First, I guess the conditions of the world are in order. It's approximately 85% the mass of earth and there is going to be a full ecosystem with all the usual types of bugs, insects, trees, and predators. The ecosystem obviously won't be the exact same as on Earth, but there will be the usual things like pollinators, seed/fruit-bearing plants, decomposers, several trophic levels etc. Oh, and atmospheric composition will be very close to Earth too.

Now, how would birds be different, physiologically, from what we are used to seeing on Earth? I realize there are a huge variety of birds from finches to eagles, but they all share some similarities.

I believe, the lighter gravity, assuming a world approximately the same diameter as Earth, would lead to less atmospheric pressure, right? So there would be less air for their wings to find purchase on. To me, that means birds would either have to be lighter or would have to work harder to fly.

But part of me is conflicted because with less gravity there will be less strain on the bird's skeletal system and it probably would have evolved to be lighter/weaker than whatever its bird counterpart is on Earth. And that lighter skeletal system might make up for the less dense air. So I don't really know which way to write this, whether birds should look too different from their counterparts here. Maybe there would be plenty of smaller birds but not as many larger birds of prey? Or the inverse of that situation?

Could any ornithology-minded individuals chime in? Or anyone that knows the types of physics that could help me figure out if something like a ~15% reduction in g-force would result in like an 80% reduction in atmospheric desnity or whatever. That's a little beyond my education haha.

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    $\begingroup$ Smaller gravity doesn't necessarily mean a thinner atmosphere. Titan is roughly the size of Earth's moon and yet it has a thicker atmosphere than Earth, if I recall correctly. $\endgroup$
    – F1Krazy
    Commented Jan 15, 2018 at 14:20
  • $\begingroup$ "I believe, the lighter gravity, assuming a world approximately the same diameter as Earth, would lead to less atmospheric pressure, right?" Not necessarily. Titan is smaller than Earth and has a much thicker atmosphere. $\endgroup$ Commented Jan 15, 2018 at 14:20
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    $\begingroup$ Or look at Venus, which is about the size of Earth but has a much thicker atmosphere. $\endgroup$
    – user
    Commented Jan 15, 2018 at 14:26
  • $\begingroup$ @MichaelKjörling Composition (which the OP has stated is close to earth's) comes into atmospheric pressure quite a bit. So it might be closer to Earth's than if we're dealing with the carbon dioxide rich atmospheres of Venus. True it isn't necessarily the same, temperature, rotation speeds, etc, come into it too. $\endgroup$ Commented Jan 15, 2018 at 16:24
  • $\begingroup$ Related question, with the reverse situation: higher surface gravity. $\endgroup$
    – HDE 226868
    Commented Jan 15, 2018 at 19:03

2 Answers 2


There will be little difference. The lower atmospheres are the same.

Let's assume that the atmospheric pressure, $P$, follows a simple exponential scale height model: $$P=P_0\exp\left(-\frac{z}{H}\right)$$ where $P_0$ is the pressure at ground level, $z$ is altitude, and $H$ is the scale height, given by $$H\equiv\frac{kT}{Mg}\propto\frac{1}{g}$$ We can then write $$P=P_0\exp\left(-\frac{zM}{kT}g\right),\quad P'=P_0'\exp\left(-\frac{zM}{kT}g'\right)=P_0'\exp\left(-\frac{zM}{kT}0.85g\right)$$ Therefore, if we set $P_0'=P_0$, $$\frac{P}{P'}=\frac{\exp\left(-\frac{zM}{kT}g\right)}{\exp\left(-\frac{zM}{kT}0.85g\right)}=\frac{\exp\left(C\right)}{\exp\left(0.85C\right)}=\exp\left(0.15C\right)$$ where $$C\equiv-\frac{zMg}{kT}=-\frac{z}{H}$$ For $C=0$, at the surface, $P=P'$. For $C\approx-1$ - near an altitude of $8500\text{ m}$, the scale height on Earth - $P\approx0.367P'$. That's a difference of a factor of $3$! Even 1 km in the air, where $C\approx-0.118$, $P=0.889P'$.

This assumes two things:

  • $M$, the mean mass of an air molecule, is the same as on Earth.
  • $T$, the temperature, is the same as on Earth.

I think that these are both likely assumptions, given what you've said about atmospheric composition. Therefore, for small altitudes - where most birds fly - you'll see pressure differences of 10% or less. I think an exponential density model would also fit, and so the same ratios should be present for density ratios. Conditions diverge at high altitudes, and so this would affect birds that fly that high, but not for most cases.

It is important to notice that, for most cases, both lift and drag forces are linear in density; therefore, a change in density of 10% should cause a change in lift of total force on a wing by 10%. This should mean that negligible - and I'm calling these small differences "negligible" - changes in density should cause similarly negligible changes in the forces governing flight.

I've been pretty much neglecting the direct effects of gravity on the birds, skipping instead to analyzing the atmosphere. This is perhaps a bit unfair to the birds; they care not just about flying, but living in general. Bird bones, for instance - which are famously hollow - aren't shaped by the atmosphere so much as by surface gravity, which has been decreased by 15%.

I suspect the only major changes in this regard would be for bone structure. Hollow bones are more easily broken, and, in general, being a bit thicker is better when it comes to surviving accidents. Therefore, I'd predict that while birds would still have thin, lightweight bones, the bones would be a bit stronger and more resistant to breaking.

With a similar atmospheric composition and structure to Earth, this planet would not cause significant evolutionary/developmental changes in birds. Surface gravity is 15% weaker but pressure is 10% higher or less at low altitudes, and these changes might cancel out, if they are significant at all.

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    $\begingroup$ I love seeing formulas on WorldBuilding. :) $\endgroup$
    – JYelton
    Commented Jan 16, 2018 at 7:43

Weaker gravity does not corretale to thinner atmosphere. Titan is quite small, yet its atmosphere is thicker than Earth's:

Titan is 50% larger than Earth's Moon, and it is 80% more massive.


Observations from the Voyager space probes have shown that Titan's atmosphere is denser than Earth's, with a surface pressure about 1.45 atm.

Birds evolved hollow bones, air sacs, and other anatomic and physiological features that made them lighter because that makes flying less costly in terms of energy. The minimum necessary adaptations for flight could be tuned down in smaller gravity, but even then... For two birds with mostly the same size, shape and physiology, a lighter one would still spend less calories flying than a heavier one. They would probably follow the same evolutionary paths, and evolve everything that they did on real Earth.

What could go different in evolution is that perhaps we would have more non-avian flying species. And maybe squirrels, snakes and common-flying dragons could achieve true flight, just like birds.

  • $\begingroup$ I see, I assumed gravity and the mix/quantity of gases that made up the atmosphere were what controlled the pressure. I thought that a less massive planet would be able to hold less of an Earth equivalent atmosphere to itself, but I can see that if there was just more atmosphere you could end up with the about the same pressure as on Earth. Thanks! $\endgroup$ Commented Jan 15, 2018 at 14:57
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    $\begingroup$ Composition, which the OP stated was the same, comes into this a lot - you seem to have still assumed no correlation between size and atmospheric pressure. $\endgroup$ Commented Jan 15, 2018 at 16:27
  • $\begingroup$ @LioElbammalf the actual quote from OP is Oh and atmospheric composition will be very close to Earth too. Also, composition is one thing, actual mass is another. If the composition and mass are the same you get lower pressure than on Earth. However, for greater mass, the atmosphere may be as thick as that of Earth, or even thicker. Viscosity will change very little compared to Earth if the pressure is around the same, even if gravity is a little weaker. $\endgroup$ Commented Jan 15, 2018 at 16:30

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