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What would birds look like if they did not have to fight gravity to fly?

The question comes from the premise put forward by some that gravity is not real and that what holds us down is pressure caused by density.

Obviously, this is not the case, but if it were, how would it (yes, conceptually) affect the way birds evolve, develop and/or fly?

[EDIT] To conceptualise a little... Let's say we had evolved on Earth and then been transplanted to an artificial planet. One with no gravity but one where the gas we are surrounded by is dense enough to actually cause some amount of pressure and thus keep us on the ground - a ground which has no gravity because it is perhaps only a metre deep, though wide enough to house every country... and then birds had evolved. (Perhaps they had been put there before we were.)

[COMMENT] This is not about whether the birds of a particular type are viable but is asking what they would look like given time enough in a particular environment.

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Fish, or possibly penguins

A world where things can fly without needing to fight gravity (I don’t want to think what the method for that is) is a world where the first fish to develop the ability to breathe out of water skips land completely and just goes straight for the air.

Even if it’s something specific to birds, you’d start to see more and more penguin like ‘torpedo’ shapes optimised for slipping through the air rather than pushing against it (hell, in this world penguins might be the most successful bird!)

You’d probably see larger control surfaces (wings/fins) than in the water, as water is denser, but without the need for surfaces to glide/flap with I can’t see why a bird wouldn’t move back to a more energy efficient form of locomotion, namely oscillating the body back and forth and using the limbs purely for control instead of relying on the limbs to do both.

ADDENDUM:

In the comments I added a link to the Festo Air Penguin, a helium filled remote control blimp. I didn’t initially add this as their design of blimp uses fins for locomotive power and only uses the nose and tail of the ‘penguin’ for control, unlike true penguins or fish which also flex their bodies to give better manoeuvrability and power, but it’s a good enough video that I’ve stuck it in the answer!

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    $\begingroup$ Fish and penguins are optimized for slipping through the water. There's a lot more resistance in water than there is in air. $\endgroup$ – Peter Shor Jan 20 at 1:51
  • $\begingroup$ True, though I think that is handled by the 3rd para. $\endgroup$ – Matt W Jan 20 at 5:49
  • $\begingroup$ To be able to float in air, air must be more dense or birds must be a lot larger. In both scenarios, air resistance matters a lot and you are getting penguin-like birds or airship-like birds - and interestingly airships are penguin-like shaped. $\endgroup$ – Pere Jan 20 at 15:33
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    $\begingroup$ @Pere In some cases airships are penguins! $\endgroup$ – Joe Bloggs Jan 20 at 16:14
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    $\begingroup$ @JoeBloggs - Awesome. This video could be an answer to the question by itself. $\endgroup$ – Pere Jan 20 at 16:26
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They would be round, because that's the best format for storing gas. They would also probably be drifters with little flight control. So...

Red!

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    $\begingroup$ @DreadfulWeather They do, but it isn't enough that you'd really call them "buoyant". They're just lighter than land animals their size. $\endgroup$ – Brilliand Jan 19 at 1:41
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    $\begingroup$ I'm seeing flotillas of round non-rubber duckies floating lazily on the breeze.... $\endgroup$ – hat Jan 19 at 10:04
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    $\begingroup$ Angry Birds just got a backstory :) $\endgroup$ – val says Reinstate Monica Jan 19 at 23:44
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    $\begingroup$ with no gravity density doesnt cause you to be pushed anywhere but "out". $\endgroup$ – Josh Vander Hook Jan 20 at 5:16
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    $\begingroup$ Yep, that is a problem. I don't know what flattards use to justify that. $\endgroup$ – Matt W Jan 20 at 5:49
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I don't see any reason why birds couldn't theoretically achieve buoyancy in the air. It's just probably extremely inefficient for animals the size of normal birds compared to winged, powered flight.

They would need a biological way of producing a gas lighter than the atmosphere, which is much more likely to be hydrogen than helium. Then, they would need a 'bag' of enough volume that its hydrogen offsets the rest of their mass.

Finally, these birds would need some means of controlling their ascent and decent, which means ballast. This is probably going to be water that they can release to fly higher. When they need to drop down, they would release hydrogen instead. They couldn't fly higher again until they had a chance to replenish both, so they would need to be able to float on the water while they produce hydrogen and take in enough water to keep them grounded until they're ready to go.

In short, they would probably look like blimps, but with flapping fins/wings instead of propellers. They would also be much more likely to be large. The square/cube law works the opposite way with things that are lighter than air. The bigger you are, the easier it is to fit in lift gas relative to surface area.

As for 'gravity not being real' and your 'pressure caused by density' alternative, I'm not really sure I understand what you mean, but I don't see how it would make a difference. In both cases, you achieve flight by being less dense than the air.

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    $\begingroup$ I hope this satisfies @ynneadwraith :) $\endgroup$ – Matt W Jan 18 at 16:44
  • $\begingroup$ Thanks @Pasqueflower. The gravity not being real thing is apparently part of the flat earth concept - I can't justify it. My understanding of current flight mechanics is that lift does not need require a body which is less dense than air - though maybe I misread your statement? $\endgroup$ – Matt W Jan 18 at 16:47
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    $\begingroup$ Lift does not require a body that is lighter than air, it just requires a larger force pointing up than gravity. But that's what buoyancy means. It's the upward force of a liquid or gas that is opposing the weight an immersed object. The magnitude of the force is equivalent to the weight of fluid that the object displaces. And this is different at different altitudes/depths, because pressure increases as you go deeper. Weight = mass x acceleration of gravity. Buoyancy = weight of displaced fluid. Whichever is higher determines which way something goes. $\endgroup$ – Pasqueflower Jan 18 at 16:54
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    $\begingroup$ "a gas lighter than the atmosphere, which is much more likely to be hydrogen than helium." - Or possibly just hot air. $\endgroup$ – 8bittree Jan 18 at 19:46
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    $\begingroup$ @8bittree - hot air has a pretty high metabolism requirement. A typical hot air balloon is something like 2-3 MW. Or something like 500 food calories / second. You could insulate... but that adds weight and increases the amount of lift you need to generate. Your best bet for hot air might be solar heated... though then the question becomes what you do at night. $\endgroup$ – TLW Jan 19 at 3:20
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As there is the physics tag, I feel that the misconceptions should be addressed.

Buoyancy is the result of gravity. If there is no gravity, there is no buoyancy.

The bigger picture is that a hot air balloon floats upwards because gravity pulls it down less than the same volume of air. So air gets to occupy the lower layer. In the smaller scale buoyancy arises because the pressure is higher in the lower layers. Thus the bottom of an object is pushed upwards more than the top is pushed downwards.

You can not reproduce that by blowing air from above. You might be able to get things staying on the ground, but the pressure gradient will be opposite - for any object the pressure from above will be higher than below. So buoyancy will push everyone downwards and being lighter won't help you. If birds would fly in such circumstances, they would be similar to those on Earth but maybe a bit thinner and having more endurance as they have to constantly fight the wind.

I answered supposing that people are held on ground by blowing wind from above as discussed in the comments. The phrase pressure caused by density by seems to have no real physical meaning, so the question as-is can't really be answered.

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  • $\begingroup$ Oh yes, I completely agree - the entire premise is stupid. As I've said in the comments, this is not my premise but the flat earthers'. I can't justify it and the problems raised because of the logical holes are not the question here. Just this one point. I guess the reasoning would come down to "even if everything about the FE were true, birds would / would not look the same as they do in real life." Then watch the flattards tie themselves in more knots. $\endgroup$ – Matt W Jan 20 at 5:57
  • $\begingroup$ @MattW Wait, Flat Earthers don't believe in gravity? That's surprising--especially because an infinite uniform slab does actually have the uniform gravitational field we approximately have on the Earth's surface. So unlike a great many other things, its existence doesn't really contradict their theory. $\endgroup$ – eyeballfrog Jan 20 at 21:45
  • $\begingroup$ @MattW: You're "birds fly because of density" argument relies on the lack of aerodynamic friction (because wings don't work the way they do - hence the need for buoyancy); but then you've also defeated how birds are able to move horizontally (left/right/up/down). And if birds are able to flap their wings to move horizontally, why wouldn't they be able to do the same vertically? What if they flap their wings to move at an angle (horizontal+vertical)? $\endgroup$ – Flater Jan 21 at 7:38
  • $\begingroup$ @eyeballfrog While I'm asserting that flatters don't believe in gravity because I've seen so many prominent flatters say that it specifically does not exist, it's not really the point to my question. That is to say that my question is not whether gravity does or does not exist or if anyone does or does not say it exists - the question is about how birds would evolve in a gravity-less environment. The specifics of the structure of our world, from the uniform opinion of the flatter movement, is not available because they don't all agree. See the first sentence in this comment, for example. $\endgroup$ – Matt W Jan 21 at 7:51
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    $\begingroup$ Whenever people insist centrifugal force isn’t real I love to tell them that buoyancy isn’t either and watch their heads explode. $\endgroup$ – Joe Bloggs Jan 22 at 8:08
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If the bird in question needs only to be buoyant then the resulting animal would float at a more-or-less constant height, unable to manoeuver to avoid predators or return to the ground to nest or feed (I will show later that this bird is vegan).

These are strong evolutionary disadvantages.

Blimps, fish and submarines are buoyant and use fins for steering, so it seems prudent to include these in its design.

Regarding altitude, the bird needs to increase or decrease the amount of lighter-than-air gas in its body. Decreasing is easy, but it needs a method of producing extra gas for lift.

Both hydrogen and methane are lighter than air, and these gases are produced when certain foods cannot be digested but instead ferment in the gut. I would therefore design the bird to have a penchant for beans and pulses, and a method of recycling its own farts.

So in summary it should look like a fish with a tube from its anus to its nostrils.

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  • $\begingroup$ I love it, but you forgot to demonstrate why the bird is vegan (though I think I can see it coming). $\endgroup$ – JBH Jan 19 at 3:59
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    $\begingroup$ +JBH Updated. Thanks for your suggestion. $\endgroup$ – Boodysaspie Jan 19 at 5:33
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    $\begingroup$ +Matt W We do indeed have a tube from our mouths to our anuses (the gastrointestinal tract). The flow is, luckily, in the opposite direction to my tube mod, burping or vomiting excepted. $\endgroup$ – Boodysaspie Jan 19 at 15:12
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    $\begingroup$ Altitude control could be affected by pressurizing the "bag" which contain the "lifting" gas. If the "bag" were surrounded by muscle, tensing and relaxing the muscles would change the density of the gas, thereby changing the lift provided by the "bag". $\endgroup$ – Julie in Austin Jan 20 at 3:50
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    $\begingroup$ Julie in Austin's bird would easily out-evolve mine. Not only is it more efficient in using, ahem, resources, it can also quickly shrink in size (and have more choice of cover on the ground) when it detects predators - an excellent camouflage technique. My only contribution would be in replenishing the bag, since no system is perpetual. $\endgroup$ – Boodysaspie Jan 20 at 20:09
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They would definitely look very silly. They would by default have to have very low momentum in relation to air resistance so they would be much less efficient in their movement than IRL birds and have much less maneuverability. A good comparison is an airplane and a hot air balloon. they would most likely look like big balloons with small propulsive appendages on their back side.

they would certainly be comical to watch go about their days.

https://www.youtube.com/watch?v=yltlJEdSAHw

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    $\begingroup$ I suspect they'd look very normal to us, given that they'd be 'everyday' things. Having said that, of course, I did state in another comment that they would need to evolve in an environment to which we get transplanted, so maybe they'd look totally nuts. $\endgroup$ – Matt W Jan 19 at 7:09

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