Round and Round

A Look at Rotations in the Animal World

I spent countless hours watching one of my sons swimming his rounds in a pool and preparing for backstroke competitions (and winning quite a few of them). At that time I began to wonder about the mechanics of the “wheeling” motions of the arms of the back-stroking swimmers. Seeing how they thrashed the water reminded me of propeller propulsion. Yet, surprisingly, true rotation, in other words turning in circles relative to some other part of the body of a machine, is virtually absent from the living world.

In animals every part of the body is usually connected to the rest of the body through muscular and connective tissue, blood vessels, nerves, etc., and an arrangement whereby a separate part freely rotates upon an axle or bearing without ending up twisted is almost impossible to realize. Although some microscopic organisms spin and somersault around, Nature does not operate with true wheels. Limbs are turning about their upper ends just like the spokes of a wheel turn about the axle, the difference being that a leg, for example, turns forward about the hip joint when the foot is off the ground, while the spokes of a wheel move continuously in one direction. A rather interesting backward and forward rotation of 180 degrees, not fully explained in all detail yet, involves the bacterial light organ of the flashlight fish Anomalops katoptron: the kidney-shaped light organ under this fish’s eye is hinged and one side it emits a greenish light while on the other side it is totally black. By turning this light organ in and out, i.e., flicking it over by 180°, the fish can either project the light outward (use it at night to see its food with) or turn it inward so that no light can get out any longer. Mechanically a 180° degree alternating repetitive rotation is involved.

In terms of its potency or usefulness, there is no difference in the propulsive effects of the two methods outlined above. A set of radially arranged muscles can lead to pseudo-rotations when the muscles contract and relax sequentially, i.e., taking turns. Rolling one’s eyes with a set of six muscles is a neat example. Another comes from the microscopically small rotifers, known also by the name wheel animalcules. They possess a crown of hair-like motile cilia. The rapid and well-coordinated sequential beating of these cilia creates the illusion of a rotating wheel, hence the name rotifer.

The marine horseshoe crab, too, can exhibit pseudo-rotations. It has a long tail spine and in an attempt to right itself when placed upside down or accidentally turned over on its back by a wave, it makes its tail spine go round and round in a circular motion. The best example of a propeller-like movement in an animal, however, comes from the hippopotamus. This huge mammal uses its stiff little tail as a rapidly turning propeller to spread and distribute its semi-liquid faeces in as wide a circle as possible when it has to defecate on land. My advice to hippo lovers therefore is, beware of the front end (it can bite), but also never stand too close to the rear end of a hippo: it does not bite, but can spring some other nasty surprise!

© Dr V.B. Meyer-Rochow and, 2018.
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