Terrestrial locomotion in animals

Terrestrial locomotion has evolved as animals adapted from aquatic to terrestrial environments. Locomotion on land raises different problems than that on water, with reduced friction being replaced by the effects of gravity.

There are three basic forms of locomotion found among terrestrial animals
 * Legged - Moving by using appendages
 * Limbless locomotion - moving without legs, primarily using the body itself as a propulsive structure.
 * Rolling - rotating the body over the substrate

Legged locomotion
Movement on appendages is the most common form of terrestrial locomotion, it is the basic form of locomotion of two major groups with many terrestrial members, the vertebrates and the arthropods. Important aspects of legged locomotion are posture (the way the body is supported by the legs), the number of legs, and the functional structure of the leg and foot. There are also many gaits, ways of moving the legs in order to locomote, such as walking, running, or jumping.

Posture
Appendages can be used for movement in a number of ways. The posture, the way the body is supported by the legs, is an important aspect. Charig 1972 identified three main ways in which vertebrates support themselves with their legs - sprawling, semi-erect, and fully-erect. Some animals may use different postures in different circumstances, depending on the posture's mechanical advantages. Interestingly, there is no detectable difference in energetic cost between stances.

The "sprawling" posture is the most primitive, and is the original limb posture from which the others evolved. The upper limbs are typically held horizontally, while the lower limbs are vertical, though upper limb angle may be substantially increased in large animals. The body may drag along the ground, as in salamanders, or may be substantially elevated, as in monitor lizards. This posture is typically associated with trotting gaits, and the body flexes from side-to-side during movement in order to increase step length. All limbed reptiles and salamanders use this posture, as does the platypus and several species of frogs which walk. Unusual examples can be founds among amphibious fish, such as the mudskipper, which drag themselves across land on their sturdy fins. Among the invertebrates most arthropods, which includes the most diverse group of animals - the insects, have a stance which might best be described as sprawling. There is also anecdotal evidence that some octopus species (such as the Pinnoctopus genus) can also drag themselves across land a short distance by hauling their body along by their tentacles (for example to pursue prey between rockpools) - there may be video evidence of this. The semi-erect posture is more accurately interpreted as an extremely elevated sprawling posture. This mode of locomotion is typically found in large lizards such as monitor lizards and tegus.

Mammals and birds typically have a fully-erect posture, though each evolved it independently. In these groups the legs are placed beneath the body. This is often linked with the evolution of endothermy, as it avoids Carrier's constraint and thus allows prolonged periods of activity (Bakker 1988). The fully erect stance is not necessarily the "most-evolved" stance; evidence suggests that crocodilians evolved a semi-erect stance in their forelimbs from ancestors with fully erect stance as a result of adapting to a mostly aquatic lifestyle (Reilly & Elias 1998), though their hindlimbs are still held fully erect. For example, the mesozoic prehistoric crocodilian Erpetosuchus is believed to have had a fully erect stance and been terrestrial.

Number of legs
The number of locomotory appendages varies much between animals, and sometimes the same animal may use different numbers of its legs in different circumstances. The best contender for unipedal movement is the springtail, which while normally hexapedal, hurls itself away from danger using its furcula, a tail-like forked rod that can be rapidly unfurled from the underside of its body.

A number of species move and stand on two legs, that is, they are bipedal. The group that is exclusively bipedal is the birds, which have either an alternating or a hopping gait. There are also a number of bipedal mammals. Most of these move by hopping – such as the macropods and various jumping rodents. Only a few mammals such as humans and the giant pangolin commonly show an alternating bipedal gait. Also cockroaches and some lizards may run on their two hind legs. Macropods such as kangaroos are the only mammalian example of tripedal movement. They have thick muscular tails and when moving slowly may alternate between resting their weight on their tails and their two hind legs.

With the exception of the birds, all terrestrial vertebrate groups with legs are mostly quadrupedal – the mammals, reptiles, and the amphibians usually move on four legs. There are many quadrupedal gaits. The most diverse group of animals on earth, the insects, are included in a larger taxon known as hexapods, most of which are hexapedal, walking and standing on six legs. Exceptions among the insects include praying mantises and water scorpions which are quadrapeds, their front two legs having been modified for grasping, some butterflies which use only four legs, and some kinds of insect larvae which may have no legs (e.g. maggots), or additional prolegs (e.g. caterpillars).

Spiders and many of their relatives move on eight legs - are octopedal. However, some creatures move on many more legs. Terrestrial crustaceans may have a fair number - woodlice having fourteen legs. Also, as previously mentioned, some insect larvae such as caterpillars and sawfly larvae have up to five (caterpillars) or nine (sawflies) additional fleshy prolegs in addition to the six legs normal for insects. Some species of invertebrate have even more legs, the unusual velvet worm having stubby legs under the length of its body, with around several dozen pairs of legs. Centipedes have one pair of legs per body segment, with typically around 50 legs, but some species having over 200. The terrestrial animals with the most legs are the millipedes. They have two pairs of legs per body segment, with common species having between 80 and 400 legs overall – with the rare species Illacme plenipes having up to 750 legs. Animals with many legs typically move by waves of motion travelling along their legs.

Leg and foot structure
The legs of tetrapods, the main group of terrestrial vertebrates, have internal bones, with externally attached muscles for movement, and the basic form has three key joints: the shoulder joint, the knee joint, and the ankle joint, at which the foot is attached. Within this theme there is much variation in form. An alternative form of vertebrate 'leg' to the tetrapod leg is the fins found on amphibious fish. Also a few tetrapods, such as the macropods, have adapted their tails as additional locomotory appendages.

The basic form of the vertebrate foot has five toes, however some animals will have evolved fewer than this, and some early tetrapods had more; Acanthostega had eight toes. Feet have evolved many forms depending on the animal's needs. One key variation is where on the foot the animal's weight is placed. Most vertebrates&mdash;the amphibians, the reptiles, and some mammals such as humans and bears&mdash;are plantigrade, walking on the whole of the underside of the foot. Many mammals, such as cats and dogs are digitigrade, walking on their toes, the greater stride length allowing more speed. Digitigrade mammals are also often adept at quiet movement. Birds are also digitigrade. Some animals such as horses are unguligrade, walking on the tips of their toes. This even further increases their stride length and thus their speed. A few mammals are also known to walk on their knuckles, at least for their front legs. Knuckle-walking allows the foot (hand) to specialise for food gathering and/or climbing, as with the great apes and the extinct chalicotheres, or for swimming, as with the platypus. In animals where feet have evolved into functional hands, hand walking is also possible.

Among terrestrial invertebrates there are a number of leg forms. The arthropod legs are jointed and supported by hard external armor, with the muscles attached to the internal surface of this exoskeleton. The other group of legged terrestrial invertebrates, the velvet worms, have soft stumpy legs supported by a hydrostatic skeleton. The prolegs that some caterpillars have in addition to their six more-standard arthropod legs have a similar form to those of velvet worms, and suggest a distant shared ancestry.

Gaits
Animals show a vast range of gaits, the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits, and leaping gaits. In one system, there are 60 discrete patterns: 37 walking gaits, 14 running gaits, and 9 leaping gaits.

Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. In an informal sense, running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension. Technically, however, moments of suspension occur in both running gaits (such as trot) and leaping gaits (such as canter and gallop). Gaits involving one or more moments of suspension can be found in many animals, and compared to walking they are faster but more energetically costly forms of locomotion.

Animals will use different gaits for different speeds, terrain, and situations. For example horses show four natural gaits, the slowest horse gait is the walk, then there are three faster gaits which, from slowest to fastest, are the trot, the canter, and the gallop. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main human gaits are bipedal walking and running, but they employ many other gaits occasionally, including a four-legged crawl in tight spaces.

In walking, and for many animals running, the motion of legs on either side of the body alternates, i.e. is out of phase. Other animals, such as a horse when galloping, or an inchworm, alternate between their front and back legs. An alternative to a gait which alternates between legs is hopping or saltation, where all legs move together. As a main means of locomotion, this is usually found in bipeds or semi-bipeds. Among the mammals saltation is commonly used among macropods (kangaroos and their relatives), jerboas, springhares, kangaroo rats, hopping mice, gerbils, and sportive lemurs. Certain tendons in kangaroo hind legs are very elastic, allowing kangaroos to effectively bounce along conserving energy from hop to hop, making hopping a very energy efficient way to move around in their nutrient poor environment. Saltation is also used by many small birds. Frogs and fleas also hop.

Most animals move in the direction of their head. However there are some exceptions. Crabs move sideways, and naked mole rats which live in tight tunnels underground can move backward or forward with equal facility. Crayfish can move backward much faster than they can move forward.

Gait analysis is the study of gait in humans and other animals. This may involve videoing subjects with markers on particular anatomical landmarks and measuring the forces of their footfall using floor transducers (strain gauges). Skin electrodes may also be used to measure muscle activity.

Limbless locomotion
There are a number of terrestrial and amphibious limbless vertebrates and invertebrates. These animals, due to lack of appendages, use their bodies to generate propulsive force. These movements are sometimes referred to as "slithering" or "crawling", although neither are formally used in the scientific literature and the latter term is also used for some animals moving on all four limbs. All limbless animals come from cold-blooded groups; there are no endothermic limbless animals, i.e. there are no limbless birds or mammals.

Lower body surface
Where the foot is important to the legged mammal, for limbless animals the underside of the body is important. Some animals such as snakes or legless lizards move on their smooth dry underside. Other animals have various features that aid movement. Molluscs such as slugs and snails move on a layer of mucus that is secreted from their underside, reducing friction and protecting from injury when moving over sharp objects. Earthworms have small bristles (setae) that hook into the substrate and help them move. Some animals such as leeches have suction cups on either end of the body allowing two anchor movement.

Type of movement
Some limbless animals, such as leeches, have suction cups on either end of their body, which allow them to move by anchoring the rear end and then moving forward the front end, which is then anchored and then the back end is pulled in, and so on. This is known as two-anchor movement. A legged animal, the inchworm, also moves like this, clasping with appendages at either end of its body.

Limbless animals can also move using pedal locomotary waves, rippling the underside of the body. This is the main method used by molluscs such as slugs and snails, and also large flatworms, some other worms, and even earless seals. The waves may move in the opposite direction to motion, known as retrograde waves, or in the same direction as motion, known as direct waves. Earthworms move by retrograde waves alternatively swelling and contracting down the length of their body, the swollen sections being held in place using setae. Aquatic molluscs such as limpets, which are sometimes out of the water, tend to move using retrograde waves. However terrestrial molluscs such as slugs and snails tend to use direct waves. Lugworms and seals also use direct waves.

Most snakes move using lateral undulation where a lateral wave travels down the snake's body in the opposite direction to the snake's motion and pushes the snake off irregularities in the ground. This mode of locomotion requires these irregularities to function. Another form of locomotion, rectilinear locomotion, is used at times by some snakes, especially large ones such as pythons and boa. Here large scales on the underside of the body, known as scutes are used to push backwards and downwards. This is effective on a flat surface and is used for slow, silent movement, such as when stalking prey. Snakes use concertina locomotion for moving slowly in tunnels, here the snake alternates in bracing parts of its body on it surrounds. Finally the caenophidian snakes use the fast and unusual method of movement known as sidewinding on sand or loose soil. The snake cycles through throwing the front part of its body in the direction of motion and bringing the back part of its body into line crosswise.

Rolling
Although animals have never evolved wheels for locomotion (see Rotation in living systems), a small number of animals will move at times by rolling their whole body. Rolling animals can be divided into those which roll under the force of gravity and those which roll using their own power.

Gravity assisted
Web-toed salamander. This 10 cm long salamander lives on steep hills in the Sierra Nevada mountains. When it's disturbed or startled it coils itself up into a little ball often causing it to roll down hill (García-París & Deban 1995).

Namib wheeling spiders (Carparachne spp.), found in the Namib desert, will actively roll down sand dunes. This action can be used to successfully escape predators such as the Pompilidae tarantula wasps which lays its eggs in a paralyzed spider so the larvae have enough food when they hatch. The spiders flip their body sideways and then cartwheel over their bent legs. The rotation is fast, with the golden wheel Spider (Carparachne aureoflava) moving up to 20 revolutions per second, moving the spider at 1 metre per second. At this speed the spider appears only as a blurred ball. Video of a wheeling spider in the Namib can be found at. More videos of Namib wheeling spiders, showing wheeling and wasp/spider interactions can be found at. This spider was studied by Dr Joh Henschel of the Gobabeb Training & Research Centre.

Pangolins, a type of mammal covered in thick scales rolls into a tight ball when threatened. Pangolins has been reported to roll away from danger, by both gravity and self-powered methods. A pangolin in hill country in Sumatra, in order to flee from the researcher, was observed to run to the edge of a slope and then curl into a ball and roll down the slope, crashing through the vegetation, and covering an estimated 30 metres or more in 10 seconds (Tenaza 1975).

Self-powered
Caterpillar of the Mother-Of-Pearl Moth, Pleuroptya ruralis. When attacked, this caterpillar will touch its head to its tail and roll backwards, up to 5 revolutions at about 40 cm per second, which is about 40 times its normal speed.

Nannosquilla decemspinosa, a species of long-bodied, short-legged Mantis Shrimp, lives in shallow sandy areas along the Pacific coast of Central and South America. When stranded by a low tide the 3 cm stomatopod lies on its back and performs backwards somersaults over and over. The animal moves up to 2 meters at a time by rolling 20-40 times, with speeds of around 72 revolutions per minute. That is 1.5 body lengths per second (3.5 cm/s). Researchers estimate that the stomatopod acts as a true wheel around 40% of the time during this series of rolls. The remaining 60% of the time it has to "jumpstart" a roll by using its body to thrust itself upwards and forwards.

Pangolins have also been reported to roll away from danger by self-powered methods. Witnessed by a lion researcher in the Serengeti in Africa, a group of lions surrounded a pangolin, but could not get purchase on it when it rolled into a ball, and so the lions sat around it waiting and dozing. Surrounded by lions, it would unroll itself slightly and give itself a push to roll some distance, until by doing this multiple times it could get far enough away from the lions to be safe. Moving like this would allow a pangolin to cover distance while still remaining in a protective armoured ball.

Limits and extremes

 * The fastest terrestrial animal is the cheetah, which can attain maximal sprint speeds of approximately 104 km/h (64 miles/hour).