Saturday, August 01, 2009

Free wheeling

Inchworms inch. Whirligigs whirl. Sidewinders wind. Grasshoppers hop. It's called getting around, and the animal kingdom has devised a thousand ways to do it.

Fleas have springs in their legs that store up energy and then release it explosively with a trigger mechanism. Hovering insects do a thing with their wings called "clap and fling." Bushbabies leap from tree to tree with their bushy tails as balance.

Animals swim, fly, creep, slither, glide, slide, and walk on water. The only mode of transport not widely exploited by nature is the wheel, at least not this side of Dr. Seuss, who imagined a fish with a pinwheel tail.

Why not? There is no more efficient way for humans to travel than on a bike. Why didn't humans evolve with wheels? Homo bicyclus.

The answer has to do with the nature of the wheel.

A wheel (and its cousin the propeller) must turn freely and continuously about its axle. But animals need connections between their parts for the transfer of nutrients, blood, and nerve impulses.

As it happens, nature did invent the continuously turning wheel. Certain bacteria swim by spinning a single whiplike flagellum. This tiny appendage is attached to a base that fits into a cavity in the bacterium's body, and -- astonishingly -- it spins continuously, driven by a biochemical motor. Nutrients move between the bacterium and its rotating appendage by diffusion; that is, molecules drift across the gap.

But this will work only for bacteria-sized creatures. As an animal gets bigger, the ratio of surface area (length squared) to volume (length cubed) decreases. Diffusion, which depends on surfaces, can no longer supply the larger volumes with nutrients. Physical connections -- pipes and wires, if you will -- are necessary. So it would seem that Dr. Seuss's fish with a pinwheel tail is impossible.


(More tomorrow.)