So, this is one thing I never fully understood. There are a lot of reasons for a flying creature to be limited in mass, from energy consumption to material strength. However there seems to be a reason that goes along the lines that over a certain mass, no matter what, they can't generate enough power to counteract gravity.

What is the theoretical reason behind this limitation?

  • $\begingroup$ The design of the “wing” and its materials... $\endgroup$ – Solar Mike Sep 21 '19 at 20:11

One of the physical limits to biological flight is muscle physiology.

Muscle force output is proportional to muscle physiological cross sectional area (PCSA) multiplied by its specific tension (Gans, 1982):

$$F = \text{PCSA} \times \text{Specific Tension}$$

PCSA is basically just the cross-sectional area of a muscle adjusted for its architecture. Pennate muscles have higher PCSA than parallel fibered muscles, because more fibers are packed into the area (at the cost of shorter functional distances). Specific tension is the force output per unit area. For a single species, specific tension is essentially constant.

So the way to make more force is to make more PCSA. But muscles don't scale only in area, they scale in volume as well. Increased volume increases mass as a cubic power, but muscle area only scales as a square power. So any increase in area that requires a volume increase (which is inevitable), will quickly outstrip any gains in force output.

Note that this doesn't mean that very large flying vertebrates were not possible: the largest pterosaurs (e.g., Quetzalcoatlus) had ~15 m wingspan and perhaps weighed 200 kg and the largest birds (e.g., Argentavis) had ~7 m wingspan with a mass over 50 kg. Both, however, were likely gliders.

Gans C. 1982. Fiber architecture and muscle function. Exerc Sport Sci Rev 10:160–207.

  • $\begingroup$ There's also the aerobic/anerobic issue. If you look at the largest flighted birds, they actually use powered flight for very short periods, usually to get airborne. Then they will soar (or like the albatross, use lift from ocean waves), which uses only a miniscule amount of muscle power for control. Just as humans in sailplanes or hang gliders can stay aloft for many hours in favorable conditions, if they're towed up or launch from a high place. $\endgroup$ – jamesqf Sep 22 '19 at 4:17

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