Animal locomotion

Being a sessile animal, unable to move from place to place, makes life difficult for a wide variety of reasons. For example, if food is not available in a very limited area, death may come very quickly. As a result, a great many living animals, from unicellular microbes to multicellular higher organisms, have developed the ability to move from one place to another as needed. This ability for self-directed movement is called locomotion.

Locomotion is under conscious (or voluntary) control. In essence, it serves to allow living creatures to move in chosen directions toward food, away from danger, toward mates, and so on. Locomotion is essential for the optimization of the lives of animals. This can be seen in two ways—most living animals are capable of locomotion, but it is particularly well developed in large, higher organisms.

The Physiology of Locomotion

Many unicellular animals, such as protozoa, carry out locomotion via hairlike organelles called cilia and flagella, or by amoeba-like motion. Their movement is not complex. Multicellular animals crawl, walk, run, swim, glide, or fly. The basis for these much more complex activities is the cooperative operation of their skeletons and muscles, along with other processes needed to suit each organism to its environment. In addition, nerve impulse transmission coordinates locomotion.

The skeleton and the muscles act as the levers that can move to produce body motion in chosen directions and as the machines that produce the motive ability required for locomotion. Skeletons are of two types. The external, bony, jointed exoskeletons of arthropods (insects and crustaceans) surround them like medieval suits of armor. This design limits the dexterity of arthropods and the complexity of the motion possible for them. In contrast, the bony endoskeletons of vertebrates are much more complex and produce the ability for much more complicated and dexterous locomotion.

The muscle involved in locomotion can be exemplified by vertebrate striated skeletal muscle. This type of muscle is called striated because its fibers appear to be banded when viewed under light microscopes. Such muscle is under the conscious (voluntary) control of any higher organism containing it and wishing to move toward a desired place. Striated muscle is either red or white. Red striated muscle has a very good blood supply—the basis for its color—and obtains energy during oxidizing foods into carbon dioxide and water. This occurs in its many mitochondria, subcellular organelles that carry out oxidative, energy-getting processes. White striated muscle has less blood supply and less mitochondria. It obtains most of its energy by converting food to lactic acid without much use of oxidation or mitochondria.

All locomotion uses the contraction and relaxation of muscle or related systems, which can be thought of as muscle waves. In the unicellular eukaryotes (such as protozoa), cilia and flagella are used. These structures are cell organelles that are whiplike and identical in their makeup. However, their size and number per cell differ. One protozoan type may have on its surface hundreds of cilia each ten to twenty microns long. Another species can have one or two flagella up to 250 microns long.

Both cilia and flagella cause movement by producing a wavelike motion transmitted all through the organelle outward from its base. Their action uses organized protein microtubules in a network, interconnected by arms and spokes reminiscent of fibrils and crosslinks of striated muscle. The mechanism of their action also uses a sliding mechanism initiated by a protein called dynein. Cilia and flagella are also found in multicellular eukaryotes, including humans. For example, flagella propel human sperm and are found in human lungs, where they mediate the removal of dust and dirt from the airways.

Amoeboid motion also occurs in protozoa. Here, their protoplasm flows in the direction of desired motion, forming pseudopods, followed by forward locomotion of the cell. This type of locomotion seems to depend on cytoplasm transitioning between fluid endoplasm and gel-like ectoplasm. Pseudopod formation occurs due to the action of microfilaments of actin, one of the contractile proteins also found in striated muscle. Thus, the contraction mechanism may be similar to that in striated muscle.

The various forms of locomotion include crawling, walking, running, swimming, gliding, and flying. These locomotive processes all use muscle waves based on actin and myosin fibrils. However, the complete systems involved become more and more complex due to increases in the need for locomotion, crawling on Earth’s surface, walking or running on land, swimming in lakes and oceans, and gliding or flying in the air. The increases in complexity are mostly due to the need for the involvement of more and more muscles. To aid in studying the complexity of animal locomotion, researchers developed a robot that accurately mimics these movements in any animal moving at any speed by predicting neuron and muscle patterns that control such movements. Named Drosophibot, this technology is useful in studies involving animals that are too small or too big to conduct some tests. It also has applications for understanding the locomotion of extinct animals.

Crawling, Walking, and Running

Crawling is movement on land without legs due to waves of muscular activity traveling along a crawler’s body. Two crawling creatures are snakes and snails. When a snake crawls, it curves its body into bends around stones and ground irregularities. These bends travel backward along its body in muscle waves. The waves push against the stones and irregularities and move the snake forward. Snails, in contrast, usually crawl on a long foot covered with mucus, using waves of muscle contraction that travel along the foot. Snail locomotion depends on their interesting mucus, which resists gentle pressure as if it were a solid, but, when pressed hard, behaves like a liquid. The muscle waves of snail feet are designed so that foot portions moving forward exert pressure enough to liquefy the mucus, while parts that move backward do not.

All living organisms capable of walking or running have skeletons because the processes are intricate and require the use of a relatively large number of muscles in a complex fashion. Walking is locomotion at a slow pace when there is no need for speedy movement. Running enables organisms to move much more quickly. It is done when predators threaten or if there is an unthreatening reason to travel more quickly than usual, for instance, to obtain some choice food or a mate.

The complexity of walking and running is made clear by the coordination required. For example, when four-footed animals walk, at least one forefoot and one hind foot are always kept on the ground. In walking bipeds, at least one foot is on the ground at all times, and at intervals, both feet are on the ground. In running, there are times when both feet leave the ground simultaneously. Four-footed mammals walk slowly but often attain higher speeds by trotting, cantering, or galloping. In each of these gaits, there are differences in the operation of the legs. For example, in a gallop, both forefeet are set down at one instant, and both hind feet are set down at the next instant. This is not due to chance; rather, it allows the back muscles to contribute to the work done and spread the overall effort through the body.

Another aspect of the differences between walking and running is the interaction of the feet with the ground. Some animals, including humans, walk or run with the entire bottom of the foot on the ground; this is known as plantigrade locomotion. In contrast, most quadrupeds carry out these actions with only their toes on the ground, known as digitigrade locomotion. Such differences in the ways vertebrates walk and run add to the complexity of the processes and the number of muscles used. The rate at which an animal moves also depends on its foot size. The larger the foot area that touches the ground while walking or running, the slower the animal. The complexity of running activity adds to total speed. For example, cheetahs can run at a speed of sixty-five to seventy miles per hour, partly by adding to their gait a series of leaps at chosen intervals. The added leaps help to produce the ability to move three times as fast as exceptional human athletes can run one-hundred-meter dashes.

Swimming, Gliding, and Flying

Organisms that swim in water range from tiny ciliated or flagellate microbes to blue whales that are over one hundred feet long. Their locomotive operations are complex and use oarlike rowing, hydrofoil motion of propellers, body undulation, and jet propulsion. Oar and propeller actions have some similarities, but oars push in the direction of a desired movement, while propellers work at right angles to blade motion. Propeller action is due to lift caused by hydrofoil action in the water. Propeller-mediated animal locomotion is more complex than walking or running, especially in the case of whale flukes. These hydrofoils, in addition to complex motion, must be tilted at angles that provide more lift than drag under widely varying conditions.

In animals that swim by undulation, movements are made like the crawling of a snake. Here, the resistance to motion of poorly compressible water makes it easier for the swimmer to slide forward along its long axis than sideways along its short axis. The squid exemplifies those organisms that move by jet propulsion. A squid does this by drawing water into its mantle and squirting it out forcefully from an organ called a funnel. The squirt is aimed forward when a squid wants to swim backward, or backward when it seeks to swim forward.

One of the problems associated with locomotion by swimming is that most tissues in swimming organisms are more dense than water. Such organisms would sink, lacking ways to raise body buoyancy. These needs add to the complexity associated with swimming, compared with walking or running on land. Ways in which to counter sink include swimming with all fins extended like airplane wings, as sharks do, to add buoyancy by increasing the surface area of the body. Also, swimming animals, such as the bony fish, adjust their densities to match that of the water via gas-filled swim bladders in their body cavities. Fish also may swim in groups to maximize the energy exerted in locomotion. Scientists measured energy consumption and movements, like tail beats per minute, to test the energy efficiency of swimming in groups. Results showed that energy consumption and tail beats were lower each minute in fish swimming in a group. Further research confirmed this finding, giving it the name vortex phase matching. However, climate change and pollution can alter the locomotion abilities of fish, and some toxins reduce their mobility.

Only insects, birds, and bats are capable of locomotion by true flight. However, some other animals glide for short distances. Best known are flying squirrels, which glide from tree to tree. Also, flying fish use their large pectorals as wings of a sort. They leap out of the oceans with their fins spread and glide through the air for up to half a minute. As is the case with whale flukes in water, a glider supported by lift on its wings is slowed and pulled down by drag. Thus, most gliding organisms do not stay in the air for long. Their time suspended aloft depends upon maintaining the appropriate angle of attack of their wings. They can adjust glide speed by changing these angles of attack. Increasing this angle slows them, and decreasing it allows them to speed up.

Soaring, or gliding for long distances, is possible only for animals aloft in quickly rising air. Few animals other than birds and butterflies have wings capable of sustained gliding. The needed air currents are found mostly over hillsides or coastal cliffs and in thermals, which are currents of hot air rising from the ground heated by the Sun. Vultures soaring to scan for carrion can travel for distances up to fifty miles. Butterflies, gulls, and albatrosses also soar for long distances. Long-distance soaring is very often used on migration trips to conserve energy for other activities.

Ways to increase glide or soar distance include tilting the body to suit different angles of attack and moving wings forward or backward. These actions provide streamlining and move the center of gravity of the body. Gliding and soaring animals make turns by giving one wing a higher angle of attack than the other. Furthermore, gliding is usually done with legs retracted to increase streamlining. The legs are lowered like the landing gear of airplanes when a braking effect is required. Clearly, locomotion in the air is even more difficult than swimming, involving much more complex interactions of the muscles and the skeleton.

Power, needed for most sustained flight, is obtained when flying animals flap their wings. There are two main animal flight types—high-speed flight and hovering flight. In the high-speed flight of birds, bats, and insects, the body moves forward at the same speed as the wings. The wings beat up and down as the animal moves forward through the air. Each downstroke produces lift, propels the flying animal forward, and supports it in the air. The angle of attack of the upstroke is increased to produce very little lift. This is because lift, acting upward at right angles to the wing’s path, would slow high-speed flight.

In hovering flight, the body is stationary, and the wings move very quickly, as when hummingbirds feed. Animals use two hovering techniques. Most often, their wings are kept horizontal and beat straight backward and straight forward. They turn upside down for each backstroke, adjusting the angle of attack so lift is obtained. The hum of hummingbirds is due to the frequencies with which their wings beat (twenty-five to fifty-five cycles per second). Insects buzz, whine, or drone according to the sound produced by the higher frequencies at which their wings beat. In some cases, as in pigeons, hovering is accomplished when the wings clap together at the end of an upstroke. The clap enhances airflow into the space formed as they separate and so enhances lift. Initial wing-clap hovering in pigeons causes the sound first made when a flock flies away together.

Principal Terms

Airfoil: The wing of a flying animal or airplane that provides lift or thrust needed for flight

Angle of Attack: The angle at which an airfoil meets the air passing it

Eukaryote: A higher organism whose cells have their genetic material in a membrane-bound nucleus and possess other membrane-bound organelles

Lift: The force enabling flight; it occurs when an airfoil makes air passing it move to lower air pressure above the airfoil to values below that beneath the airfoil

Locomotion: The ability of an organism to move from one place to another as needed

Mitochondrion: A subcellular organelle that converts foods to carbon dioxide, water, and energy

Sessile: An organism that is not capable of moving from its point of origin

Striated Muscle: Voluntary or skeletal muscle, capable of conscious enervation

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