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(maybe this should be part of the "anatomically correct" series. If so, feel free to edit accordingly, as I don't know how to add it to that list)

Could a roughly human sized, human weight, bipedal, anthropomorphized flying squirrel achieve significant gliding capabilities?

Details, limitations, clarifications, etc:

  1. With the understanding that lower weight makes flight/gliding easier, they can be slightly shorter than humans, on average, and more slender as well, but I want to stay away from avian hollow bone structures and similar non-mammalian adaptations. More slender bones, like bats, are acceptable. Because I suspect someone will ask for specific numbers, let's say the average height for an adult male is 5' (1.524 meters), and an adult female averages 4'8" (1.32 Meters). The lower end of "normal" BMI for a 5' human male is just under 100 pounds (45.3 kg), so lets make that the upper end of a normal healthy weight for a healthy specimen of this species, bordering on overweight.

  2. While the overall body plan should be humanoid, adjustments can be made to limb length, if it's aerodynamically required to achieve gliding capability. But the (hind) legs should not be any longer, proportional to the rest of the body, than an actual human's legs, so they should only be shortened, if needed, not lengthened. And the arms (forelegs) should not be any shorter, proportional to the rest of the body, than an actual human's arms, so they should only be lengthened, if needed, not shortened. Also, the arms can be as long as, but not longer than, the legs. Summary: legs can be shortened, arms can be lengthened, but only if needed for aerodynamic purposes, otherwise they should stay as close to human proportions as possible.

  3. Gliding should be accomplished by with the aid of a patagium, a parachute-like membrane that stretches from wrist to ankle. No third set of appendages/wings/etc., and no massive re-imagining of the two existing sets of limbs outside of what an actual flying squirrel has (neither set can be converted to wings, or any similarly drastic change). The patagium should be mostly collapsible/fold-able/retractable/etc. to achieve a "mostly human shape" when it's not in use, just as an actual flying squirrel can achieve a "mostly squirrel shape".

  4. Tail should not be longer than about 3' (1 meter).

  5. Should be capable of an average glide ration of 2 (that's 2 feet of horizontal distance traveled for each 1 foot of elevation lost)

  6. The setting is Earth. No exotic gravity, atmosphere, etc.

  7. Disregard evolutionary improbabilities. Assume the evolution is/was possible, however likely or unlikely it might be. In other words, it is outside the scope of this question to declare implausibility due to evolutionary restrictions.

Are there any known aspects of real-world biology that prohibit such a creature from existing? If so, what are they? If not, what is the most likely form this creature would take within these parameters?

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    $\begingroup$ Wingsuit flying is an actual thing... Not fantasy. "With training, wingsuit pilots can achieve sustained glide ratio of 2.5:1 or more." (Wikipedia) $\endgroup$ – AlexP Sep 20 at 15:12
  • $\begingroup$ @AlexP Yes, and part of my inspiration for this question. However, wingsuits are decidedly not biologically part of the animal/creature using them, and I can't, in good confidence, make the leap from synthetic certainty to biological plausibility. $\endgroup$ – Dalila Sep 20 at 15:14
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    $\begingroup$ You don't see much footage of wingsuit flyers landing in wingsuit mode, which is an important consideration. They generally use some kind of parachute for that bit. $\endgroup$ – Starfish Prime Sep 20 at 15:15
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    $\begingroup$ @notovny Yeah, I found the same number. And I also found 3.1 for Northern Flying Squirrels. I figured "2" was reasonable, as a nice round number, and being close to the lower end of actual nature's numbers (that I could find) to give it the best chance of being plausible. $\endgroup$ – Dalila Sep 20 at 15:28
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    $\begingroup$ @StarfishPrime I hadn't considered the impact of landing, but that's just the kind of consideration that prompted me to ask the question. To find out what aspects I don't know about the topic, in case any of them prohibit plausibility $\endgroup$ – Dalila Sep 20 at 15:34
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Could a roughly human sized, human weight, bipedal, anthropomorphized flying squirrel achieve significant gliding capabilities?

Sure. As mentioned in the comments, wingsuit gliding is a Thing--the tricky bit is landing.

Wingsuits, which are directly analogous to biological patagia, can achieve decent glide ratios--but they also require rather high speeds to function effectively, which makes them rather dangerous. To make them plausible as a biological adaptation, we need to make it possible to slow down without an extra parachute, and land without significant risk of death.

Key to that are reducing weight and increasing lifting area and lift per area, and improving control to avoid injurious crashes. You've already got the reduced weight. So, let's look at increasing lifting efficiency and control.

Shortening the legs should not be strictly necessary--and, after all, shortening the legs does result in less total area across which to stretch a patagium. But, the area provided by longer legs isn't actually the most useful area--unless the legs are held straight out to the side in flight (typical for a flying squirrel, but less comfortable for a humanoid body plan), they mostly contribute depth the gliding surface, not width. And wide, shallow wings are better for slow gliders than short, deep wings. So, shortening the legs will help--it'll help get down to your weight goal without significantly harming gliding lift capacity. Again, we already know that wingsuits basically work, so the precise amount of change isn't really critical--go as far as you're willing to, and every bit will help.

Lengthening the arms can only help. There already exist apes that have arms that hang all the way down to the ground from their shoulders, or farther, so clearly that's within the range of what a humanoid body plan can accommodate. If you want to keep the proportions closed to "actual human", though, there is another option, which also help with keeping the lift membrane folded out of the way when not in use: extend the pinky. You still have 3 fingers and a thumb left over for normal human-y uses, and when not gliding the lengthened pinky can be folded down against the forearm, along with the collapsed gliding membrane. That gives you a 50% increase in wing width before you make any changes to the proportional arm length. The addition of three more joints to the leading edge of the lift surface also gives you considerably more control at higher speeds, reducing the inherent danger of human-scale gliding and making it easier to achieve safe, running or perching landings at near-wingsuit speeds. Running landings, of course, would be easier if you preserve the longer legs. Perching landings would require a target surface that's at a suitable height for the glider to bleed off speed by climbing towards it and then grabbing on.

So, my expectation for the form of such a creature would be:

  1. Slightly, but not extremely, shortened legs, to help reduce weight without significantly negatively impacting running ability.
  2. Slightly, but not extremely, lengthened arms--maybe going down to about knee length, rather than hip length. That increases wing width a bit without increasing the weight contributed by arm bones too much.
  3. Additional wing width provided by a modified pinky that folds along the forearm down to the elbow.

The creature would fly mostly like a wingsuit glider, but at slightly reduced average speeds due to the increased wing width and with better control provided by the added finger joints along the leading edge of the wing. They could, of course, achieve higher tops speeds if they wanted just by folding the pinky back during flight to reduce the drag profile. Ideal landings would be achieved by

  1. Pitching the body upwards and and climbing to bleed speed before grabbing on to a surface at a convenient height. This is good for landing on trees, cliff faces, rising hillsides, etc.
  2. Dropping the body to near vertical and pointing the pinkies forward to produce a large drag force with minimal lift, quickly bleeding speed and allowing for running contact with the ground.

Somewhat less elegant landings would be achieved by spreading the gliding surface as wide as possible to reduce speed near the ground, then performing a "tuck and roll".

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    $\begingroup$ Wing suits have been proposed as a resolution of the Fermi Paradox. xkcd.com/962 $\endgroup$ – puppetsock Sep 20 at 18:24
  • $\begingroup$ A wingsuit is not going to work. When gliding, a wingsuited person is still falling vertically at about 40 mph (and moving forward by 100 mph). That's equivalent to a fall from 53 ft, and is likely to main or kill you on impact. Furthermore, the wingsuit won't even be at flying speed until you have fallen at least 334 ft. $\endgroup$ – Dan Hanson Sep 21 at 18:52
  • $\begingroup$ @DanHanson The question doesn't specify a minimum launching point height, so needing to fall at least 334ft shouldn't be a problem. Bleeding vertical velocity before landing is indeed a problem, but that's what increasing the lifting area by over 50% and aiming for perch landings are for. $\endgroup$ – Logan R. Kearsley Sep 21 at 20:31
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Assuming these future humans have to land on the ground and/or grab onto trees or something (like squirrels), the answer is that this is probably not possible.

Wingsuits are the closest thing to what you are talking about, and the slowest vertical speed a wingsuiter can manage is about 40 mph. Achieving a 2-1 glide ratio can be done (even a person without a wingsuit can glide somewhat) if you are going fast enough. The problem is stopping.

Here are some numbers: A wingsuit glides at about 100 mph. It takes 334 ft dropping straight down to reach that speed. At best glide, the wingsuited skydiver hits the ground with a horizontal speed of 100 mph and a vertical speed of 40 mph - about the speed reached when falling 53 ft. This would very likely be fatal. And wingsuit flying has a frighteningly high fatality rate even though each landing is supposed to be by parachute. Accidental contacts with the ground by a wingsuited flyer are almost always fatal.

There is a guy called the "Jetman" who has built a suit that contains a 12ft wide rigid wing. He has four jet engines attached, and it can fly at about 180 mph. But even with the long wing he has to use a parachute to land.

If you want your humans to glide and land at a speed that doesn't break bones, you aren't going to get much smaller than a Rogallo wing hang glider type apparatus.

If your future humans use their gliding from very tall cliffs or airplanes, and they only use their gliding to get to a point where they can dive into water or something, then perhaps the wingsuit-like skin flaps would work.

But if you are envisioning them jumping from tree to tree or being able to glide around and land on the ground, I don't believe any combination of your requirements will work. At a minimum I think you would need a Rogallo-wing type of structure at least 12ft in span, or a traditional wing of 18ft or more to get acceptable descent rates for landing.

Search for 'foot launched gliders', 'smallest hang glider', and 'smallest glider' and you'll see many examples of very small flying machines. None of them come anywhere close to your requirements, unfortunately.

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