Size and Complexity: Why Animals Are the Way They Are

From bone strength and oxy­gen absorp­tion in lar­ger anim­als, to the per­ils of sur­face ten­sion and poor eye design in smal­ler ones: just some ideas to con­sider when study­ing com­par­at­ive ana­tomy and why anim­als are the way they are.

A per­fect take on the top­ic is J. B. S. Haldane’s 1928 On Being the Right Size. In this absorb­ing short essay, Haldane looks at why rhi­nos have short, thick legs; why the smal­lest mam­mal in Spitzber­gen is the fox; and, primar­ily, how the size of an anim­al determ­ines almost everything about its ana­tomy.

There is a force which is as for­mid­able to an insect as grav­it­a­tion to a mam­mal. This is sur­face ten­sion. A man com­ing out of a bath car­ries with him a film of water of about one-fiftieth of an inch in thick­ness. This weighs roughly a pound. A wet mouse has to carry about its own weight of water. A wet fly has to lift many times its own weight and, as every­one knows, a fly once wet­ted by water or any oth­er liquid is in a very ser­i­ous pos­i­tion indeed. An insect going for a drink is in as great danger as a man lean­ing out over a pre­cip­ice in search of food. If it once falls into the grip of the sur­face ten­sion of the water—that is to say, gets wet—it is likely to remain so until it drowns. […]

The high­er anim­als are not lar­ger than the lower because they are more com­plic­ated. They are more com­plic­ated because they are lar­ger. Just the same is true of plants.

As is typ­ic­al of Haldane, he fin­ishes with some­thing a bit more polit­ic­al than ana­tom­ic­al, stat­ing that “just as there is a best size for every anim­al, so the same is true for every human insti­tu­tion”. Some­thing to con­sider.

via The Browser