The Stomatopod and the Wheel
One of the intriguing questions about nature is the fact that no one has yet discovered an organism that actively uses wheels or wheel-like motions to get around. The simple argument of course is that it is either biologically difficult to evolve such a mechanism, or that "there ain't highways in the wild, man," and legs (or their equivalent) are the optimal way of getting around.
In science fiction, I remember at least two stories which feature aliens who make use of wheeled locomotion. The first is a short story called "Arena" by Fredric Brown, where two combatants (a human and his alien counterpart) are plucked from their corresponding space armadas by a higher race and made to fight one another. The winner gets to see his race spared, while the loser's species will be eliminated by the abductors (guess who won?)
The second story is a novel about a genetic engineer who goes from world to world peddling his wares. One of the species he volunteers to sell to the inhabitants of a world that cherishes animal vs animal blood sports is an unusual creature called a "rolleram". This animal is an immense 6 metric ton ball-like organism that can roll at up to 50 km/hr and crush its prey by running over it ("Tuf Voyaging" by George RR Martin).
Stomatopods are elongated organisms with relatively "stumpy" legs who live mainly in shallow waters. Water, of course, is denser than air, so it's no great feat for these crustaceans to zip along the ocean floor with no problems. Unfortunately, the occasional storm and whatnot may sometimes drop them right in the middle of a beach, and here the creature's swimmerets and legs are of almost no use whatsoever.
Instead of patiently waiting for the Baywatch crew to rescue them, some stomatopods have evolved a rather ingenious solution to the problem. The small stomatopod Nannosquilla decemspinosa lives along the Pacific coast of Panama, and is frequently washed on shore by waves. This mantis shrimp is even more elongated looking than the typical specimens that are sometimes found in live rock, and its legs are completely inadequate when it comes to getting the animal safely back to its ocean burrow.
|Posture of N. decemspinosa during roll maneuvers. Adapted from Full et al. (1993)|
Incredibly, N. decemspinosa moves across the sand by backwards somersaulting, moving as far as 2 meters at a time by rolling 20-40 times, with speeds of around 72 revolutions per minute at 1.5 body lengths per second (3.5 cm/s). Researchers estimate that the stomatopod functions as a true wheel around 40% of the time during this series of rolls, with the remaining 60% being those times when it has to "jumpstart" a roll by using its whole body as a single "leg" to thrust itself upwards and forwards. It must be a sight to see this little creature zooming along the beach and into the waiting water....
|Image showing movement of N. decemspinosa as it rolls along a beach. Adapted from Full et al. (1993).|
Full R, Earls K, Wong M, and Caldwell R. (1993). Locomotion like a wheel? Nature 365, 495.
Web Site Author: A. San Juan
Site Created February 3, 1998