LOCOMOTION

in ethology, any of a variety of movements among animals that results in progression from one place to another. All locomotion systems require both propulsion and control mechanisms that vary with the animal's habitat, which may be aquatic, fossorial (underground), terrestrial, or aerial (including arboreal). In all of these situations, the forces of gravity and drag that resist upward and forward movement, respectively, are found in different degrees. Most animals have evolved muscular systems to counteract these forces as well as skeletons that prevent their bodies from collapsing. Animal locomotion is classified as either appendicular, accomplished by special appendages, or axial, achieved by changing the body shape. In passive locomotion, an animal depends on the environment for mobility; the remora, for example, attaches itself to another fish. Aquatic protozoans (one-celled animals) move by ciliary or flagellar appendages, both acting like oars, or by pseudopods, footlike appendages into which the body empties itself and then extends the appendage again. Aquatic invertebrates move along the bottom by ciliary movement or muscular contraction; by walking, like crabs and lobsters; or by the contractanchorextend method, which is best developed in leeches. Swimming may be accomplished by either hydraulic propulsion (jellyfish and scallops) or undulation (eels and fishes). Undulatory locomotion is also used by aquatic vertebrates that have walking appendages (salamanders and lizards), but most aquatic tetrapods move by the appendicular motion of hind legs or flippers. Most aquatic birds are propelled by webbed feet. Fossorial animals move by building tunnels. Soft-bodied invertebrates burrow by peristaltic action (worms) or the contractanchorextend method (clams). Some invertebrates, chiefly mollusks, can bore through rock by chemical or mechanical means. Fossorial vertebrates may be amphibians, reptiles (usually with axial locomotion), or mammals that rely mostly on strongly developed forelegs and claws. Terrestrial locomotion among invertebrates has developed mainly in the arthropods (e.g., insects, spiders, and crustaceans). Because the legs of these animals provide support as well as locomotion, limb movements must be synchronized in order to maintain the animal's balance. Locomotion takes place in two stages: the propulsive or retractive stage, in which the body pivots forward over the stationary leg and foot, and the recovery or protractive stage, beginning with foot lift-off and ending with footfall, in which the body remains stationary. Saltatory, or leaping, animals (rabbits, kangaroos, and many insects) have hind legs about twice as long as their most anterior front legs. Crawling animals move by peristaltic or contractanchorextend motion, except for those without limbs. Each group of arboreal animals has a unique method of climbing, but all have strong grasping ability and are able to maintain their centre of gravity near the object being climbed. Grasping is achieved by prehensile tails or toes or by scales that enable the animal to cling to the surface. Climbing birds hold on with their feet and brace themselves with stiffened tail feathers; some use their bills. Arboreal leaping is mechanically similar to the terrestrial type, while brachiation (swinging) is accomplished by the forelegs. Gliding may be either gravitational, used by amphibians, reptiles, and mammals, or soaring, which is confined to birds. Lift-off is achieved by flapping, and height is regulated by air currents. Soaring is either static, at relatively high levels over land, or dynamic, at lower levels and usually over water. True flight, developed in insects, birds, and mammals, is achieved by the forward thrust of lateral appendages and is independent of gravitational pull or air currents. Thrust is produced by simultaneous rotation of left and right wings in a circle or figure eight. The wing movement of insects is caused by pulsations of the thorax rather than by muscles. Patterns of flight and wing movements are distinctive for each species. Hovering flight is found in some birds, mainly hummingbirds, but most birds and bats use propulsive flight. in ethology, any of a variety of movements among animals that results in progression from one place to another. Additional reading Eadweard Muybridge, Animals in Motion (1899, reissued 1957), is a classic work on the subject. R.A.R. Tricker and B.J.K. Tricker, The Science of Movement (1966), is a useful introduction to the physics of locomotion. James Gray, Animal Locomotion (1968), provides a synthesis of most aspects of invertebrate and vertebrate locomotion. R.B. Clark, Dynamics in Metazoan Evolution (1964), discusses locomotor patterns of invertebrates with hydrostatic skeletons. Heinrich Hertel, Structure, Form, and Movement (1966; originally published in German, 1963), treats the mechanics of flight and undulatory swimming.