The speed, manner, and ease with which animals move depends directly on the compactness of the material and its cohesiveness. Many aquatic animals can swim through semisolid mud or muck suspensions, which lack compactness. Some lizards and snakes that live in an arid environment can swim through friable sand, which is compact but lacks strong cohesiveness. Although these swimming movements can be considered a form of fossorial locomotion, the following discussion considers only locomotor patterns in which most of the activity of the animals involved is confined to tunnels that they leave behind.
Fossorial invertebrates
Burrowing or boring invertebrates have evolved a number of different locomotor patterns to penetrate soil, wood, and stone, of which soil or mud is the easiest to penetrate. The soft-bodied invertebrates, such as worms and sea cucumbers, burrow either by peristaltic locomotion or by the contract–anchor–extend method. Their hydrostatic, or fluid, skeleton, combined with their circular and longitudinal musculature, permits controlled deformation of their shape, which allows them to squeeze into narrow spaces and then enlarge the spaces, thus creating a burrow or tunnel. Worms with a protrusible proboscis (a tubular extension of the oral region) generally burrow by the contract–anchor–extend method. Contraction of the circular muscles in the posterior half of the body drives the body fluids forward, causing the proboscis to evert (turn outward) and forcing it into the soil. When the proboscis is fully everted, the part of the body (collar) directly behind it dilates and anchors the proboscis in the soil. The entire body is then pulled forward by the longitudinal muscles and reanchored. This pattern produces the very jerky and slow forward progression typical of most fossorial locomotion.
Peristaltic locomotion, which is generated by the alternation of longitudinal- and circular-muscle-contraction waves flowing from the head to the tail, is similar to the above pattern. Forward progression is more continuous, however, because of the contraction waves. The sites of longitudinal contraction are the anchor points; body extension is by circular contraction. The pattern of movement is initiated by anchoring the anterior end. As the longitudinal contraction wave moves posteriorly, it is slowly replaced by the circular contraction wave. The anterior end slowly and forcefully elongates, driving the tip farther into the surface as the circular contraction wave moves down the body. The tip then begins to dilate and anchor the anterior end as another longitudinal contraction wave develops. This sequence is repeated, and the worm moves forward. Reversing the direction of the contraction waves enables the worm to back up.
Burrowing bivalve mollusks, such as clams, use the contract–anchor–extend locomotor mode. Such bivalves have a large muscular foot that contains longitudinal and transverse muscles as well as a hemocoel (blood cavity). The digging cycle begins with the extension of the foot by contraction of the transverse muscles. The siphons (tubular-shaped organs that carry water to and from the gills) are closed, and the adductor muscle of the shell contracts, thereby forcing blood into the tip of the foot and causing it to dilate. With the tip acting as an anchor, the longitudinal muscles then contract, pulling the body down to the anchored foot. Frequently, the longitudinal muscles contract in short steps and alternate between the left and right sides; this causes the shell to wobble and penetrate deeper as it is pulled down.
Some invertebrates are able to bore through rock. Most of the rock borers are mollusks; they bore either mechanically by scraping or chemically by the secretion of acid. The piddock, or angel’s wing, bivalves, for example, attach themselves to a rock with a sucker-like foot. The two valves, held against the rock, grind back and forth by the alternate contraction of two adductor muscles; the grinding slowly produces a tunnel.
Fossorial vertebrates
The fossorial vertebrates are found in three classes: amphibians, reptiles, and mammals. Although some fishes and birds dig or bore shallow burrows, they can hardly be considered truly fossorial, as are moles or earthworms. Locomotion of fossorial amphibians and reptiles tends to be axial; it is appendicular only in mammals. Fossorial mammals have strong forelegs with a tendency toward flattening; their hands and particularly the claws are enlarged. Forelegs show the greatest modification in such species as moles and gophers, whose entire lives are spent in burrows. These animals tend to dig with a breast stroke, either synchronously or alternately, by extending the foreleg straight forward in front of the snout and then retracting it in a lateral arc. The loosened soil is compacted against the side walls of the burrow. In those fossorial species that dig burrows as nests but forage above the ground—many rodents, such as prairie-dogs, ground squirrels, and groundhogs—the digging movements tend to be dorsoventral with alternating limb movement. The forelegs are extended forward and then retracted downward and backward; the loosened soil passes beneath the body and is frequently pushed to the surface.
Fossorial reptiles and amphibians are usually legless, or the legs are so reduced that they serve no locomotor function; in most species, the head is flattened dorsoventrally, and the snout extends beyond and somewhat over the mouth. Burrowing is accomplished by one of three patterns analogous to the contract–anchor–extend locomotion of invertebrates. In the most common of these, the snout is driven straight forward along the bottom of the tunnel, the head is then raised, and the soil is compacted to the roof. The head tends to be laterally compressed in animals that use the other two patterns. In one of these patterns, the snout is shoved forward and then swung from side to side; in the other, the snout is rotated as it swings from side to side and seems to shave the walls of the tunnel.
Terrestrial locomotion
Walking and running
Only arthropods (e.g., insects, spiders, and crustaceans) and vertebrates have developed a means of rapid surface locomotion. In both groups, the body is raised above the ground and moved forward by means of a series of jointed appendages, the legs. Because the legs provide support as well as propulsion, the sequences of their movements must be adjusted to maintain the body’s centre of gravity within a zone of support; if the centre of gravity is outside this zone, the animal loses its balance and falls. It is the necessity to maintain stability that determines the functional sequences of limb movements, which are similar in vertebrates and arthropods. The apparent differences in the walking and slow running gaits of these two groups are caused by differences in the tetrapodal (four-legged) sequences of vertebrates and in the hexapodal (six-legged) or more sequences of arthropods. Although many legs increase stability during locomotion, they also appear to reduce the maximum speed of locomotion. Whereas the fastest vertebrate gaits are asymmetrical, arthropods cannot have asymmetrical gaits, because the movements of the legs would interfere with each other.
Cycle of limb movements
The cycle of limb movements is the same in both arthropods and vertebrates. During the propulsive, or retractive, stage, which begins with footfall and ends with foot liftoff, the foot and leg remain essentially stationary as the body pivots forward over the leg. During the recovery, or protractive, stage, which begins with foot liftoff and ends with footfall, the body remains essentially stationary as the leg moves forward. The advance of one leg is a step; a stride is composed of as many steps as there are legs. During a stride, each leg passes through one complete cycle of retraction and protraction, and the distance that the body travels is equal to the longest step in the stride. The speed of locomotion is the product of stride length and duration of stride. Stride duration is directly related to retraction: the longer the propulsive stage, the more time is required to complete a stride and the slower is the gait. A gait is the sequence of leg movements for a single stride. For walking and slow running, gaits are generally symmetrical—i.e., the footfalls are regularly spaced in time. The gaits of fast-running vertebrates, however, tend to be asymmetrical—i.e., the footfalls are irregularly spaced in time.
The different gaits of insects are based on the synchrony of leg movements on the left (L) and right (R) sides of the animal. The wave of limb movement for each side passes anteriorly; the posterior leg protracts first, then the middle leg, and finally the anterior leg, producing the sequence R3 R2 R1 or L3 L2 L1. There is no limb interference, because the legs of one side do not have footfalls along the same longitudinal axis. The slowest walking gait of insects is the sequence R3 R2 R1 followed by the sequence L3 L2 L1. As the rate of protraction increases, the protractive waves of the right and left sides begin to overlap. Eventually, the top speed is reached when the posterior and anterior legs of one side move synchronously. This gait occurs because the protraction times for all legs are constant, the intervals between posterior and middle legs and between middle and anterior legs are constant, and the interval between posterior and anterior legs decreases with faster movements. Other gaits are possible in addition to those indicated above by altering the synchrony between left and right sides.
The limb movements of centipedes and millipedes follow the same general rules as those of insects. The protraction waves usually pass from posterior to anterior. Because each leg is slightly ahead of its anteriorly adjacent leg during the locomotory cycle, one leg touches down or lifts off slightly before its anteriorly adjacent one. This coordination of limb movement produces metachronal waves, the frequency of which equals the duration of the complete protractive and retractive cycle. The length of the wave is directly proportional to the phase lag between adjacent legs.
Whereas the millipedes must synchronize leg movements to eliminate interference, the tetrapodal vertebrates must synchronize leg movements to obtain maximum stability. Four legs are the minimum requirement for symmetrical terrestrial gaits. Although bipedal (two-legged) gaits require extensive structural modifications of the body and legs, they still retain the leg-movement sequence of tetrapodal gaits. The basic walking pattern of all tetrapodal vertebrates is left hind leg (LH), left foreleg (LF), right hind leg (RH), right foreleg (RF), and then a cyclic repetition of this sequence, which is equivalent to the slow walking gait of insects but with the middle legs removed. Unlike the insects, however, vertebrates can begin to walk with any of the four legs and not just the posterior pair. The faster symmetrical gaits of vertebrates are obtained by overlapping the leg-movement sequences of the left and right sides in the same manner as insects; for example, an animal can convert a walk to a trot by moving diagonally contralateral legs (those on opposite sides) simultaneously, or to a pace by moving the ipselateral legs (those on the same side) simultaneously. Many other symmetrical gaits occur between the walk and the pace and the trot, which are extreme modifications of the walk.
Cursorial vertebrates
Cursorial (running) vertebrates are characterized by short, muscular upper legs and thin, elongated lower legs. This adaptation decreases the duration of the retractive–protractive cycle, thereby increasing the animal’s speed. Because the leg’s cycle is analogous to the swing of a pendulum, reduction of weight at the end of the leg increases its speed of oscillation. Cursorial mammals commonly use either the pace or the trot for steady, slow running. The highest running speeds, such as the gallop, are obtained with asymmetrical gaits. When galloping, the animal is never supported by more than two legs and occasionally is supported by none. The fastest runners, such as cheetahs or greyhounds, have an additional no-contact phase following hind foot contact.
In cursorial birds and lizards, both of which are bipedal, the feet are enlarged to increase support and the body axis is held perpendicular to the ground, so that the centre of gravity falls between the feet or within the foot-support zone. The running gait is, of course, a simple alternation of left and right legs. In lizards, however, bipedal running must begin with quadrupedal (four-footed) locomotion. As the lizard runs on all four legs, it gradually builds up sufficient speed so that its head end tilts up and back, after which it then runs on only its two hind legs.
