Animals have evolved an amazing variety of ways to get around. There are animals with no legs; animals with one appendage that serves as a “leg” (snails, clams); animals with two, four, six, or eight legs; animals with dozens of legs; even animals with hundreds of legs. There are animals that move constantly, and animals that stay in one place for their entire adult life. There are animals that swim purposefully and animals that drift wherever the currents take them. Animals slither, crawl, flap, glide, and swim. Some animals spend their entire life underground, whereas others spend almost their entire life in the air. All of these are different modes of animal locomotion.
Locomotion is not the same as movement. All animals move, but not all animals locomote. In ethology , or the study of animal behavior, locomotion is defined as movement that results in progression from one place to another. Animals that spend all or nearly all their entire adult life in one place are called sessile . Animals that move around are called motile.
Locomotion has evolved to enhance the animal’s success at finding food, reproducing, escaping predators, or escaping unsuitable habitats. Typically, the animal uses the same mode of locomotion for all these functions, but there are exceptions. For example, a squid normally swims forward or backward by undulating (rhythmically waving) finlike flaps on the sides of its body. However, when startled, the squid expels water through a nozzle and jets backward. Shrimp have a similar behavior. They normally swim using modified appendages called swimerettes. When avoiding a predator, they contract their powerful tail muscles and rapidly move backward through the water. Even some normally sessile animals use crude forms of locomotion to escape predators. Scallops can clap their shells together to produce a sort of jet propulsion. Some cnidarians (such as sea anemones) can break free from their attachment point and then use an undulating motion to swim away from a slow-moving predator.
PRINCIPLES OF LOCOMOTION
Locomotion can be passive or active. Each has its advantages and disadvantages.
Passive locomotion is the simplest form of animal locomotion. This behavior is exhibited by jellyfish and a few other animals. In this form of locomotion, the environment provides the transportation. The advantage is that no muscular effort is required.
The disadvantage of this type of locomotion is that the animal is at the whim of wind and wave. It goes where the current takes it. A somewhat different form of passive locomotion is exhibited by the remora (the name for various species of fish in the family Echeneidae ). Remora attach themselves (harmlessly) to a larger fish or sea turtle and thus go wherever the larger animal goes. However, remora are perfectly capable of swimming on their own.
Most animals exhibit active locomotion at some stage of their life cycle. To move purposefully from place to place, animals must have a means of providing propulsion and a means of controlling their movement.
In most cases animals use some sort of muscle tissue attached to a structure to contract and generate the force required to move. This muscle could be attached to a leg bone, causing the animal to jump, as in a frog, or it could contract a chamber, causing a jet of water to propel the animal, as in a squid.
The amount, type, and location of contractions are controlled by a nervous system. The nervous system can be as simple as the nerve web in hydra or as complex as the elaborate and highly specialized human nervous system. Nervous system control produces rhythmic movements of the appendages or body that result in locomotion.
Active locomotion can be appendicular or axial.
In appendicular locomotion, various appendages such as legs, wings, and flippers interact with the environment by pushing or flapping to produce the propulsive force.
Axial locomotion occurs when the animal modifies its body shape to achieve motion. For example, squid contract their large body cavity and forcefully expel water through a nozzle, producing a form of jet propulsion. Eels produce rhythmic ripples down the lengths of their bodies. Leeches stretch out their bodies, extending their anterior ends forward. They then anchor and draw their posterior ends forward by shortening and thickening their bodies.
Whether passive or active locomotion is used, the physical environments occupied by animals fall into four broad categories, each requiring unique forms of locomotion. The four environments are fossorial (underground), terrestrial (on the ground), aerial (in the air, including arboreal , on tree-dwelling), and aquatic (in the water). Each environment has similar restraints on motion: mass or inertia, gravity, and drag. Drag is any force that tends to restrict movement.
In fossorial locomotion, drag is the most important factor restricting forward motion. If the soil is very loose, some animals (insects and lizards) can “swim” through. This form of locomotion is quite rare. Most fossorial animals must burrow or dig tunnels. Some dig as they go, pushing the soil behind them. However, most fossorial animals build permanent tunnels. Once the tunnel is constructed, the mode of locomotion in the tunnel is indistinguishable from terrestrial locomotion.
Animals that spend part of their time in the air (bats, birds, flying insects) need powerful muscles to maintain flight against the force of gravity. Animals that burrow underground or that move about on the surface also require strong muscles to balance the force of gravity. Thus animals that live in aerial, fossorial, or terrestrial environments have evolved strong skeletal systems. Muscles must also overcome inertia to propel the animal forward. The more massive the animal, the more inertia it has.
Many aquatic animals are weightless in water. The buoyancy of the water exactly balances their weight. So muscular effort is not required to maintain their position. However, these animals must still exert muscular effort to initiate motion. Because water has substantial drag, muscular effort is also required to maintain motion. Some animals have negative buoyancy.
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They sink to the bottom if they stop swimming. Animals with negative buoyancy must expend muscular energy to remain at a given level in the water. An animal with positive buoyancy floats to and rests on or near the surface and must expend muscular energy to remain submerged.
Because the amount of drag due to movement through water is substantial, animals that need to move quickly must have a very streamlined shape. Drag results mainly from the friction of the water as it flows over the surface of the animal. Drag is also caused by water sticking to the surface of the animal.
Many fish have evolved a special mucous coating that protects the skin and also reduces friction. The flow of water over the skin of the animal is usually lamellar, which means different layers of the water flow at different speeds relative to the animal. The slowest layer of flow is the one next to the body surface.
Moving away from the surface, each layer moves a little faster until the speed of the water flow over the animal is matched at the last layer. Turbulence reduces lamellar flow and increases drag, ultimately limiting the speed of the animal through the water. Dolphins have evolved a gel-like layer just under the skin that tends to absorb turbulence and restores lamellar flow, thus allowing them to swim at a higher speed.
The viscosity of air is much lower than that of water, producing much less drag. However, lamellar flow of air, especially across the wing surfaces, is even more critical. Lift is provided by the shape of the wing. Lift results from air flowing faster across the upper surface than across the lower surface of the wing. Turbulence eliminates lamellar flow and lift is reduced.
Fossorial animals dig burrows, bore into the soil, or construct tunnels. Constructing tunnels or burrows requires that the material be compact and stick together. Semisolid mud or loose sand will not support a burrow. Lizards that “swim” through loose sand or amphibians that swim through mud do not leave tunnels or burrows. While these behaviors could be considered fossorial, they are not discussed here.
Burrowing invertebrates have evolved a number of ways to dig through material. Some worms use the contract-anchor-extend method of locomotion. Contraction of the muscles in the rear half of the body pushes the body forward and causes the proboscis to protrude. When the proboscis is fully extended, the worm anchors the proboscis in the soil and pulls the rest of its body forward. This process is repeated, producing a slow and erratic forward motion.
Clams and some other burrowing mollusks use a variation of the contract-anchor-extend method. They extend a muscular “foot” into the soil. Blood is pumped into the foot, causing it to swell and thus forming an anchor. Then the muscle contracts, pulling the clam down into the soil.
Many worms, such as earthworms, use peristaltic locomotion. This form of locomotion is generated by the alternation of longitudinal waves and circular-muscle-contraction waves flowing from the head to the tail. The movement is similar to the contract-anchor-extend method, but each peristaltic wave produces separate anchor points. So several segments of the worm may be moving forward at the same time.
Fossorial vertebrates include amphibians, reptiles, and mammals. Locomotion of fossorial amphibians and reptiles is usually axial. Fossorial locomotion of mammals is appendicular. Moles are a good example of fossorial mammals.
They have strong, flat forelegs with large, strong claws. Moles dig by extending a foreleg straight ahead in front of the snout and then sweeping it to each side. The loosened soil is pushed against the sidewalls of the burrow. Many rodents dig burrows for nesting but forage above ground.
These animals dig by alternately extending their forelegs forward and downward. The loosened soil is pushed backward under the body. The animal may back up through the burrow, pushing the soil out to the surface.
This is the form of locomotion humans use to get around. However, few species use the pure bipedal locomotion of humans. Most animals use four or more legs. Only arthropods and vertebrates have evolved the ability to move rapidly on the ground using legs. Both groups of animals raise their bodies above the ground and use their legs to propel themselves forward.
The legs provide both support and propulsion, so the animal must maintain balance as it moves. The sequence and patterns in which the various legs move is determined by the need to maintain balance. More legs create greater stability, but the fastest vertebrates and invertebrates use six or fewer legs.
Both arthropods and vertebrates use a similar pattern of walking or gait. A foot is planted on the ground and the body is pushed or pulled forward over the foot. The foot remains stationary as the body moves forward. Then the body remains stationary as the foot is lifted and the leg moves forward. For walking and slow running, gaits are generally symmetrical . The footfalls are regularly spaced in time. Fast-moving vertebrates, such as horses, have an asymmetrical but regularly repeating gait.
Insects tend to move their six legs in a simple pattern, lifting and replacing each leg in turn followed by the leg in front of it. Then the legs on the other side are moved. Forward motion always begins with the posterior legs. In slow walking, only one leg is lifted at a time. The limb movements of centipedes and millipedes are similar to those of insects, but with many more legs and simultaneous waves of movement that progress from the posterior end to the anterior end on both sides of the animal.
Four-legged vertebrates must synchronize leg movements to maintain balance. The basic walking pattern of all four-legged vertebrates is left hind leg, left foreleg, right hind leg, and right foreleg. This cycle is then repeated. The faster symmetrical gaits of vertebrates are obtained by overlapping the leg-movement sequences of the left and right sides.
Verterbrates that can run are known as cursorial. They have short, muscular upper legs and thin, elongated lower legs. This adaptation reduces the mass in the lower leg, allowing it to be brought forward more quickly. For slow, steady-running, cursorial vertebrates use a gait known as trotting. All-out running is known as galloping.
The gallop is an asymmetrical gait. When galloping, the animal is never supported by more than two legs. Horses at full gallop have all four legs off the ground at the same time during part of the gait. This fact was first demonstrated by Eadweard Muybridge, the American photographer and motion picture pioneer, using highspeed photography involving multiple cameras. His groundbreaking, eleven-volume work, Animal Locomotion, was published in 1899.
Cursorial birds and some lizards use bipedal locomotion. These animals have evolved large feet to increase support. The axis of the body is held perpendicular to the ground. Cursorial birds and lizards have long tails for balance, so that the center of gravity of the animal always falls between its feet. The running gait is, of course, a simple alternation of left and right legs. Lizards begin with four-footed locomotion and switch to bipedal as speed increases.
The locomotor pattern of hopping is found in both invertebrates and vertebrates. Invertebrates include a few insects, such as grasshoppers and fleas. Vertebrates include tailless amphibians, kangaroos, rabbits, and a few rodents. All hopping animals have hind legs that are approximately twice as long as the forelegs.
Frogs jump by first flexing their forelegs and tilting their bodies upward. The hind legs are swung out from the sides of the body. When the upper hind leg is perpendicular to the body, the hind leg is forcefully straightened out and the animal is launched upward at a 30° to 45° angle.
Rabbits, kangaroos, and all other mammals move their legs vertically when they jump, instead of horizontally. The hopping gait of rabbits is quadrupedal . A jumping rabbit stretches forward and lands on its forefeet. As the forefeet touch, the back flexes, and the hind end rotates forward and downward. The hind feet touch down next to the forefeet, and a new jump begins. Kangaroos take off and land on their hind feet. The back is not arched and the front legs are used only for balance. All of the muscular effort required for jumping is provided by the powerful hind legs.
THE ILLUSTRATION BELOW SHOWS HOW THE KANGAROO HOPS.
Invertebrates that crawl use either peristaltic or contract-anchor-extend locomotion. Limbless vertebrates use serpentine, rectilinear, concertina, or sidewinding locomotion. The most common pattern is serpentine locomotion, used by snakes, legless lizards, and a few other species. Rectilinear locomotion is used by most snakes, occasionally by large snakes all the time, and by fossorial limbless vertebrates when burrowing. Concertina and sidewinding locomotion are largely confined to snakes.
In serpentine (snakelike) locomotion, the body moves in a series of curves. In serpentine motion the entire body moves at the same speed. All parts of the body follow the same path as the head. Propulsion is by a lateral thrust in all segments of the body in contact with projections of the surface.
AN ILLUSTRATION OF CONCERTINA LOCOMOTION
Concertina locomotion is used when the surface is too slick for serpentine locomotion. The snake moves its body into a series of tight, wavy loops. These provide more friction on the slick surface. The snake then extends its head forward until the body is nearly straight or begins to slide backward. The snake then presses its head and upper body on the surface, forming a new frictional anchor, and pulls the posterior regions forward.
Sidewinding locomotion is a specific adaptation for crawling over loose, sandy soils. It may also have the added advantage of reducing contact with hot desert soils. Like serpentine locomotion, the entire body of the snake moves forward continuously in a series of sinuous curves. These curves are sideways to the direction of motion of the snake.
The track made by a sidewinding snake is a set of parallel curves roughly perpendicular to the direction of movement. The unique feature of sidewinding is that only two parts of the body touch the ground at any instant. The remainder of the body is held off the ground. To begin, the snake arches the front part of the body forward and forms a loop leaving only the head and the middle of the body in contact with the ground.
The snake then moves in a sinuous loop, causing the contact point to move backward along the snake’s body as each body segment loops forward. As soon as enough body length is available, the animal forms another loop and begins the next cycle. Each part of the body touches the ground only briefly before it begins to arch forward again.
In snakes, rectilinear motion is completely unlike the other forms of locomotion. The body is held relatively straight and glides forward in a manner similar to the motion of snails. The belly of the snake is covered by rows of wide, overlapping scales. Each scale is attached to two pairs of muscles, both of which are attached at an angle to ribs ahead of and behind the scale. Waves of contraction move from the front of the snake toward the back, lifting and moving each scale forward in turn. Then the scale is pulled rearward, but the edge of the scale digs into the surface, propelling the snake forward.
AN ILLUSTRATION SHOWING RECTILINEAR, SIDEWINDING AND SERPENTINE
AERIAL AND ARBOREAL LOCOMOTION
Animals have evolved many ways of moving without touching the ground. Aerial locomotion includes gliding, soaring, and true flight. Animals who move through trees are known as arboreal.
Each group of arboreal animals has a unique adaptation for climbing. Arthropods weigh little so they show few specialized climbing adaptations. Most arthropods, especially insects, can climb. The heavier vertebrates have many climbing adaptations.
Arboreal frogs and lizards are slender-bodied animals whose climbing gait is essentially the same as their terrestrial gait. The tips of the toes on arboreal frogs are expanded into large, circular disks, which increase the contact area.
The digits of arboreal lizards are spread out. On the bottom of each of these spatula-shaped digits are claws and one or two rows of elongated scales. Chameleons have two more specialized adaptations. Their tails are able to grasp objects (prehensile), and their digits have fused into two groups of opposable digits. Chameleons can tightly grasp a thin limb.
Brachiation and leaping
Most arboreal animals must occasionally leap across a gap between trees or branches. The leaping motion is essentially the same as terrestrial leaping, although landing is trickier. Brachiation is using the arms to swing from limb to limb. A few primates have developed highly specialized adaptations for brachiation, although all monkeys brachiate to some extent. Primates that use this form of locomotion have extremely long, powerful arms or forelimbs.
In gliding, the animal coasts from a high point to a low point, losing elevation constantly. Gliding animals include amphibians, reptiles, and mammals. The small animals known as flying squirrels demonstrate this behavior. A flying squirrel will climb to near the top of one tree and launch itself into space, gliding to a lower branch on the next tree, then climbing to the top and repeating the process as often as necessary. Gliders have adaptations that allow them to increase the width of their bodies. In the flying squirrels flaps of skin extend from the front limbs to the back. Frogs, snakes, and lizards are able to flatten their bodies. Some gliding lizards have elongated ribs that open like a fan.
Soaring is a very different process. Birds who are able to soar are much better gliders than any of the gliding animals. They are able to soar because of their instinctive or learned ability to take advantage of columns of rising air to gain altitude. A vulture will soar in circles in a rising column of air to a high altitude, then glide to the next rising air column. In this way, vultures are able to stay aloft for hours with almost no muscular effort.
Three living groups of animals possess true flight: insects, birds, and mammals. They can propel themselves upward and forward by flapping their wings. Each of these groups evolved this ability independently of the others. A fourth group, the extinct winged reptiles known as pterosaurs, may have been capable of true flight or only of soaring and gliding. The aerodynamics of flight are basically the same for all flying animals. However, the mechanical details are quite different among the groups. While all three groups propel themselves forward by flapping their wings, many species of birds also include extensive gliding and soaring to conserve energy.
Animals that live in aquatic environments exhibit many different forms of locomotion. Some animals crawl or burrow into the bottom of a body of water. Others swim through the water using a variety of different appendages. Still others float freely, following the currents wherever they go. Aquatic organisms range in size from microscopic to the blue whale, the largest animal that has ever lived.
Aquatic invertebrates swim through the water, crawl along the bottom, or burrow into the bottom. In swimming, muscular activity propels the animal by pushing against the water. On the bottom, muscular activity moves the animal around by interacting with the bottom. Some bottom dwellers simply crawl around on the bottom in a manner exactly like terrestrial locomotion. Others take advantage of the weightless environment to move in ways unique to the water environment.
Aquatic invertebrates have developed two distinct modes of swimming. One mode uses hydraulic propulsion. Jellyfish are a good example of this type of locomotion. They have umbrella-shaped bodies, with the “handle” of the umbrella containing the digestive system. The outer margin of the top of the umbrella, or medusa, is a band of muscles that can contract rapidly.
As the muscles contract (just like closing an umbrella) water is expelled forcefully and the jellyfish is propelled along. Scallops use a similar locomotion. They are the best swimmers among bivalves, but at its best, the motion is jerky and poorly controlled. It is used mostly to escape predators. Rapid clapping movements of the two shells create a water jet that propels the scallop.
Cephalopods, such as the squids and octopi, are also mollusks that use water-jet propulsion. Adult cephalopods have lost most of their heavy shell. Many squid are excellent swimmers and can swim forward or backward by undulating flaps along each side of their bodies. All cephalopods are much better swimmers than any other species of mollusk.
The mantle of cephalopods encloses a cavity that contains the gills and other internal organs. It also includes, on its bottom surface, a narrow opening called a siphon. When the circular muscles surrounding the cavity simultaneously contract, water is forced through the siphon. This propels the cephalopod in a direction opposite to the direction of the siphon. Thus the siphon also provides directional control.
Some fishlike animals use a purely undulatory motion to move themselves. Almost all fish use undulatory movement to some extent and supplement that motion with muscular effort by fins.
An eel swims by undulating its entire body in a series of waves passing from head to tail. This type of movement is called anguilliform (eel-like) locomotion. During steady swimming, several waves simultaneously pass down the body from head to tail. The waves move faster as they approach the animal’s tail.
While eels have a body with a fairly blunt anterior and constant diameter for the rest of the length of the body, most fish have a body that tapers at both anterior and posterior ends. For these fish, undulatory motion is not the most efficient. So most fish exhibit carangiform locomotion, in which only the rear half of the body moves back and forth.
The fastest swimming fish use this method of locomotion, so it is apparently the most efficient one. In contrast, ostraciiform locomotion uses only the tail fin to sweep back and forth. This is slower and apparently less efficient.
Whales and other cetaceans use undulatory body waves, but the waves move the whale’s body up and down instead of from side to side. The elongated tail region of whales produces a form of carangiform locomotion apparently as effective as that of the swiftest fish. Fish, whales, and other aquatic vertebrates have some arrangement of fins distributed around their bodies.
They all have a caudal (tail) fin, vertical in fish and horizontal in cetaceans. Aquatic vertebrates also have a large dorsal fin and a pair of large fins (or flippers) on the sides of their bodies close to the front. The caudal fin is the primary means of locomotion. The lateral fins do most of the steering. The dorsal fin or fins provide stability.
Tetrapodal vertebrates (four-legged vertebrates) that use undulatory locomotion include crocodilians, marine lizards, aquatic salamanders, and larval frogs. However, adult frogs and other tetrapods primarily use appendicular locomotion. Many aquatic tetrapods move primarily by using the hind legs. However, sea turtles, penguins, and fur seals have evolved short hind legs with webbed feet used primarily as rudders. These animals use their powerful forelegs, which have evolved into flippers.
Diving birds, such as cormorants and loons, are propelled by their webbed hind feet. Loons are the best adapted for diving. Their body, head, and neck are elongated and slender; the hind legs have moved far back to the posterior end of the body; the lower legs are short; and the feet are completely webbed.
Frogs and some freshwater turtles have elongated rear legs with enlarged, webbed feet. Other aquatic turtles (such as snapping turtles) are relatively poor swimmers. These turtles walk on the bottom of the lake or stream with limb movements very similar to those used on land except that they can move faster in water than they can on land.
Many mammals have swimming movements identical with their terrestrial limb movements. Most aquatic mammals—such as sea otters, hair seals, and nutria—use their hind legs and frequently their tails for swimming. The feet have some degree of webbing. Fur seals and polar bears swim mainly with forelimbs.