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I'm writing about an alien species of lean-bodied, long-limbed bipeds who have vestigial gliding membranes stretching between their elbows and hips (female) or knees (male); see: https://christopherlbennett.wordpress.com/aliens-of-the-arachne-troubleshooter-universe/#Biauru In writing a swimming scene, I realized I have no idea how the membranes would affect their swimming ability. I never learned to swim, so I don't know much about it.

I assume the membranes would create significant drag in the water, since that's what gliding membranes are basically for. So how to compensate for that drag and allow swimming as fast as possible? Is there a stroke that would let the membranes push the water back and improve their swimming speed/force? Or is the goal to minimize the drag as much as possible, say, by keeping the membranes parallel to the direction of motion? Should they stretch their arms forward to keep the membranes taut, or keep them pressed back against their sides and paddle with their feet? (Their legs are very powerful. They normally move by hopping like an indrid lemur instead of walking, so their leg muscles are probably adapted to move in sync instead of opposition, which might result in a sort of sinuous, mermaid-like stroke.)

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  • $\begingroup$ Consider how rays (such as a manta ray) swim. $\endgroup$
    – Jon Custer
    Apr 7 at 17:31
  • $\begingroup$ Not helpful without specifics. How does that translate to the movement of an approximately humanoid body? I'm not sure it would even work, since a manta's fins can flex along their whole length, while in this case we're talking about non-muscular membranes stretching back from rigid upper arms like a human's. $\endgroup$ Apr 7 at 18:06
  • $\begingroup$ Questions using superlatives (e.g. "best") are a hard sell on this Stack because what makes something "best" is always story-based (a specific reason for closing questions). To select a "best" you need to explain the conditions for the best answer, such as the specific circumstances (are we in a pool? the ocean? a strong current? rainy weather?). Consider humans: there isn't a "best" swimming stroke for humans. There are commonly-known strokes (doggy paddle, front crawl), strokes for competition (breast stroke), low-impact strokes (back stroke), etc. (*Continued*) $\endgroup$
    – JBH
    Apr 7 at 19:35
  • $\begingroup$ ... So, I'd recommend you edit your post to remove the word "best" (and any similar superlative). It might also be helpful to ask us if there are real world examples of creatures you can look at to help you describe your species' swimming characteristics (and @JonCuster's idea isn't a bad one, it would make a good answer if, as you asked, it was fleshed out with some explanation of why it works). Ultimately, it will be you who creates your species, we're just trying to help you overcome a problem. $\endgroup$
    – JBH
    Apr 7 at 19:37
  • $\begingroup$ I don't think other creatures would help, since these are basically lanky humanoids in shape aside from the gliding membranes. Basically, it's as if a human were trying to swim with the underarm webbing Spider-Man deployed in HOMECOMING (gothamsn.com/…). But since I don't swim, I can't envision how that would affect one's drag in the water, or how to compensate for it. I'm hoping an experienced swimmer can more readily imagine such a thing. $\endgroup$ Apr 7 at 21:28

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Assumptions: Since it is not specified otherwise, I am assuming that these creatures:

  1. Have a similar density to humans
  2. Have a similar range of neck motion to humans
  3. Need to take breaths roughly as often as humans
  4. Are using these swimming techniques unaided, ie without masks, snorkels, flippers or other accessories.

Given those assumptions, there's good news and bad news. The good news - not directly related to the question you have asked - is that by trapping air under their underarm membranes they will be able to stay afloat more easily than humans in calm water. They have no need to tread water, they can just trap air and float more easily than humans.

The bad news is that with the massive drag from those same underarm membranes when outspread, fast swimming will rely on keeping the arms and associated membranes tight against the body as you suspected - the drag from the membrane would almost completely counteract any force exerted by the arm and would be exhausting. (I'm basing this on experience swimming with normal clothing on. If anyone has a squirrel suit that they are willing to get wet, please let me know how it goes, but make sure you have assistance on hand in case the experiment goes as badly as I suspect it would.) This means that their swimming speeds are likely to be much slower than humans - this paper suggests that on average humans swimming freestyle exert 66-71% of their force against the water from their arms.

With that out of the way and after looking at a quick video clip of how lemurs leap around, I suggest that a powerful frog kick would be the fastest stroke for the creatures to use - basically breast stroke but with the arms tight alongside the body instead of out front. Without knowing the exact buoyancy of different parts of the body I cannot be sure how easy it would be for the creature to get their head out of the water to breath on each stroke - if the head is too heavy then they may need to trap a bit of air in their underarm membranes. When/if technological aids are invented, a snorkel would be a huge advantage so they can just kick along without needing to hold their head up and tilted back. Water resistance scales with the square of speed, though, so these creatures are probably looking at being roughly half as fast as human swimmers, depending on exactly how powerful and efficient their legs are at frog kicking.

One further bit of bad news though after reading the (nicely written) linked description of these creatures - the heat-dissipating dorsal crest is going to work even more efficiently in water than in air, which means that these creatures are going to get cold quickly in the water. (The bit about involuntary blushing suggests that they cannot consciously decide to stop blood flow through the dorsal crest.) As a result, in addition to being comparatively slow swimmers these creatures will also have low endurance in all but the hottest of waters.

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  • $\begingroup$ Thanks, that's very helpful. A lot of good details. $\endgroup$ Apr 8 at 0:31

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