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Animal locomotion
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In biomechanics, animal locomotion is the study of how animals move. Most animals move in order to find food, a mate, escape predators, find suitable microhabitats, etc. The ability to do so is therefore essential to their survival, and the nature of this relationship often governs the selective pressures that shape a particular organism's locomotion.
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In biomechanics, animal locomotion is the study of how animals move. Most animals move in order to find food, a mate, escape predators, find suitable microhabitats, etc. The ability to do so is therefore essential to their survival, and the nature of this relationship often governs the selective pressures that shape a particular organism's locomotion. For instance, a migratory animal which moves hundreds or even thousands of miles (such as the Arctic Tern) is more likely to have a mechanism of locomotion which costs very little energy per unit distance, while an animal which primarily moves in order to quickly escape predators (such as a frog) is likely to have costly but very fast locomotion, since it can always eat more if it escapes.
Locomotion requires energy to overcome friction, drag, inertia, and gravity, though in many circumstances some of these factors are negligible. In terrestrial environments gravity must be overcome, though the drag of air is much less of an issue. In aqueous environments however, friction (or drag) becomes the major challenge, with gravity being less of a concern. Although animals with natural buoyancy need not expend much energy maintaining vertical position, though some will naturally sink and must expend energy to remain afloat. Drag may also present a problem in flight, and the aerodynamically efficient body shapes of birds highlight this point. Flight presents a different problem from movement in water however, as there is no way for a living organism to have lower density than air. Limbless organisms moving on land must often contend with surface friction, but do not usually need to expend significant energy to counteract gravity.
Newton's third law of motion is widely used in the study of animal locomotion: if at rest, to move forwards an animal must push something backwards. Terrestrial animals must push the solid ground, swimming and flying animals must push against a fluid or gas (either water or air). The effect of forces during locomotion on the design of the skeletal system is also important, as is the interaction between locomotion and muscle physiology, in determining how the structures and effectors of locomotion enable of limit animal movement.
Introduction
Animals move through a variety of fluids, such as water, air and mud. Some, for example seals and otters, move through more than one type of fluid. In some cases locomotion is facilitated by the substrate on which they move. Forms of locomotion include:
Through a fluid medium
Swimming
In the water staying afloat is possible through buoyancy. Provided an aquatic animal's body is no denser than its aqueous environment, it should be able to stay afloat well enough. Though this means little energy need be expended maintaining vertical position, it makes movement in the horizontal plane much more difficult. The drag encountered in water is much higher than that of air, which is almost negligible at low speeds. Body shape is therefore important for efficient movement, which is essential for basic functions like catching prey. A fusiform, torpedo-like body form is seen in many marine animals, though the mechanisms they employ for movement are diverse. Movement of the body may be from side to side, as in sharks and many fishes, or up and down, as in marine mammals. Other animals, such as those from the class Cephalopoda, use jet-propulsion, taking in water then squirting it back out in an explosive burst. Others may rely predominantly on their limbs, much as humans do when swimming. Though life on land originated from the seas, terrestrial animals have returned to an aquatic lifestyle on several occasions, such as the fully aquatic cetaceans, now far removed from their terrestrial ancestors.
Flight
Gravity is a major problem for flight through the air. Because it is impossible for any organism to approach the density of air, flying animals must generate enough lift to ascend and remain airborne. Wing shape is crucial in achieving this, generating a pressure gradient that results in an upward force on the animal' body. The same principle applies to airplanes, the wings of which are also airfoils. Unlike aircraft however, flying animals must be very light to achieve flight, the largest birds being around 20 kilograms. Other structural modifications of flying animals include reduced and redistributed body weight, fusiform shape and powerful flight muscles.
Rather than fly, some animals simply reduce their rate of falling by gliding. Flight has independently evolved at least four times, in the insects, pterosaurs, birds, and bats. Gliding has evolved on many more occasions. The advantage gliding provides to arboreal animals provides a bridge for the evolution of flight.
On a substrate
Terrestrial
Forms of locomotion on land include walking, running, hopping or jumping, and crawling or slithering. Here friction and buoyancy are not longer an issue, but a strong skeletal and muscular framework are required in most terrestrial animals for structural support. Each step also requires much energy to overcome inertia, and animals can store elastic potential energy in their tendons to help overcome this. Balance is also required for movement on land. Human infants learn to crawl first before they are able to stand on two feet, which requires good coordination as well as physical development. Humans are bipedal animals, standing on two feet and keeping one on the ground at all times while walking. When running, only one foot is on the ground at any one time at most, and both leave the ground briefly. At higher speeds momentum helps keep the body upright, so more energy can be used in movement. The number of legs an animal has varies greatly, resulting in differences in locomotion. Many familiar mammals have four legs; insects have six, while spiders have eight. Centipedes and millipedes have many sets of legs. Some have none at all, relying on other modes of locomotion.
Other animals move in terrestrial habitats without the aid of legs. Earthworms crawl by a peristalsis, the same rhythmic contractions that propel food through the digestive tract. Snakes move using several different modes of locomotion, depending upon substrate type and desired speed. Some animals even roll, though typically not as a primary means of locomotion.
Some animals are specialized for moving on non-horizontal surfaces. One common habitat for such climbing animals is in trees, for example the gibbon is specialized for arboreal movement , traveling rapidly by brachiation. Another case is animals like the snow leopard living on steep rock faces such as are found in mountains. Some light animals are able to climb up smooth sheer surfaces or hang upside down by adhesion. Many insects can do this, though much larger animals such as geckos can also perform similar feats.
On water
While animals like ducks can swim in water by floating, some small animals move across it without breaking through the surface. This surface locomotion takes advantage of the surface tension of water. Animals that move in such a way include the water strider. Water striders have legs that are hydrophobic, preventing them from interfering with the structure of water. Another form of locomotion (in which the surface layer is broken) is used by the Basilisk lizard.
Energetics
The energetics of locomotion involves the energy expenditure by animals in moving. Energy consumed in locomotion is not available for other efforts, so animals typically have evolved to use the minimum energy possible during movement. However, in the case of certain behaviors, such as locomotion to escape a predator, performance (such as speed or maneuverability) is more crucial, and such movements may be energetically expensive. Furthermore, animals may use energetically expensive methods of locomotion when environmental conditions (such as being within a tunnel) preclude other modes.
The most common metric of energy use during locomotion is net cost of transport, defined as the calories needed above baseline metabolism to move a given distance, per unit body mass. For aerobic locomotion, most animals have a nearly constant cost of transport - moving a given distance requires the same caloric expenditure, regardless of speed. This constancy is usually accomplished by changes in gait. The net cost of transport of swimming is lowest, followed by flight, with terrestrial limbed locomotion being the most expensive per unit distance. However, because of the speeds involved, flight requires the most energy per unit time. This does not mean that an animal that normally moves by running would be a more efficient swimmer, however; these comparisons assume an animal is specialized for that form of motion. Another consideration here is body mass—heavier animals, though using more total energy, require less energy per unit mass to move. Physiologists generally measure energy use by the amount of oxygen consumed, or the amount of carbon dioxide produced, in an animal's respiration.
Methods used
Several diverse methods are used to study animal locomotion, addressing various different questions.
Kinematics is the study of the patterns of movement, and is usually accomplished by videotaping an animal's movement (possibly at over 2000 frames per second for high-speed movement) then digitally tracking markers placed on the animal. From this information, one can determine velocity, acceleration, joint angles, and the timing of kinematic events relative to one another, which can quantify ecologically relevant abilities (how fast an animal can run, how steep of a slope it can climb) or can aid in formulating further hypotheses about the movement.
Electromyography (EMG) is a method of detecting the electrical activity that occurs when muscles are activated, thus determining which muscles are used when in a given movement. This can be accomplished either by surface electrodes (usually in large animals) or implanted electrodes (often wires thinner than a human hair). Furthermore, the intensity of electrical activity can correlate to the level of muscle activity, with greater activity implying (though not definitively showing) greater force.
Force plates are platforms, usually part of a trackway, which can measure the magnitude and direction of forces of an animal's step. If used with kinematics and a sufficiently detailed model of anatomy, inverse dynamics solutions can determine the forces not just at the contact with the ground, but at each joint in the limb.
Sonomicrometry uses a pair of peizo-electric crystals implanted in a muscle or tendon to continuously measure the length of a muscle or tendon. This is useful because surface kinematics may be off due to skin movement, and if an elastic tendon is in series with the muscle, the muscle length may not be accurately reflected by the joint angle.
Tendon force buckles measure the actual force produced by a single muscle by measuring the strain of a tendon. After the experiment, the tendon's elastic modulus is determined, and this allows scientists to determine the exact force a given muscle produces. However, this can only be used on muscles with long tendons.
Particle image velocimetry is used in aquatic systems to measure the flow of fluid around and past a moving aquatic organism, allowing fluid dynamics calculations to determine pressure gradients, speeds, etc.
Fluoroscopy allows real-time X-ray video, for precise kinematics of moving bones. Markers which are opaque to X-rays can allow simultaneous tracking of muscle length.
All of the methods can be combined - studies frequently combine EMG and kinematics to determine "motor pattern", the series of electrical and kinematic events which produce a given movement.
See also
Further reading
- McNeill Alexander, Robert. (2003) Principles of Animal Locomotion. Princeton University Press, Princeton, N.J. ISBN 0691086788
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