I'm in the process of creating flying creatures big enough to be mounted, but there's a problem: the bigger the size, the heavier the weight, the heavier the weight, the more difficult (or even impossible) it is to fly. And some of my creatures have 6 limbs. With that in mind I remembered that there are gases lighter than air and that birds have hollow bones filled with air pockets, so I got the idea that my creatures' hollow bones are filled with some gas lighter than their own. Can a creature fill bones with gas that is lighter than air?

It has to be a gas that the animal can get from nature or produce with its own body, ideally it is not toxic or flammable (imagine if someone shoots a fire arrow at a pegasus and it explodes). If you can, you can talk more than one gas (if they are gases that the animal produces with its body, then there is an alternative for carnivores and herbivores). My heaviest creature can weigh up to a maximum of 450kg.

  • $\begingroup$ Sadly basically every gas that is lighter than air is flammable, other than nitrogen and one or two others I can't remember, all of which not directly usable by life as we know it. Perhaps the nitrogen-binding microbes can be used somehow $\endgroup$
    – Lemming
    Nov 30, 2021 at 14:53
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    $\begingroup$ You'll note one thing in common among all types of lighter-than-air aircraft (hot air balloons, blimps, zeppelins, weather balloons, etc.) - they are all very big compared to the payload they lift. $\endgroup$ Nov 30, 2021 at 15:53
  • $\begingroup$ @Lemming as air is mostly nitrogen, nitrogen can't be (more than a tiny bit) lighter than air. Helium is the canonical example but is inert meaning a plausible biological system wouldn't be able to concentrate it even if it found a source $\endgroup$
    – Chris H
    Nov 30, 2021 at 16:43
  • $\begingroup$ @Nuclear Hoagie, the objective is not to make the animal fly only with gas, but to make it lighter so the wings are not purely ornamental. $\endgroup$ Nov 30, 2021 at 18:54
  • $\begingroup$ Example: try adding a 20 meter wingspan to a horse and make his bones hollow. It will still be very heavy and the wings will hardly be anything other than pretty ornaments. $\endgroup$ Nov 30, 2021 at 18:59

5 Answers 5


Yes, but it will make no significant difference to the weight of the animal.

The gas that immediately springs to mind is methane with a molar mass of 16.04 g compared to air at 28.97 g/mol. One mol occupies 22.4L at standard temperature and pressure, so 22.4 litres of methane would be 13g lighter than air.

An albatross weighs 7Kg, I have failed to find an estimate for the volume of the bones but I think one litre is an overestimate. Taking that, methane rather than air would only make 1/2 a gram difference to the total weight.

Methane is flammable, but as it is encased within the bone which is encased within muscle which is covered with flammable feathers I do not think this is your major problem.

  • $\begingroup$ Upvoted, but why not hydrogen or helium? Or even a vacuum? It still won't matter much on these scales, but using the lightest most abundant material or nothing at all seems to give the answer the most merrit. $\endgroup$
    – Trioxidane
    Nov 30, 2021 at 19:07
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    $\begingroup$ I am not familiar with any biological process that generates elemental hydrogen, but those that produce methane are well known. I cannot envisage any way a biological process could create a vacuum. $\endgroup$
    – User65535
    Dec 1, 2021 at 10:48

Not worth it.

The only two gases sensibly lighter than air are hydrogen and helium.

Of the two, hydrogen forms explosive mixture with air at practically any concentration, therefore you are left with helium.

Helium is a noble gas, and is produced by nuclear decay, via alpha particles capturing two electrons. Being a noble gas doesn't react with other substances, and being so light is also not trapped in Earth's gravity well. The only plausible way for your creature to produce it would be to feed on radioactive substances.

All these problems apart, the lifting capacity you might get is very meager: based on this table, a 10 inches/25 cm round balloon filled with helium can lift a mere 6 grams. It's easier for the creature to get lighter by emptying its bowels before taking off.

Just for curiosity, lifting an 80 kg person would take a tad more than 5700 regular balloons


Even if the gas you handwave is as light as a vacuum (as opposed to hydrogen, which is roughly 1/14 of the density of air), it will take substantial volumes of it to displace enough air to lift a creature with significant muscle, bone, organ, and skin mass.

Air, at standard conditions, masses about 29 g for 22.4 liters (that's the volume of one mole, so it has a mass of one gram for each atomic mass unit -- and air, at 79% nitrogen, 20% oxygen, and small amounts of carbon dioxide and argon and traces of other stuff has an average molecular weight barely over 29) -- so that, minus the weight of your lifting gas, is the most weight that buoyancy can lift.

Since hydrogen weighs 2 g (and a tiny fraction, accounting for the very small percentages of deuterium and tritium in natural hydrogen) for the same volume, a 22.4 liter volume of hydrogen can lift about 27 g -- including whatever structure contains the hydrogen, of course.

This is the reason we don't see animals using hydrogen for lift. It's easy for biological systems to produce hydrogen, if it's needed -- but the amount of hydrogen needed to lift a human, at around 75 kg, comes to 22.4x75,000 = 1,680 cubic meters -- that is, the human would have to contain an additional 1680 cubic meters of volume, filled with hydrogen, without increasing its body mass at all.

Even if you had a fictional lifting gas that has no mass, you'd only reduce that volume a little; 29-ish g per 22.5 liters instead of 27.

This is completely impossible (the weight of the skin to contain the gas would exceed the lift of the gas) for any creature heavier than a good-size beetle -- and much heavier creatures can fly faster, with greater control, and lift more "payload" with wings.

  • $\begingroup$ 29 g for 22.4 liters is a strange way of saying 1.3 grams per litre or 0.17 ounces per US gallon. (Yes, that's the molar volume, I know.) $\endgroup$
    – AlexP
    Nov 30, 2021 at 15:25
  • $\begingroup$ Nothing strange about making the concept easy. Molecular weight = grams per 22.4 L at STP. $\endgroup$
    – Zeiss Ikon
    Nov 30, 2021 at 15:32
  • $\begingroup$ I know that. But I still think that you should explain why you chose the seemingly arbitrary volume. (Most people don't remember their high-school physics or chemistry in such detail, even those who went to the kind of high school were such things were taught.) $\endgroup$
    – AlexP
    Nov 30, 2021 at 15:33
  • $\begingroup$ Good suggestion, @AlexP -- did it. $\endgroup$
    – Zeiss Ikon
    Nov 30, 2021 at 15:51

Hot Air or (even better) Steam:

Hot air is a lifting gas. Water vapor is lighter than air. Combine the two, and what do you have? Steam.

  • On the specific topic of encasing lifting gas inside of a rigid case like bone: while it's not impossible to have a biologically derived rigid gas-containing structure composed of carbon nanotubules or the like to allow vacuum or steam at less-than-boiling pressures, it's biologically less likely, and I don't think you should go that route unless you aren't overly concerned about plausibility. Most rigid containment will vastly outweigh the lifting gasses associated with it.

This can be very problematic. The biggest barrier is to have a life form that can tolerate exposure to pressurized water at the boiling point. But life forms are endlessly creative. There could be a thin layer of material that insulates the organism from heat. It could be a grown structure, no longer alive, that creates a natural sealed heat & pressure-resistant bag. The earliest hot air balloons were made of paper and silk, so I'm guessing some reasonable material could be grown, extruded, or manufactured naturally that would work. Or the whole organism could be adapted to high internal temperatures.

Then you need to be able to generate a lot of heat. This could mean the organism actually starts fires somehow and harvests the heat produced, OR that the organism produces it's own combustible (which could itself be a lifting gas, streamlining the process). Biologically, hydrogen peroxide can be rapidly metabolized to steam (thank you Logan R. Kearsley).

The animal may also have some means of external digestion/fermentation, trapping flammable gasses in an external storage (like a giant grown balloon...) and eliminating the need to carry around a large bulk of undigested food while providing a supply of methane either to burn or as LTA gas. There's nothing to say it has to only be one or the other.

But nothing will prevent your organism from tending to look like a giant parade character - big, bloated, un-maneuverable, with lots of long small limbs enabling interaction. The upside is that if your organism doesn't constantly fly, especially if the heat is obtained from an external source, they can always dump the steam (possibly as a form of defense or offense as well) and become a heavier-than air animal again.


  • $\begingroup$ The easiest way to produce steam biologically is by first producing hydrogen peroxide, and then catalyzing its decay. That's what bombardier beetles do, for example. $\endgroup$ Nov 30, 2021 at 18:48

A different approach... An anti-lung.

An organ in the creature that is collapsed at rest and expands into a vacuum at the contractions of the muscles around it. A vacuum is the most buoyent thing you'll get in an atmosphere, though you'll still need a relatively large amount of space displaced for it to be truly effective.

There's also the problem of osmosis and various other gas and fluid transfer mechanics that organisms have that'll make it so that the anti-lung doesn't remain empty for long so even if a creature could make flight easier for itself with this it'll not remain effective forever.

A different different approach...

EVERY cell of the creature is(somehow) partly balloon. Each cell has only a tiny bit of it be an actually useful cog of the metabolic machine, while the rest of its(expanded) volume is actually either (1) empty space or (2) a concentration of methane or hydrogen or some other LTA gas. This would have two effects, them being that the creature would end up being a lot larger than its weight suggests, and that the entire creature would be naturally buoyant, making larger wings, stronger muscles, hollower bones, etc, for a larger flying creature a little unnecessary since its own volume would be doing all of the heavy lifting for it.

Unfortunately this would lead to a relatively squishy creature, and depending on if the cell balloons are vacuum spaces or gas pockets, it might end up being a ridiculously explosive creature too, but hey, it's an(albeit unlikely to ever occur) option.

  • $\begingroup$ See my answer -- still too big, more than 14000 liters to lift 75 kg. $\endgroup$
    – Zeiss Ikon
    Nov 30, 2021 at 15:04
  • $\begingroup$ @ZeissIkon So you're saying you don't want a dragon that lifts itself with two anti-lung balloons for wings and uses its tail as a propeller? How absurd... XD $\endgroup$
    – Lemming
    Nov 30, 2021 at 15:07
  • $\begingroup$ I'm saying I'd have less trouble suspending disbelief if you just say it levitates magically. $\endgroup$
    – Zeiss Ikon
    Nov 30, 2021 at 15:11
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    $\begingroup$ On a cell-by-cell basis, this greatly limits the abilities of cells to specialize, and a lot of mass is taken up by cell membranes. I SUSPECT but don't know that the mass of muscle needed to generate a vacuum would be heavier than the lift from the vacuum. $\endgroup$
    – DWKraus
    Nov 30, 2021 at 17:55

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