On Being the Right Size

It’s been a while since I’ve updated. I come and go from this blog. It’s usually a combination of how much work I have and how lazy I’m feeling (I think this an issue shared by many people).

I want to write about size a little more, specifically to mention great essay by the evolutionary biologist JBS Haldane called On Being the Right Size which talks about how big animals can get.

While there are instances which Nature seems to take on a fractal attitude and repeat itself at many order of magnitude, things like animals are usually constricted to be a certain size and no bigger. The reason can be simple physics, like the ratio of the volume of a bone to its cross-sectional area, and how much weight such bones can withstand before breaking. Haldane writes spectacularly about it in his essay:

“…consider a giant man sixty feet high—about the height of Giant Pope and Giant Pagan in the illustrated Pilgrim’s Progress of my childhood. These monsters were not only ten times as high as Christian, but ten times as wide and ten times as thick, so that their total weight was a thousand times his, or about eighty to ninety tons. Unfortunately the cross sections of their bones were only a hundred times those of Christian, so that every square inch of giant bone had to support ten times the weight borne by a square inch of human bone. As the human thigh-bone breaks under about ten times the human weight, Pope and Pagan would have broken their thighs every time they took a step. This was doubtless why they were sitting down in the picture I remember. But it lessens one’s respect for Christian and Jack the Giant Killer.”

So here we have an example where the size of something is not arbitrary at all! Curiously, our size is actually set by the size of the planet. On the moon, a giant could probably walk around without breaking his thighs because of how much weaker gravity is. So this actually creates a funny inverse relationship- giant planets could probably only support very small beings (if their bones were made out of calcium like ours are) or invertebrates of some kind. Godzilla and giant monsters are out of the question! They are banished to the small moons and asteroids.