Animal locomotion

Lore & Background

Aerial locomotion includes active flight, which has evolved independently in insects, pterosaurs, birds, and bats. Gliding, a form of heavier-than-air flight without thrust, has evolved more often and is seen in animals like flying fish, flying frogs, and sugar gliders. Soaring birds use rising air currents to maintain flight without flapping. Ballooning is a method used by spiders, where they secrete silk to travel at high altitudes. Terrestrial locomotion includes walking, running, hopping, jumping, dragging, and crawling, requiring strong skeletal and muscular support.

Reader's Guide

Animal locomotion is a fundamental aspect of ethology, illustrating how diverse evolutionary pressures have shaped movement strategies across habitats. The study reveals key adaptations: aquatic animals often have fusiform bodies to reduce drag, while flying animals require lightweight structures and powerful muscles. Passive locomotion, such as sailing in jellyfish or ballooning in spiders, shows how some species exploit environmental forces. The variety of methods—from the side-to-side oscillation of fish to the jet propulsion of squid—highlights the ingenuity of natural selection. Understanding these mechanisms aids in fields like biomechanics and conservation, as locomotion is critical for survival, migration, and predator avoidance. The independent evolution of flight in multiple lineages underscores its adaptive value, while gliding's frequent emergence suggests a common pathway from arboreal life. Overall, animal locomotion demonstrates the remarkable diversity of solutions to the universal challenge of moving through different environments.

Did You Know?

• Some crabs, notably Portunidae and Matutidae, can swim using flattened walking legs.
• The stomatopod Nannosquilla decemspinosa can escape by rolling into a self-propelled wheel at 72 rpm.
• Active flight evolved independently at least four times: in insects, pterosaurs, birds, and bats.

The Spectrum of Movement Strategies

Animals employ an extraordinary range of techniques to travel from one location to another, and these fall broadly into self-propelled and passive categories. Self-propelled methods include running, swimming, jumping, flying, hopping, soaring, and gliding. Passive locomotion, by contrast, relies on the environment for transportation: some jellyfish sail on currents, spiders kite on the wind, certain beetles and spiders roll, and some organisms simply ride on the backs of other animals, a phenomenon known as phoresis. The reasons animals move are varied—seeking food, finding mates, locating suitable microhabitats, or fleeing predators—but the energy economics of movement have been profoundly shaped by natural selection. Migratory species like the Arctic tern, which cover vast distances, tend to possess locomotion mechanisms that are extremely energy-efficient per unit of distance traveled. In stark contrast, animals that must burst into speed to escape predators often rely on locomotion that is energetically expensive but delivers rapid acceleration. This trade-off between endurance and speed is one of the most fundamental design pressures in the animal kingdom.

Life in and on Water

Aquatic environments impose unique physical demands, and animals have evolved a diverse toolkit to meet them. In open water, buoyancy helps maintain vertical position, but horizontal movement is costly because water generates far more drag than air. A fusiform, torpedo-like body shape is a widespread adaptation. Fish typically produce thrust by oscillating their bodies side-to-side, the wave culminating in a large tail fin, while pectoral fins handle finer, slower maneuvers. Marine mammals undulate vertically instead. Some species, including the spotted ratfish and batiform fish like electric rays and skates, rely primarily on pectoral-fin propulsion, a style termed labriform swimming. Cephalopods, meanwhile, use jet propulsion—drawing water in and expelling it in a rapid burst. On the seafloor, echinoderms crawl via tube feet that act as suction pads, and crabs generally walk sideways because of how their legs articulate. One stomatopod, Nannosquilla decemspinosa, escapes by rolling into a self-propelled wheel and somersaulting backward at 72 rpm over distances exceeding two meters. At the surface, the by-the-wind sailor (Velella) rides the wind on a small rigid sail, while water striders exploit surface tension with hydrophobic legs to cross the water without piercing it.

Conquering the Air

Flight is arguably the most demanding form of locomotion because gravity must be continuously overcome. No organism can match the density of air, so flying animals must generate sufficient lift to remain airborne. Wings moving through the air produce an upward force, but the animals themselves must be extraordinarily light—the heaviest living fliers are birds weighing around 20 kilograms. Beyond lightness, flying animals benefit from reduced and redistributed body mass, a fusiform body shape, and powerful flight muscles, sometimes supplemented by physiological adaptations. This repeated, independent origin underscores how strongly natural selection favors the ability to take to the skies. For animals that cannot sustain active flight, gliding offers a partial alternative. Semi-arboreal species reduce their rate of descent by spreading out and catching air, a form of heavier-than-air flight that requires no wing flapping.

The Machinery of Motion

The anatomical structures animals deploy for movement—cilia, legs, wings, arms, fins, tails—are often collectively called locomotory organs or locomotory structures, and their design is intimately tied to the physical medium in which the animal operates. In water, where drag is substantial, morphology becomes critical for efficiency; the fusiform shape seen in so many aquatic species is a direct response to the need to cut through a dense fluid. In air, the priority shifts to minimizing weight while maximizing lift, which is why flying animals carry powerful flight muscles yet must keep overall mass low. On land and at the water's surface, the challenge is different again: water striders, for instance, have evolved hydrophobic legs that let them exploit surface tension without breaking the water's skin, while the basilisk lizard takes a different approach by actively breaking through the surface layer. Even in the most constrained environments, such as the seafloor, specialized structures like the tube feet of echinoderms or the flattened swimming paddles of Portunidae crabs demonstrate how form follows the specific mechanical demands of the habitat. In every case, natural selection has fine-tuned these locomotory structures to balance the competing needs of speed, endurance, and maneuverability.