Rotating locomotion in living systems
However, true wheels and propellers—despite their utility in human vehicles—do not play a significant role in the movement of living things (with the exception of the corkscrew-like flagella of many prokaryotes).
[3][4] Several species of elongate organisms form their bodies into a loop to roll, including certain caterpillars (which do so to escape danger),[3][5] tiger beetle larvae,[6] myriapods, mantis shrimp, Armadillidiidae, and Mount Lyell salamanders.
[5][9] These species may roll passively (under the influence of gravity or wind) or actively, typically by altering their shape to generate a propulsive force.
[14] Although the Latin name rotifer means 'wheel-bearer', these organisms do not have any rotating structures, but rather a ring of rhythmically beating cilia used for feeding and propulsion.
[5] Among animals, there exists a single known example of an apparently freely rotating structure, though it is used for digestion rather than propulsion: the crystalline style of certain bivalves and gastropods.
This allows for the energetically-favorable transport of protons across the membrane into the cytoplasm (or mitochondrial matrix) through special subunits of ATP synthase, whose subsequent rotation powers the conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP)—the energy storage molecule—via the addition of one inorganic phosphate (Pi).
[33] Some eukaryotic cells, such as the protist Euglena and animal sperm, possess a convergent, evolutionarily distinct[34] flagellum-like structure known as a cilium or undulipodium.
[35]: 1105 Navicula, a type of diatom, may locomote using a band of free-flowing mucilage strands, in the manner of a tracked vehicle.
[40] Adaptations are produced incrementally through natural selection, so major phenotypic changes will not usually spread within populations if they decrease the fitness of individuals.
Richard Dawkins describes the matter: "The wheel may be one of those cases where the engineering solution can be seen in plain view, yet be unattainable in evolution because it lies [on] the other side of a deep valley, cutting unbridgeably across the massif of Mount Improbable.
The basal body that is now the rotary motor, for instance, might have evolved from a structure used by the bacterium to inject toxins into other cells.
In either a passive or driven case,[Note 2] the wheel (and possibly axle) must be able to rotate freely relative to the rest of the machine or organism.
Unlike animal joints, which have a limited range of motion, a wheel must be able to rotate through an arbitrary angle without ever needing to be "unwound".
In animals, motion is typically achieved by the use of skeletal muscles, which derive their energy from the metabolism of nutrients from food.
[53] Gerhard Scholtz of Humboldt University of Berlin asserts that a similar secreted lubricant or dead cellular material could allow a biological wheel to rotate freely.
If the tissues that make up a wheel are living, they will need to be supplied with oxygen and nutrients and have wastes removed to sustain metabolism.
[5][27] Wheels incur mechanical and other disadvantages in certain environments and situations that would represent a decreased fitness when compared with limbed locomotion.
In his book Hen's Teeth and Horse's Toes, Stephen Jay Gould explains this curiosity of history, asserting that, in the absence of maintained roads, camels required less manpower and water than a wheeled cart pulled by oxen.
This limitation of wheels can be seen in the realm of human technology: in an example of biologically inspired engineering, the forest machinery company Timberjack tested tree harvesters with six legs from 1999 to 2011.
[60]: 419 Work by vehicle engineer Mieczysław G. Bekker implies that the distribution of irregularities in natural terrains is log-normal; that is, small obstacles are far more common than larger ones.
[27] Anatomist Michael LaBarbera of the University of Chicago illustrates the poor maneuverability of wheels by comparing the turning radii of walking and wheelchair-using humans.
[63] At speed, a vehicle can become dynamically unstable—that is, it can be tipped over by an obstacle smaller than its static stability limit, or by excessive acceleration or tight turning.
[1] The hoop snake, a creature of legend in the United States and Australia, is said to grasp its tail in its mouth and roll like a wheel towards its prey.
[66] Buer, a demon mentioned in the 16th-century grimoire Pseudomonarchia Daemonum, was described and illustrated in Collin de Plancy's Dictionnaire Infernal as having radially-arranged arms on which it rolled.
[69] Rolling creatures are also featured in works by comic author Carl Barks,[70] science fiction writers Fredric Brown,[71] George R. R. Martin,[72] and Joan Slonczewski,[73][74] and in the Sonic the Hedgehog video game series.
L. Frank Baum's 1907 children's novel Ozma of Oz features humanoid creatures with wheels instead of hands and feet, called Wheelers.
[77] In the latter half of the 20th century, wheeled or wheel-using creatures featured in works by fantasy and science fiction writers including Clifford D. Simak,[78] Piers Anthony,[79] David Brin,[80] K. A. Applegate,[81] Philip Pullman,[82] and writing partners Ian Stewart and Jack Cohen.
