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Other animals have developed features, such as the webbed feet of ducks, that assist their muscles in generating greater thrust through the water.
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Animal locomotion occurs in only a limited number of forms, including swimming, flying, walking, running, crawling, sliding, or jumping. In all cases, animals experience certain constraints to locomotion. For example, animals must overcome frictional forces (drag) generated by the air, water, or surface of the Earth. In addition, all forms of locomotion require energy to provide thrust, defined as the forward motion of an animal in any environment, and/or lift, which is movement against gravity.
In water, the greatest challenge to locomotion is the density of the water, which is much greater than that of air. The resistance to movement posed by the density of water increases exponentially as the speed of locomotion increases, which is one reason why many fish swim at relatively slow speeds. Overcoming this resistance requires considerable muscular effort. Most swimming animals, including fish, amphibians, reptiles, and marine animals have evolved streamlined bodies that reduce drag and make swimming more efficient. An energetic advantage to swimming is that fish and other swimmers do not need to provide lift to overcome gravity. Because the density of the water is similar to that of an animal's body, water provides buoyancy, which helps support the animal's weight.
The mechanism of swimming is similar among many different vertebrates. Most fish, for example, contract posterior skeletal muscles to move the tail end of the animal back and forth. This pushes water backward and propels the fish forward. Other fins provide additional thrust and enable changes in direction. Likewise, amphibians and marine reptiles rely predominantly on their hind legs for propulsion through the water. Confining most of the swimming muscles to the rear of an animal's body has other advantages. With the rear end devoted to movement, the front end is free to explore the environment, fight off aggressors, or find food.
Due to streamlining, the relatively slow speed of most swimmers, and the buoyancy of water, swimming is the cheapest form of locomotion in terms of energy spent. In contrast, locomotion on land is, on average, the energetically costliest means of locomotion. Flying may seem costlier, but it is not. Many migratory birds can travel hundreds of miles daily for many days. No terrestrial animal could possibly match such a feat by walking or running.
Whereas gravity is not an important factor for locomotion in swimming animals, terrestrial animals must overcome gravity each time they take a step. Of even greater importance to walking or running animals, though, is the necessity of accelerating and decelerating the limbs with every step. In essence, each step is like starting a movement from scratch, without the luxury of occasionally gliding through water or air as fish and birds do. This challenge is even greater when an animal moves uphill or over rough terrain. Apart from mollusks, which move along the surface of the Earth on a layer of secreted mucus, and snakes, which undulate along the ground on a portion of their ventral body surface, most terrestrial animals limit the amount of contact with the ground while moving, thereby minimizing the amount of friction they encounter.
Flying is a highly successful means of locomotion, having evolved on four occasions: in pterosaurs (extinct reptiles), insects, birds, and mammals (bats). The advantages to flying are numerous. Animals can escape land-based predators, scan their surroundings over great distances, and inhabit environments such as high cliffs that may be inaccessible to non-flying animals. The mechanisms of flying, however, require animals to overcome gravity and air resistance, which makes flying energetically costlier than swimming but still cheaper than running on land. As with swimming, resistance to flight is decreased by streamlined bodies. However, earthbound animals have one advantage over their flying cousins-they can grow to much larger sizes than animals that fly. In flying vertebrates, lift and thrust are provided by pectoral and back muscles that move the wings. The pectoral muscles are so powerful and massive that they constitute as much as 15-20 percent of a bird's total body mass and up to 30 percent in humming-birds, which use their wings not only to fly but also to hover. The requirement for larger, strong pectoral muscles is one reason why the remaining body mass of flying vertebrates is limited.
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