Everything about Animal Locomotion totally explained
In
biomechanics,
animal locomotion is the study of how
animals
move. Not all animals move, but locomotive ability is widespread throughout the animal kingdom. As all animals are
heterotrophs, they must obtain food from their environment. Some animals such as
sponges are
sessile, and move the fluid in which they live through their body (this is known as
filter feeding). However, most animals must move around to find food, a mate, and so forth. Ability to do so efficiently is therefore essential to their survival.
Locomotion requires
energy to overcome
friction and often
gravity as well. In
terrestrial environments gravity must be overcome, though the friction of air is much less of an issue (except for
crawling animals like worms, for which friction is much higher). 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. Friction 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's no way for a living organism to have lower
density than air.
Much of the study is an application of
Newton's third law of motion: 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).
Classification
Animals move through a variety of fluids, such as water, air and mud. Some may move through more than one, such
seals and
otters. 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's impossibly 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're 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.
Animals that crawl or slither must use more energy due to the higher friction levels.
Earthworms crawl by a
peristalsis, the same rhythmic contractions that propel food through the digestive tract.
Snakes move differently, undulating from side to side or lifting and repositioning their
scales.
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. Animals that swim expend less energy per unit of body mass per meter traveled. Flying animals expend more, however running terrestrial animals actually expend more energy for the distance traveled than those that fly. Flying animals use the most energy per unit time, however. This doesn't 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's
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.
Energy consumed in locomotion isn't available for other efforts, so animals have evolved to be highly efficient in movement. Having said that, some animals move through different environments, such as the mudskipper pictured above, so their movement will be below optimum for any given environment. In this case the optimum reached is a trade-off between the different forms of locomotion.
Further Information
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