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I was generally considering the idea of a flying species that uses lighter than air (LTA) gas to help provide lift, allowing it to have a larger mass than flying species can usually attend. I used the concept in my suggestion for justifying a phoenix.

Is there an evolutionary reason why this is unlikely to evolve naturally, or would prove disadvantageous? If I had a world where non-explosive LTA gas was more common is it likely that use of it to assist with flight would be common?

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    $\begingroup$ I think the main problem you have to solve is choosing a gas that is LTA in its elemental form AND can be found in organic compounds. Since your animal would have to metabolize or isolate it through a cellular process after, say, eating something. Or breathing and filtering it out of the air? $\endgroup$ – Ross Aug 16 '17 at 14:34
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    $\begingroup$ Let's see. Hydrogen is out of the questions because explosions and it would also leak out. Helium is impossible for an organism to isolate. Humanity only discovered it relatively recently on the sun! How about a very dense and heavy atmosphere, so the opposite of what you suggested? I do not know why such an atmosphere would form naturally since it would be made out of rare to super-rare elements such as xenon, fluorine or sulfur compounds for example, but it sounds to me like a better approach. Please be aware that no matter what, this would be extremely inefficient and slow $\endgroup$ – Raditz_35 Aug 16 '17 at 14:37
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    $\begingroup$ You'll have to say what non-explosive gas you have in mind, because there aren't any likely candidates. So whatever you come up with is going to have either serious drawbacks or serious implausibility. $\endgroup$ – Mike Scott Aug 16 '17 at 14:44
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    $\begingroup$ Have you ever heard a helium filled dragon roar? They died out from embarrassment. $\endgroup$ – Darren Bartrup-Cook Aug 16 '17 at 16:30
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    $\begingroup$ I have deleted the [hard-science] tag and replaced it with [science-based] as it is more appropriate. This will get better answers. $\endgroup$ – a4android Aug 17 '17 at 1:47
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Your main problem with lighter than air gasses is that the lift provided by gas is very small compared to the enclosed volume.

For an earthlike atmosphere you have an average density of air at sea level: $$\rho_{air} = 1.292 kg/m³$$

If you take your average human being, your density is somewhat a lot more than that. If you go into a swimming pool, stretch out fully and lay yourself in the water you will most likely barely stay afloat. That is because on average, the human body has a density slightly less than that of water:

$$\rho_{human} \approx 985 kg/m³ \approx 762*\rho_{air} $$

So your average human is 762 times more dense then air. (Hence we firmly stand on the ground). Even if you take most avian species of earth, your average density will be several hundred times more than that of air. Meaning without them flapping and spreading their wings, they would plummet to the ground just like yourself.

If your animal is to float by itself without any additional lift provided by wings, then your animal as a whole must have an average density of less than the air at sea level.

The maximum lift you could achieve - theoretically - would be to enclose a total vacuum within a solid body. Then your lift would be $1.292kg$ for each $m³$ of air you displace - at sea level. We have yet to find any substance that can enclose that space and not exceed the weight limit without being crushed by the atmospheric pressure. Thus we use lighter than air gasses which exert the same pressure but have a much lighter density. The best candidates for that are Hydrogen $\rho_H = 0.090 kg/m³$ and Helium $\rho_{He} = 0.178 kg/m³$.

They acutally reduce your maximum lift compared to a complete vacuum but don't require your containing structure to be as sturdy, thus your maximum lift will be about 8-16% less than with a vacuum. Hydrogen, while readily available, obviously has the major disadvantage of being highly combustible when mixed with Oxygen. Helium on the other hand is rather rare compared to Hydrogen.

Still if we assume we are using Helium, your maximum lift will be: $1.292kg/m³ - 0.178kg/m³ = 1.114 kg/m³$

The most efficient body to enclose any kind of volume is a sphere. It has the highest volume content for the lowest surface area. The volume of a sphere is calculated by $V_{sphere} = \frac{4}{3} \pi r³$ while the surface is $A = 4 \pi r²$.

The maximum mass for skin, organs, muscles etc. can thus only be $M = (\rho_{air} - \rho_{gas}) * V$. The mass of the creature is however also the surface area of the creature times its skin thickness times the skin density (wihch must also contain all organs etc). If we assume a human like density creature $M = \rho_{creature} * A * r_{creature}$.

If we bring both together:

$$ (\rho_{air} - \rho_{gas}) * V = \rho_{creature} * A * r_{creature} $$

Now since $\rho_{air} - \rho_{gas} \approx \frac{1}{700} \rho_{creature}$:

$$ \frac{1}{700} * \frac{4}{3} \pi r³ \approx 4 \pi r² * r_{creature} $$ or shortened: $$ r_{creature} \approx \frac{r}{2100} $$

Thus the creature's thickness would only be about $\frac{1}{2100}$th of the total volume enclosed... or for a creature with $2 m$ diameter, its skin could only (at best) have an average thickness of $0.5mm$ if it has the average density of human flesh. It would only get a reasonable thickness to withstand the elements if you make the creature have a 100 m radius - larger than the airship Hindenburg and even then, the creature on average would only be 5 cm thick (which is about the thickness of a slender human arm)... and it would still look like a balloon.

This doesn't sound very feasible, considering the amount of blood needing to be pumped, the amount of food needed to digest for such a huge body etc.

The only possibility for a sturdy enough skin to survive the elements and have reasonable space left for adequately sized organs would be a much denser atmosphere that can lift more mass for an equal volume.

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  • $\begingroup$ Commercially available leather starts at about 2 ounces/square foot or 0.61 kg/m^2. Other organs such as frogs throat sacs and similar membranes suggest that lighter weights than that may be achievable in live animals. Conceivably a lifecycle of water-borne juveniles progressing to airborne adults is possible, even with modest size like 5m, you could have a bladder 1mm thick carrying a payload body of about 1kg containing heart, brain, etc?? $\endgroup$ – Ben Aug 17 '17 at 8:48
  • $\begingroup$ Don't for get, it's average thickness. For every extremity, it has to get thinner. For digestive organs, heart, lungs, every muscle all that means that at some other point, it has to be much much thinner somewhere else or it will go over the weight limit. A 5 meter diameter creature could at maximum weigh about as much as the average human being. Now stretch your skin out to envelop a 5 meter diameter sphere, I guess the result is less than pleasant... and that is not considering what happens when a storm hurls debris at you... now if you double or triple the air density however... $\endgroup$ – Adwaenyth Aug 17 '17 at 9:07
  • $\begingroup$ Instead, imagine a creature the size of a large rat, attached to a bladder 5m in diameter. $\endgroup$ – Ben Aug 17 '17 at 9:32
  • $\begingroup$ The problem is that this bladder has to be living tissue that has to be supplied with blood, nutrition etc. Otherwise an injury there will permanently ground the creature as the injury can't be repaired. $\endgroup$ – Adwaenyth Aug 17 '17 at 9:48
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    $\begingroup$ @dsollen A problem arises there with aerodynamics. Most efficient in lighter than air flight is a rather bulbous or spherical shape to minimize surface area and maximize payload. Heavier than air tries to minimize drag in order to achieve higher speed and less energy usage to maximize wing lift and thus payload. Combining both is rather difficult because properties wanted for one form of flight adversely affect the other. $\endgroup$ – Adwaenyth Aug 17 '17 at 12:22
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Most fish have swim bladders that allow them to control their bouyancy so they can sink without losing energy swimming

The swim bladder normally consists of two gas-filled sacs located in the dorsal portion of the fish, although in a few primitive species, there is only a single sac. It has flexible walls that contract or expand according to the ambient pressure. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. By adjusting the gas pressurising organ using the gas gland or oval window the fish can obtain neutral buoyancy and ascend and descend to a large range of depths. Due to the dorsal position it gives the fish lateral stability.

enter image description here

Advantage:

  • This animal can actually float like a balloon so it doesn't waste a lot of energy compared to other animals like birds

Disadvantages:

  • While a fish can find light air inside water, this animal should almost float forever since they're in the atmosphere (the top), or a pretty long time until they find some gas locations.
  • As a zeppeling, this animal would need a massive air bag to float, this means this animal would be way less maneuverable compared to a winged animal, being an easy prey for flying predators.

So your animal would be almost a living balloon rather than a sentient lifeform, something like an air jellyfish. It can probably develope toxins or defensive traits to help it survive, but it would be too easy to catch.

enter image description here

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    $\begingroup$ "it would be too easy to catch" for predators that can reach the kind of altitudes these gentle giants live in. And even if a predator can fly that high, unless they can survive there for several minutes or pierce the gas sacs, they won't be able to enjoy the bulk of their meal. $\endgroup$ – John Dvorak Aug 16 '17 at 17:10
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    $\begingroup$ Speaking of easy prey, how come sharks don't feast on whales on a daily basis? $\endgroup$ – John Dvorak Aug 16 '17 at 17:12
  • $\begingroup$ @JohnDvorak we are talking about aliens right? Pretty sure there are winged aliens that can reach those altitudes, otherwise the jellyfishes would spread blocking sunlight in entire zones. $\endgroup$ – Lunar Heretic Aug 16 '17 at 17:17
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    $\begingroup$ Don't worry about the jellyfish overpopulation. They have to eat occasionally, and there's not much food in these altitudes, so they have to descend occasionally. Unless the plot hook demands they do overpopulate anyways, that is... ;-) $\endgroup$ – John Dvorak Aug 16 '17 at 17:20
  • $\begingroup$ @JohnDvorak well that was something I stated. They must float forever or go down for resources. It would be hard for them to have food just if there are aerial plants too $\endgroup$ – Lunar Heretic Aug 16 '17 at 17:25
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A lot of answers are cautioning against using hydrogen or methane because of the explosive properties of those gasses - but in reality, there wouldn't be much danger. Those gasses are dangerous around humans, but that's because humans live in a terrifying danger zone. We use fire to prepare food, use lightning to power tools, and surround ourselves with objects and spaces that are seem designed to create dangerous static charge (wool socks and synthetic carpets in a dry house? Seriously?).

Most creatures avoid all those things out of fear. Sure, lightning strikes might be a problem, but then again, it doesn't matter if you're filled with hydrogen or not, really. Apart from that, hydrogen is fairly safe, as long as it is contained.

Producing hydrogen is possible, probably through "fermentative hydrogen production" to make biohydrogen, or biologically produced hydrogen.

I think it's entirely possible for a flying creature to use hydrogen to maintain flight. Just... don't light a match if one is descending.

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There are a couple reasons I can think of that would discourage, but not altogether prevent, the evolution of such fauna:

Flammability

Gases such as methane and hydrogen would fall under this category. While both could be produced organically/with water and used to provide lift (assuming an atmosphere like Earth's), it would take rather large quantities to actually be of use for flight, at which point the risk of combusting could easily result in deaths of members of the species. If they traveled in groups, one accidentally exploding could kill off the entire group in an explosive chain reaction.

Energy Demands

Sufficiently hot air or steam/water vapor is a safer method to produce lift, but likely not nearly enough to serve as the primary means of flight, more likely just an aid. Furthermore, the caloric requirement for this species to maintain their means/supplement to flight would be much greater than another species of comparable mass, or else flight would only be feasible to those members of the species with a great enough food supply to keep these gases heated. The undernourished members would be forced to take their chances on the ground.

Rare/Inorganic Gases

Another option could be noble gases like helium or neon, but the latter is fairly heavy among lighter-than-air gases, and both are comparatively rare on Earth. If your world had these gases in abundant supply somewhere, like from underground vents, and your species had the capability of consuming and storing these gases long term, this solution could be possible. Of further note is that the young of this species would begin their lives flightless, and would have to consume these gases to gain their flight capabilities.

Anatomical Structure

Finally, I presume this species would look very bulbous from sac-like structures large enough to offset their weight. The skin/membrane that contains the gas would have to be strong enough to withstand breaches from incidental contact with pointed objects (e.g. tree branches, coarse rocks). I exclude teeth from this as most standard animals bitten a large predator probably wouldn't survive the encounter, either, so this flight-capable species would need to adapt some sort of escape mechanism to prevent this sort of thing from-- wait...

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    $\begingroup$ Hydrogen and methane aren't actually explosive unless mixed with proper ratios of oxygen. For hydrogen, the range is fairly large, but even so a "swim" bladder filled with pure hydrogen would not present an explosion risk, or even a significant combustion risk. Note that the Hindenburg did not explode due to the hydrogen catching fire. Hydrogen fueled the flame once the outer skin had already started burning, but was not the cause of ignition nor explosive. $\endgroup$ – Logan R. Kearsley Aug 16 '17 at 20:36
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There are two major reasons for it to be complicated rather than plainly impossible.

  • Although it's moderately easy to use a volume of gas lighter than the environing medium for animals that live underwater because gases are lighter than water, it becomes a lot trickier when the medium is also made of gases. You need a gas that is not only less dense than the average density of the medium, but also that it's denser enough to carry the solid parts of your animal. Indeed, the difference in density between H2 and H20 is pretty significant while the difference between two gases will be less noticeable. Therefore, you'll need the lightest gas possible or close to it and so much of it that it's enough to make the whole body to float. Since your animal can't use the ambiant gas cocktail for obvious reasons, it must be a - way - lighter gas, that it will be your animal's responsibility to produce. That may be more or less difficult, but the choice is already restrained by density so it has to be a relatively easy to synthetize one among the lightest. Another option is void, which is lighter than anything else, but it requires a strong and rigid container or the outer pressure will crush your lifeform. That stuff doesn't come as light so it might be counterproductive.
  • Even after the issues described above are solved, you basically end up with a baloon. An animal that floats in the air. While birds have great and precise control over their three dimensional speed and direction thanks to their wings that they can move in the way they want and orient accordinglingly to their needs, your animal won't have many option : producing or releasing gas and eventually directing the release in some particular direction, that's all. That makes your lifeform something like a plant or a jellyfish : something that's carried away and reacts by chemical/electrical reaction instead of instinct or choice. That may be possible, but not that fun.

All things considered, that stuff applies to Earth, but you can decide that the gas cocktail in your atmosphere is heavy, therefore easing the conditions so that your idea is easier in your world that it would be on Earth - the reason why it only exists underwater on Earth.

My conclusion is that you can't apply that idea to a complex lifeform without either introducing a heavy atmosphere or handwaving some stuff.

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  • $\begingroup$ This is the only answer I feel ok with up-voting. It gets close to the mathematical problem involved here, that being: how much light gas do you need in order to float in the atmosphere, although it doesn't actually get to the math. That said, Mythbusters covered this one. $\endgroup$ – Draco18s Aug 17 '17 at 4:19
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One thing you need to bear in mind is Graham's Law: the rates of diffusion of gases are inversely proportional to the square root of their masses.

In other words, the lighter a gas is, the more likely it is to leak out of whatever container it's in. This means, you can't just collect the gas once and expect it to last the rest of the organism's life; it has to continuously collect the gas throughout its lifetime, and the lighter the gas, the more it leaks, and the more of it that needs to be collected.

Also, consider that if the gas is stored within the body, it carries the risk of reacting with the organism's body, possibly acting as a carcinogen. Alternately, if it is stored outside the body, it is effectively a giant blister, just like bubble wrap. Neither is a safe alternative, especially if the gas is flammable.

Lastly, air is a mixture of all available gas. If an LTA gas is commonly available, it means that the density of air is also lower.

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  • $\begingroup$ LTA gasses in the atmosphere can't be commonly available by definition $\endgroup$ – Andrey Aug 16 '17 at 19:26
  • $\begingroup$ Well done, sir, you beat me to the punch. Leakage is the main reason why organisms don't use LTA gas-bags to float or fly. The containment of any gas isn't easy. Biological membranes and tissues are poor at creating hermetically sealed vessels. $\endgroup$ – a4android Aug 17 '17 at 1:55
  • $\begingroup$ A living creature will most likely consume some form of organic matter. Organic matter besides containing carbon, oxygen, sulfur etc. also contains hydrogen. So leakage would not be that much an issue if it just can consume enough food to produce the hydrogen again. $\endgroup$ – Adwaenyth Aug 17 '17 at 8:23
  • $\begingroup$ @Adwaenyth: a) What would be the energy requirements to generate a sufficient mass of $H_2$ to equal a significant fraction of the organism's mass? Since we don't know the organism's mass, we can't begin to guess. b) 2g of $H_2$ has a volume of 24 $l$ under 1 atm pressure. That's the volume of a small pail. For an organism weighing several kg, you'd need a much larger volume or much higher pressure, and you have to do it without reinforcing the bladder, as additional weight iscounterproductive. Add the fact that gases're more prone to leak at higher pressures... How much $H_2$ would you need? $\endgroup$ – nzaman Aug 17 '17 at 13:18
  • $\begingroup$ @nzaman regarding the equations in my own answer to lighter than air flight, to get a mass of $1.202kg$ afloat you need $0.09 kg$ of Hydrogen. So it is a ratio of roughly $13.3 : 1$ of body mass to hydrogen produced to stay afloat - or roughly 8% of mass must be converted to hydrogen. $\endgroup$ – Adwaenyth Aug 17 '17 at 13:23
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If a flying species wanted to use a lifting gas as part of its biology, a major problem would be finding a source that is accessible on the surface. After all, being 'lighter than air' any free H, He, Ne etc. are prone to drifting upwards and to atmospheric escape.

Consider the various lifting gases and why they'd be impractical:

Noble gases are found trapped underground, in quantities too minute to support evolution by nearby creatures into using them.

Methane could be created as organic by-product but is flammable. Even if the creature somehow developed a fireproof coating for its 'floating organ', it would still have to contend with explosion due to static electricity. Well, impervious fireproof materials would also be much heavier than normal organic matter and get trapped in a cycle of needing more volume of methane which needs more heavy material to cover it!

Steam may be easy to generate in most regions, but it takes a lot of energy to turn water into steam. Rising up and then encountering a cold squall would lead to condensation ending in a long fall. Again, having to evolve a burn proof, insulating (to protect other organs), impervious membrane will be a tough challenge for any would-be steampunk species.

As for vacuum - the pressure differential will crush anything not sufficiently thick. A single puncture would be disastrous as well - more so than other options where the enclosing layer might have time to heal up.

Energy and food requirements would be prohibitive as well:

The reason why a flying species wants lift in some form (whether from 'lighter than air' gas or other sources) is to reduce energy expenditure to maintain altitude. Even if it somehow braved the odds, or had help from intelligent design (whether gods or genetic engineering) to have a 'floating organ'; the effort to maintain its condition would far surpass the simple expedient of using lightweight structures with large surface area, aka 'wings'.

The massive amount of extra weight and complexity of such an organ, would also necessitate much greater food intake - which won't be found in the atmosphere, so the creature has to come down near the surface for its food anyway. This negates the only positive factor of floating over flying, which is greater efficiency in maintaining the same high altitude.

Edit: Realized later that my answer draws a lot from the same 'lifting gas' entry as MSet's existing one, but I'll keep the overlap section as some things as stated a bit differently.

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One useful alternative to lighter than air gases would be to act as a solar balloon:

https://en.wikipedia.org/wiki/Solar_balloon

A Solar balloon is black and absorbs sunlight, so the air inside it gets hot, so it becomes a hot air balloon that doesn't need a burner.

The downside is that it only works during the day. You also get a lot less lift than hydrogen would give you, but without the technical problems that hydrogen would give you, like escaping and being difficult to replace.

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    $\begingroup$ I imagine this means there would be evolutionary pressure for dark colors even for something that produces lighter than air gasses. $\endgroup$ – JollyJoker Aug 17 '17 at 7:38
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The speculative nonfiction book The Flight of Dragons is entirely about this very topic. Hard to find nowadays, but well worth the read even if you don't agree with the thesis.

While positing that dragons flew by producing hydrogen in a sort of honeycomb of sacs (and that firebreathing was a method of burning off excess hydrogen in a controlled fashion to reduce the risk of sudden explosions), the author identifies numerous drawbacks to this type of biology, chief among them:

  • In order to be light enough to fly, they'd have to be fairly weak in terms of muscles and skeleton, and easily punctured by any sharp object.
  • Being thus vulnerable, they would tend to require caves or other sturdy lairs, the competition for which would make it almost impossible for them to ever be very numerous.
  • Young dragons wouldn't even be able to fly until they reached a size where the volume of the sacs would be enough to lift the rest of them off the ground.

In short, while a LTA flying animal would be fearsome due to its size if nothing else, it would probably also be fairly fragile in both a literal and ecological sense.

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  • $\begingroup$ The author is Peter Dickinson. The book is a piece of delightful speculation. Originally published in 1979 this is part of the reason it's hard to find. $\endgroup$ – a4android Aug 17 '17 at 1:51

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