the ribs downwards. And you end up with a flier kind of like this:

DnD Gold Dragon, art by Todd Lockwood

Versus a regular six-limbed sort of flap (it's big so I didn't past it in). https://imgur.com/4Vje41e

But again, note how much length is given up to be a continuous wing to let rib-like projections work. I think this version is much more anatomically feasible than the first option, fitting the rib sails directly above the ribs to the sides of the spinal process. Though you might end up with a humanoid creature that is built very deep and broad of chest, with a buffalo hump. They would be humanoid but not humanlike in shape.

The "buffalo hump" need not stand out. Buffalo have their hump because their neural spines are tall to support the weight of their heads. You can see the same with rhinos, which do not look particularly humpy, but have tall neural spines as well.

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    $\begingroup$ First of all, I'm well-aware that Draco is incapable of flight and merely a glider. I'm talking something with a similar structure that is capable of flapping. It is a short-limbed, longer-bodied species, regarding the issue of lift. However, you do raise some excellent points. Rib-like structures are fine, and provide an advantage in this case, namely that it allows longer wings extending down the tail as seen in your animation. (The species was always going to have a tail, this just adds functionality.) The broad chest and buffalo hump to anchor muscles also fit the design well. $\endgroup$ – Cowrie Mar 1 '18 at 22:13
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    $\begingroup$ You could also have them dwell in a much more oxygen rich environment so that muscles are able to be more powerful and efficient. These aliens could also live on a lower gravity planet than earth with a thicker atmosphere. $\endgroup$ – TCAT117 Mar 2 '18 at 4:41

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