Terrestrial locomotion in animals
A number of animals have evolved so as to be able to travel over the ground.
Terrestrial locomotion has evolved many times as animals moved onto the land from the water.
There are three basic forms of locomotion found among terrestrial animals
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Legged - Moving by using
appendages
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Slithering - moving using the bottom suface of the body
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Rolling - rotating the body over the substrate
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
stance - the way the body is supported by the legs, the number of legs - from two to many, and the
foot shape - broad, on toes, etc. There are also many
gaits - ways of moving the legs in order to locomote - such as
walking,
running, or
hopping.
Stance
Appendages can be used for movement in a number of ways. The
stance, 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 - the
sprawling stance, the
semi-erect stance, and the
fully-erect stance. Some animals may use different stances in different circumstances, depending on the stances mechanical and energetic advantages.
The most basic is the
sprawling stance. Here the legs are used to drag the body over the land. This is the earliest form of use of legs on land.
Amphibious fish such as the
mudskipper drag themselves across land on their sturdy fins. Many
reptiles and
amphibians, some or all of the time, use this method of locomotion. Among
invertebrates there is anecdotal evidence that some
octopus species (such as the
Pinnoctopus genus), sometimes to pursue prey between rockpools, can also drag themselves across land a short distance by hauling its body along by it tentacles, see [
1]. There may be video evidence of this [
2].
The second form of stance found among legged terrestrial animals is the
semi-erect stance. Here the legs are to the side, but the body is held above the substrate. This mode of locomotion found among some
reptiles and
amphibians. It is also the main stance of the
crocodilians. A few mammals, such as the
platypus also use this stance. Among the
invertebrates most
arthropods, which includes the most diverse group of animals - the
insects, have a stance which might best be described as semi-erect.
Finally there is there is the main form of stance of
mammal and
birds, the
fully-erect stance. In these groups the legs are placed beneath the body. This is often linked with the evolution of
endothermy (Bakker 1988). The fully-erect stance not-necessarily the 'most-evolved' stance, evidence suggest that crocodilians evolved a semi-erect stance from ancestors with fully-erect stance as a result of adapting to a mostly aquatic lifestyle (Reilly & Elias 1998). For example the
mesozoic prehistoric crocodilian
Erpetosuchus is believed to have had a fully erect stance and been terrestrial [
3].
Number of legs
The number of legs 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 typically
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 fair number of species move and stand on two legs, that is, are
bipedal. The group that is exclusively bipedal is the
birds, which have an alternating gait. There are also a number of bipedal mammals. Most bipedal mammals move by hopping - 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 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 are mostly
quadrupedal - the
mammals, the
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 which are quadrapeds, their front two legs having been modified for grasping, and some kinds of insect
larva who 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 have up to six additional fleshy
prolegs in addition to the six legs standard to insects. Some species of invertebrate have many more legs, the unusual
velvet worm having stubby legs under the length of its body.
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, relatives of the
centipedes. They have two pairs of legs per body segment, with common species having between 80 and 400 legs overall. However, the rare species
Illacme plenipes can have up to 750 legs. Animals with many legs typically move by waves of motion travelling over their legs.
Foot
The basic form of the vertebrate
foot is similar. However where on the foot the animals weight is placed can vary greatly between vertebrates. Most vertebrates, the amphibians, the reptiles, and some mammals such as
humans and
bears 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 [
4]. 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 posible.
Gaits
Animals show a vast range of
gaits, the order that they place and lift there legs when locomoting.
Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals.
running where at some points in the gait all feet are off the ground can be found in many animals, this is a faster but more energetically costly way of moving. There are often many different gaits intermediate between walking and running, this depends on the animals, for example the main
horse gaits are
walk,
trot,
canter, or
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 occaisionaly, for example in cramped spaces they may
crawl on all fours.
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, and
gerbils. 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.
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.
References
Charig, A.J. (1972) The evolution of the archosaur pelvis and hind-limb: an explanation in functional terms. In Studies in Vertebrate Evolution (eds K.A.Joysey and T.S.Kemp). Oliver & Boyd, Edinburgh, pp.121-55.
Reilly, Stephen M. and Elias, Jason A. 1998, Locomotion in alligator mississippiensis: kinematic effects of speed and posture and their relevance to the sprawling-to-erect paradigm, J. exp. Biol 201,2559-2574.
pdfExternal links
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Adaptations of running animals*
Crocodile stance*
Tetrapod stanceThere number of terrestrial and amphibious
limbless vertebrates and invertebrates. These animals, due to lacking appendages move by slithering on their underside. Slithering is also known as crawling, although this is also used for humans moving on all fours limbs. All limbless animals come from
poikilothemic 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 animals that slither 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 protect 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 a leeches, have suction cups on either end of their body, which allows 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 know 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, and some other worms. The wave 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 contraction down their body, the swollen sections being help 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 also use direct waves.
Most snakes move using
lateral undulation where a lateral wave travels in the opposite direction of motion and pushes the snake of irregularities in the ground. This is not effective on a very flat surface. 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 burrows or among rocks, 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, where the snake moves sideways. The snake cycles through throwing the front part of its body sideways in the direction of motion and bringing the back part of it body into line.
External links
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Lecture on crawling (slithering) at Berkeley*
Animation of earthworm movement by a propagating retrograde waveAlthough animals have never evolved
wheels for locomotion (yet
bacteria have for their
flagella), a small number of animals will move at times by rolling their whole body.
Gravity assisted
Web-toed salamander. This 10cm 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. See [
5].
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. A video of a wheeling spider in the Namib can be found at [
6]. More videos of Namib wheeling spiders, showing wheeling and wasp/spider interactions can be found at [
7]. This spider was studer by Dr Joh Henschel of the Gobabeb Training & Research Centre. See [
8].
Self-powered
Caterpillar of the
Mother-Of-Pearl Moth,
Pleurotya ruralis. This research was done by John Brackenbury at University of Cambridge in the United Kingdom. When attacked, this caterpiller will touch it 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. See [
9].
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 a low tide the 3cm 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. See [
10] and [
11]. Discovered in 1979 by Roy Caldwell, an animal behaviourist at the University of California at Berkeley
