So many people, so few predators-- in the sky

People are generally slow. And if fat is flavor, as great cooks have been heard to say, we’re probably pretty tasty. So, why no giant hawks that swoop down and carry us away?

Didn’t there used to be giant pterodactyls? Yes. The largest one was as tall as a giraffe. Even then, it would’ve taken two of them to lift a whole human. So is there a biological limit to the size of a flying creature, and why would that be?

For anything that flies, keeping those flapping muscles cool is a big deal. Flight is hard work. Hollow bones, multi-chambered lungs for constant flow, and very large chest muscles are among the evolutionary advantages. It’s those muscles that are the key.

Surface area provides the cooling needed, so having enough surface area for the muscle to work and not overheat is a limiting factor. The relationship of volume to surface area becomes the question. Think of how a small wire has very little volume inside. It’s almost all surface area. That’s why multi-stranded wire was invented. Electrons travel on the surface of a wire. A big single strand is far less efficient than a multi-strand of the same size. I digress. Compare wire with a large drain pipe. The volume increases rapidly, but the surface area does not increase as fast.

Volume is cubic space, and surface area is square space. Here’s how that plays out:

As you can tell, as a muscle gets bigger, the surface area shrinks in proportion. Eventually there’s not enough skin surface to cool the muscle volume needed for flight. And that’s why, thankfully, there are no giant airborne predators hunting our cities and towns.