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Comment by rauljara | Hacker News Reader

Would love to see a pterosaur / bat version of this drone. Birds use one set of muscles to jump in the air and another to flap their wings, limiting how big they can get. That’s because, if you make your wing muscles bigger, then you need bigger leg muscles to support them, then you need bigger wing muscles to support your legs, etc. pterosaurs and bats have tiny little legs and use their “arm” (wing) muscles to do the initial jump into the air. It’s just one set of muscles that are used for both functions, which is why pterosaurs were able to get so big. It does beg the question, tho, why we haven’t seen any truly giant bats.

This pbs aeons video has a great explanation: https://youtu.be/scAp-fncp64?si=hjeWKGBI7riyjE1M

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> It does beg the question, tho, why we haven’t seen any truly giant bats.

They're mammals, birds have different respiratory system

"Flow-Through Ventilation

Unlike mammals, birds breathe through continuous one-directional flow of air through the respiratory system. We take air in and breathe it out, sort of like the tide moves in and out of a bay. As a result, our breathing system is said to be tidal. Avians have a non-tidal respiratory system, with air flowing more like a running stream."

https://birdfact.com/anatomy-and-physiology/respiratory-syst...

Nature optimizes. The bigger you get, the more you need to eat. The harder it gets to fly. Fruit bats eat fruits.

Look at the food source and you'll understand the evolution.

> Fruit bats eat fruits.

The most caloric dense source of nutrition available in nature? I don't see why that is a limitation to body size for a flying animal - quite the opposite!

Plants aren't particularly calorie-dense. Meat, on the other hand...

Look at great apes. Large land mammals in general. (Apes came to mind specifically because they usually eat fruit)

Are you aware you switched "fruit" for "plant" there?

Fruits want to be eaten, Veggies don't.

this is almost in "not even wrong" territory, but for the fact that autotrophs are definitionally the entry point for abiotic energy into edible calories for animals, and the observation that the largest terrestrial megafauna are herbivorous.

bamboo is not calorie dense to humans, because we've lost the ability to digest most of it, but pecans are absolutely more calorie dense than even fatty beef.

all else being equal, an ideal carbohydrate source is more calorically dense than an equivalent ideal lean protein source due to the balance in the thermic effect of food between the two. most mammals outside the obligate carnivores are really well optimized for getting calories from plants— this is why we have amylase in our saliva.

fruit bats are the biggest bats

not GP but I think that was the point.

also, volume grows as the cube of linear dimensions which also puts an upper limit on size, as wing surface area only grows as the square (not sure what/how lift grows relative to)

That's why mammals can't breathe at high altitudes that birds can, but I'm not sure if that affects the body plan much in terms of size. The largest birds are smaller than the largest mammals on land or at sea. Then again, lower oxygen levels compared to the past seems to be a limitation for insect sizes too (who have an even less efficient respiratory system).

I also don't think it's the warmbloodedness. There are giant mammals in general after all.

Perhaps it is because bats form large, dense colonies? There is only so many resources available in any given ecological niche, so then for any species that fills a niche one would expect those resources to be divided either among many small individuals or a few large ones. Bat evolution chose the "big colony" route, which I assume favors smaller individuals.

> The largest birds are smaller than the largest mammals on land or at sea

With all my respect to you theory I think comparing size of animals should not ignore the medium they moved in: water, land or air. Weight is (loosely but still) related to size. It’s probably not a coincidence the largest mammals lives on water where they need less energy to supper their weight, and it’s not a coïncidents the largest mammals on earth are way bigger that bats.

The biggest bats are ~1.7m which is not so far from biggest albatros (3.7m).

Also consider the biggest bird (Ostriches) can’t fly. Now I’m trying to picture a swimming gigantic bird.

An Emperor Penguin?

Right! To complete the unusual list : flying fish and... Amphibious fish! Wikipedia says there's 11 of them. Ok stop procrastinating now.

Well, fair. But birds are warmblooded too so that doesn't change much there, and on top of that the difference in requiring bigger lungs for the same amount of oxygen extraction would exactly add much weight per volume, so to speak

Robots and living animals have different limitations and constraints though: compared to separate legs and wings for animals, using one motor with some kind of gearbox to switch output from wings/propellers to legs might have a lower added cost in terms of weight . The legs can stay very skinny. The limitation would be how bulky such a gearbox would be, and how much extra kinetic energy loss it would introduce. At the same time creating functioning wings that can also work as legs sounds like it might be a huge challenge in robotics (unless there's a way to massively simplify it).

Definitely an interesting idea that should be investigated though! :)

(Also, I've seen so many "AI learns to walk" videos that I'm wondering if it could be used to find a design that would work for this task)

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