The locomotor pattern of saltation (hopping) is confined mainly to kangaroos, anurans (tailless amphibians), rabbits, and some groups of rodents in the vertebrates and to a number of insect families in the arthropods. All saltatory animals have hind legs that are approximately twice as long as the anteriormost legs. Although all segments of the hind leg are elongated, two of them—the tibial (between upper segment and ankle) and tarsal (ankle) segments—are the most elongated.

There are at least four different saltatory patterns, but all are similar in that the simultaneous retraction or extension of the hind legs is followed by an aerial phase of movement. The aerial phase in all patterns is governed by the physical principles of ballistics (the flight characteristics of an object): the height and the length of the jumps are functions of the takeoff velocity and angle. The longest jumps are attained when the takeoff angle is 45°.

Before jumping, the femur (upper segment of the hind leg) of the flea is held perpendicular to the ground, the tibia extends obliquely posterior, and the remainder of the hind leg extends posteriorly along the ground. Just prior to the jump, the middle legs flex and tilt the body upward; then the femur of the hind legs swings sharply backward simultaneously with the extension of the tibia. This retraction forces the animal upward and forward at an angle of 50°. As the flea approaches touchdown, the front legs are swung forward and downward, the middle legs are held perpendicular to the body axis, and the hind legs project obliquely posterior. The anterior two pairs of legs thus act to absorb the landing shock.

The frog jump is initiated with three simultaneous movements: the forelegs flex, and the back arches to tilt the entire body upward; the tarsus of the hind leg swings to a vertical position and locks; and the femur, extending anteriorly along the body, swings in a horizontal plane. When the femur is perpendicular to the body, the knee joint snaps open, and the frog jumps forward at a 30° to 45° angle. As the frog begins to land, the forelegs are protracted and held downward in front of the chest. The forefeet touch down first, the forelegs acting as shock absorbers. Simultaneously, the hind legs are protracted so that they can be in jumping posture by the completion of landing.

The positions and movements of the hind legs in rabbits and kangaroos are similar to those of the frog. The major difference is that rabbits, kangaroos, and all other mammals move their legs in a vertical plane instead of a horizontal plane, as do the frogs; because the femur and tibia move vertically, the tarsus need not be elevated to prevent the hind leg from hitting the ground. The saltatorial gait of rabbits is quadrupedal, whereas that of kangaroos is bipedal. A jumping rabbit stretches forward and lands on its forefeet; generally, both forefeet do not touch ground simultaneously, however. As the forefeet touch, the back flexes, and the hind end rotates forward and downward. The hind feet touch down lateral to the forefeet, and, as the back extends, a new jump begins. In contrast, the kangaroo lands on its hind feet, and the back is held fairly straight through all phases of the jump, although the body inclines forward at takeoff and posteriorly when landing.

Crawling

Invertebrates crawl either by peristaltic locomotion or by contract–anchor–extend locomotion, both of which have been described previously (see above Fossorial locomotion). Limbless vertebrates, however, crawl in one of four patterns: serpentine, rectilinear, concertina, and sidewinding. The most common pattern, serpentine locomotion, is used by snakes, legless lizards, amphisbaenids (worm lizards), and caecilians (wormlike amphibians). Rectilinear locomotion is used by the giant snakes and almost exclusively by fossorial vertebrates when burrowing. Concertina and sidewinding locomotion are largely confined to snakes.

Sidewinding

Sidewinding, which is also used when the locomotor surface fails to provide a rigid frictional base, is a specific adaptation for crawling over friable sandy soils. Like serpentine locomotion but unlike concertina locomotion, the entire body of the snake moves forward continuously in sidewinding locomotion. Although the body moves through a series of sinuous curves, the track made by the snake is a set of parallel lines that are roughly perpendicular to the axis of movement. This happens because only two parts of the body touch the ground at any instant; the remainder of the body is held off the ground. To begin sidewinding, the snake arches the anterior part of the body forward and forms an elevated loop with only the head and the middle of the body in contact with the ground. Because each part of the body touches the ground only briefly before it begins to arch forward again, the snake seems to roll forward much like a short, coiled spring. In a continuously repeating cycle, as a segment arches forward, the posteriorly adjacent segment touches down.