Surface area to Volume Ratio.

A typical ant we know and love is about 5mm long and has a mass of about 5 milligrams. The giant ants you might like to have around would be 1000 times longer. Not just longer, but 1000 times wider. Not just wider, but 1000 times taller. To calculate the new mass of the giant ant we have to multiply these all togher–a billion times the volume. At the same density, a giant ant would weigh about 5 tons. But its legs would only be wider in two dimensions. They are a million times stronger, but that is not enough–for a creature a billion times heavier. Before taking their first step they would break all their legs, leaving them immoblile and harmless. While mass increases as the cube of size, the function of its structure improves only as the square, hence the name “square-cube law”.

Elephants have no problem being 5 tons. But they don’t support themselves with just the flimsy exoskeleton that suffices for ants. We and other animals our size have internal bones to support us. This is just one example of how physics determines that animals must fundamentally change their form if they are to be much larger or smaller.

So there is no need to watch Honey, I Shrunk the Kids (1989). If our bodies shrunk to the size of an ant, we would be just as hopeless as the giant ant. As warm-blooded creatures, we humans lose body heat with our surface area, which goes as the square of our linear size. Meanwhile our total body mass decreases much faster, as the cube. Even at such a miniscule size, you would never be able to eat enough to stay warm. Whales, warm-blooded mammals of the sea, benefit from growing so large in keeping warm, especially since water conducts heat away faster than air. But they have no legs to stand on, being able to rely on their buoyancy in water. So physicists could have predicted the largest mammals would live in the sea.

Worse still for the giant ants, they bring oxygen into their system directly through their exoskeleton. It is as if you could breathe directly through your skin. That’s a lot less effort than growing lungs, but that won’t work at our size….the volume needing to be sustained by air diffusing through the surface is just too large. Animals had to develop lungs in order to grow to larger. For life, form follows not function, but rather systems serve size.