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There are many questions and answers regarding giants and how to scale up human anatomy on this website. (larger bones, better lungs and heart, lower body temperature, tighter skin at the feet, etc...)

What does it look like the other way around?

Let's say specifically that you could apply certain evolutionary pressures constantly for a period of several million years on a group of human beings, selecting for a smaller size while keeping a human level of intelligence, and then later on for flight.

What does the anatomy of such a species look like?

For example, Would they have bat wings? Or feathered wings? Could they keep their thumb, index and middle fingers short, with the ring and pinkie fingers elongating to form the wing so they could have functional hands still? How does the square cube law affect their anatomy? Do they have a very high metabolism and need to eat very often like shrews? What about their intelligence? I have read somewhere that birds have evolved more tightly packed neurons than humans to maximize intelligence with the smaller, lighter brain required for flight. Is that possible with human beings as well?

A ton of questions here but these are just examples of what I am looking for. To summarize, my question here is: what does the anatomy of a 10 inches tall or so, flying, human-descended species look like?

If the format of my question is wrong, please tell me and I will fix it as this is my second time asking a question on this website and I am unsure of how to do this properly. Thanks.

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Let's assume a size of 18 inches and compare to a 6-foot human. This is one-quarter the size, meaning one-64th the weight, all else being equal, for a mass of 2.5 to 3 lbs, or roughly twice the weight of a raven. It is also very similar to the weight of a flying fox, so we can assume similar wing sizes.

The brains of such a human bat would of course be a lot smaller than that of a full-grown human, but brain size is not the only factor in intelligence (the amount of convolutions seem to play a greater factor). Their heads could also be relatively oversized, compared to humans. I included a raven in the weight comparison above because ravens are very smart, capable of astonishing cognitive feats. Still, we should perhaps not expect full human intelligence in such a small winged humanoid, especially since survival of the species might depend more on other factors. If they retain speech, it will be more high-pitched than for normal humans.

Bats have retained a thumb that can be used to grasp stuff, and it is likely that 'fairies' would be similar, possibly even retaining one more finger for better gripping. Their feet are likely to evolve to resemble clawed hands, and a fairy could likely stand on one foot while manipulating objects with the other and the wing fingers.

Their lungs would have to be far larger, relatively, to allow for the greater energy expenditure of flight. In turn, their bellies are likely smaller, since flying with a large, full belly is difficult, and hence they would need to eat more times a day, basically snacking all the time. So we should expect large chests (with flat chests for females) and narrow waist. Legs may be folded underneath the body in flight, as for birds, or may have wing skin between thighs for greater lift.

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    $\begingroup$ One thing that I wonder is: are raven brains similar enough to human brains? I'm not a biologist, but, making an analogy to computer chips, there are some algorithms that work well for smaller inputs, yet don't scale as well as more complicated constructions. In other words, if you simply downscale human brain, it may be significantly less efficient for its size than raven's. $\endgroup$ – Alice Oct 14 '19 at 10:55
  • $\begingroup$ Humans brains aren't all that similar to bird brains, but size seems not to be a significant factor in intelligence (whales have brains 10 times bigger than human brains). If the complexity and structure of the human brain is retained in the 'fairies', they might be as intelligent as us, though size does seem to play some role. scientificamerican.com/article/does-brain-size-matter1/# $\endgroup$ – Klaus Æ. Mogensen Oct 14 '19 at 11:06
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    $\begingroup$ Oh, absolutely, size is not the primary factor. But we don't really understand why human brain can do what it can. Will the same structure really work when we take out 63 neurons out of every 64? Or will they have to develop significantly different brain structure to compensate for the loss of raw power? And if the second is the case, it can go either way: discarding all the unnecessary for survival parts and dropping below a raven, or restructuring for even more complexity and retaining at least primate-level intelligence. (Thanks for the interesting read, btw) $\endgroup$ – Alice Oct 14 '19 at 11:49
  • $\begingroup$ As I write, perhaps intelligence will not be the most important survival feature of a small, winged humanoid. However, if the asker wants intelligent 'fairies', it is arguably possible even at small size. $\endgroup$ – Klaus Æ. Mogensen Oct 14 '19 at 13:15
  • $\begingroup$ Thank you! With regards to brain size, I wonder how reproduction would be affected, I've just looked it up and female crows carry eggs for 18 days before laying them, what does a bat-fairy-human's gestation look like? I'm picturing the birth happening very early in the fetus development like for marsupials except there is no pouch for the baby to stay in. $\endgroup$ – Tennessy Oct 14 '19 at 14:21
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Let's not use the limbs as wings, aesthetically it just doesn't look like a "fairy".

Let me suggest the flying lizards work around instead.

enter image description here

As you can see they've extended their ribs & used them instead of their forelimbs like the bat, here's a look at the skeleton.

enter image description here

  1. Imagine your flying lizard has hair (or is a flying mouse) & evolves a long fringe of stiff bristles along the edge of its 'wings' giving more surface area (even if it does have gaps).

  2. Now that the hairs evolve to be flatter & begin to merge into a single sheet of thin stiff keratin (the stuff your hair & fingernails are made of).

  3. Over time the skin flaps recede & the keratin wing surface expands (less flesh to provide blood flow to, has to be an evolutionary advantage in that).

  4. The intercostal muscles associated with the ribs in question develop more strength & shift anchor points as needed & the position of the extensible ribs they're anchored to slowly shift further back & around to a more traditional position for fairy wings.

  5. Once they start gliding you've all the same evolutionary pressures as birds & they evolve many of the same evolutionary advantages for flight birds have (higer neuron count for smaller brains with the same intelligence & denser honeycombed bones etc).

Now imagine it was a miniature human instead of a lizard & there's your fairy.

If we use only 2 pairs of ribs we can give it the double wings of most insects & a really thin colourless keratin wing can look just like an insect wing.

They'll appear to have two less ribs than a normal human of course.

That covers the basic morphology & how they got there.

Instead of 'bat wings', they'll have scaled up insect wings with Keratin serving in place of an insects Chitin & the morphology of their arms & hands can be entirely unaffected.

Their wings might perhaps look much like a dragonfly's but more translucent (less transparent).

enter image description here

But where on the body will these wings be & which two ribs should we use?

As you can see from this picture the horizontal centre of balance of the human body (with swept back arms to make it slightly more aerodynamic) is basically just above the hips.

enter image description here

So (contrary to normal depictions of wings sprouting from shoulder blades) the small of the back is it seems the best position (yes, I didn't expect that either) so we use the 11th & 12th rib for our fairy wings.

enter image description here

They're not quite far enough down for perfect balance in horizontal flight but that's OK, leaves some wriggle room for a larger head & brain (to help make reasonable intelligence less implausible).

The 11th & 12th vertebra probably fuse for strength & evolve into a shape better suited for anchor points for the wing muscles, imagine a ridge or 'keel' rising from the back of the fused vertebrae & then extending into a plate that looks something like four symmetrical scapula fused together with the socket joints facing out (two on each side) resting on top of the ridge rising up (as the fairy lies on its chest) from the fused vertebrae.

So if you cut the wings off you'll be left with what looks like four small stumpy shoulders (with their arms amputated just before the bicep) in the small of it's back.

Which (sort of) approximates this (at 0:45 on the tape) on a dragonfly.

The ribs have shortened considerably of course & developed a ball to sit in the socket joint formed on the remolded vertebra & each need six muscles for a full range of motion (rotate one way, rotate the other, sweep forward sweep back, flap down, flap up).

There's only three intercostal muscles per rib but I'm confident there are plenty other small muscles that can be expanded & repurposed, the arrector pili muscles of hair follicles for a start (the ones a porcupine uses to rattle its quills) so I don't foresee a problem there.

Or we can fuse the 11th & 12th ribs for the first pair of wings & the 9th & 10th for the second pair if we have to (letting us repurpose six intercostal muscles per wing).

Of course the smaller end of your scale makes it easier for them to fly & there's obviously no reason a humanoid can't be small.

Take the pygmy marmoset for example, that's even smaller than you ask for.

enter image description here

The square cube law really isn't an issue for body design (morphology) at this size (as evidenced by the pygmy marmoset in comparison to other primates) & won't plausibly have any visible effect on it.

However, smaller animals have a greater surface area to volume ratio, so more heat is lost, which is probably the main reason smaller mammals have higher metabolic rates (to replace the lost heat), & higher metabolic rates lead to shorter lifespans & mean they need to eat more frequently.

Of course as you get really small you begin to come up against issues of plausible intelligence.

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Now some math

The average North American male is 175.5 centimeters tall & 10 inches is 25.4 centimeters.

So 6.9094488189 of your 10 inch munchkins are as tall (175.5/25.4).

So 329.860423747 of them are the same mass (6.9094488189 cubed).

A human brain is around 3 pounds which is 1360.78 grams.

So a 10 inch humans brain is 4.12532059634 grams (1360.78/329.860423747).

& are half the height of a 20 inch one so 8 of them fit in one (2 cubed).

So a 20 inch humans brain is 33.0025647707 grams (4.12532059634*8).

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The brain of the domesticated cat weighs 25–30 g & is about 0.9 percent of their body mass compared to about 2 percent in an average human and about 1.2 percent in an average dog.

Cats often perform as well as a two or three year old in cognitive tests & our 20 inch fairy has a larger brain.. with a higher neuron density per gram.

A raven's brain may weigh just over half an ounce, but it accounts for 1.3 percent of the bird's body mass, half an ounce is 14.1748 grams.

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Ounce for ounce birds have significantly more neurons in their brains than mammals or primates

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Birds pack more cells into their brains than mammals

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While Crows can perform as well as 7- to 10-year-olds in some cognitive tests.

So (with a large amount of hand waving) we might plausibly & conservatively assign a 20 inch tall fairy with an equivalent intelligence to a four or five year old human, the 4 gram brain of the 10 inch version isn't something I'd want to hazard a guess about.

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So what does it "look like"

It can look just like a normal human, but smaller, with insect wings, you probably won't even notice the missing ribs unless you examine it very closely.

Work still in progress Dang! forgot about this, I really should tidy it up & finish it off, no time now, tomorrow or later, just intelligence left to comment on I think.

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  • $\begingroup$ Thank you very much for your answer! Using keratin spikes as a base for evolving wings is a really cool idea, I wonder what the wings in question would look like, I am picturing something similar to human nails in appearance, which makes me wonder how soft or solid these wings would be. $\endgroup$ – Tennessy Oct 16 '19 at 18:52
  • $\begingroup$ The center of lift should be behind the center of mass, to maximize stability. This means that the humanoid would have erratic flight and consume a lot of energy to fly unless they were "overweight" or something. Paraphrased from one of the linked answers: "If the center of mass is located aft of the aerodynamic center, the [body] becomes inherently unstable in pitch & will require constant [adjustments] to keep it level." $\endgroup$ – jpaugh Oct 18 '19 at 18:33
  • $\begingroup$ (Stability basically means that when drag forces change, the new drag automatically pushes the flying body into the correct orientation (angle of attack) to compensate for that new drag force, without the flyer having to expend extra energy to compensate manually.) $\endgroup$ – jpaugh Oct 18 '19 at 18:39
  • $\begingroup$ @jpaugh : "The center of lift should be behind the center of mass" : So the lower down the back & the heavier the head the better, perfect, we use the lowest two ribs & scale the skull up as needed :) $\endgroup$ – Pelinore Oct 18 '19 at 18:40
  • $\begingroup$ Well... so long as I don't have to built it, lol! $\endgroup$ – jpaugh Oct 18 '19 at 19:08
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If they evolved from humans, they would look more like harpies, than fairy:

  1. Would they have bat wings? Or feathered wings? - Batwings. Feathers is dinosauros's feature.
  2. Could they keep their thumb, index and middle fingers short,... - they could.
  3. How does the square cube law affect their anatomy? - they need larger chest for larger lungs (lungs would be about half of there body volume), there bones and legs would be very thin, they would be very skiny - both for flight and due to relative increase in musle strenght
  4. Do they have a very high metabolism and need to eat very often like shrews? - Yes. Actually eating and pooping would take most of their daytime. They may have some mechanism that slows down metabolism when they are not flying. But that would mean thay need time to "warmup" before flying and "cooldown" after: drink some shugar sirop, get hot, fly, exaust energy (about half an hour at most), land, cool down (peeing and pooping in process).
  5. What about their intelligence? - they will not be fully intelligent. If they are about 0.5-0.7m (20-inch) they can have speech and communicate with normal humans, but they would be smart as small children (not childish behaviour! But, say, inability to get concept of a number, bad short memmory, etc). For smaller sizes, they would be like dogs or cats.
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  • $\begingroup$ Thank you! this makes me wonder though, both hair on mammals and feathers on dinosaurs evolved from scales, this really should be a different question but I wonder if mammals could evolve feathers or something equivalent as well. would something like pengolin scales or porcupine quills work for flight I wonder? $\endgroup$ – Tennessy Oct 14 '19 at 14:16
  • $\begingroup$ @Tennessy, mammals might develop something feathers-like. Actualy hedgehog and porcupine needle has some commonality with early dinosaurs feathers (quills). So if hedgehog would increase size to several hundreds kilograms, develop highly agrresive to each other behavior, and start using it's needle for demonstrational behavior, then survive several extinction events, get small again and learn to fly - they might develop smth. like feathers. But it is highly unlikely. $\endgroup$ – ksbes Oct 15 '19 at 6:59
  • $\begingroup$ "3. - they need larger chest for larger lungs" Uhm, maybe think about that one again, you seem to have misunderstood something about the square cube law & why really small animals (insects) don't need lungs. $\endgroup$ – Pelinore Oct 16 '19 at 17:26
  • $\begingroup$ @Pelinore, insects does not need lungs not because of there size, but becase they are insects - they have no closed blood system and they are "coldblooded". As for lungs - they have somewhat fractal "design" (alveoles fills all the volume they can) and "square cube law" is (almost) linear for them: lungs area is proportional to volume. But that means that less volume is left for air ("wall" width is the same!) and smaller animals need to breath faster (easy done due to relative increase of musle strength) and have high lung volume ratio. And flying increase lungs volume demands even more. $\endgroup$ – ksbes Oct 17 '19 at 7:56
  • $\begingroup$ Simple fact, "the square cube law" & it's implications for gas exchange & respiration means an animal needs larger lungs as it gets bigger, not (as you've said) as it gets smaller, if you're asserting otherwise I'm going to need to see some references if you want to persuade me ;) the smaller the animal the less the % of it's body it needs to supply with oxygen with an active internal system. $\endgroup$ – Pelinore Oct 18 '19 at 15:09

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