question: How do four legged animals not constantly trip?
comment: It's the same reason that you can walk without constantly looking down at your feet. You see the terrain ahead of you and can predict when you will be walking over it and adjust your steps without having to directly watch each step.
passages: There are no three-legged animals—though some macropods, such as kangaroos, that alternate between resting their weight on their muscular tails and their two hind legs, could be looked at as an example of tripedal locomotion in animals.
   Section::::Passive locomotion.:Animal transport.
  
  Some animals change location because they are attached to, or reside on, another animal or moving structure. This is arguably more accurately termed "animal transport".
  
  Section::::Passive locomotion.:Animal transport.:Remoras.
   BULLET::::- Detour behaviour Some animals appear to have an advanced understanding of their spatial environment and will not take the most direct route if this confers an advantage to them. Some jumping spiders take an indirect route to prey rather than the most direct route, thereby indicating flexibility in behaviour and route planning, and possibly insight learning.
  
  Section::::Research questions.:Spatial cognition.:Long-distance navigation; homing.
   The medial pre-motor cortex seems to be specialized for initiating movements specified by internal rather than external cues. These movements based on internal events are called "open-loop" conditions. In contrast to lesions in the lateral pre-motor area, removal of this medial pre-motor area reduces the number of self initiated or spontaneous movements that the animal makes. Conversely, the ability to move in response to an external cue is largely intact.
  
  Section::::Parietal area 5.
   Tripedalism (from the Latin tri = three + ped = foot) is locomotion by the use of three legs. It has been conjectured that parrots (Psittaciformes) display tripedalism during climbing gaits, though this has not yet been documented thoroughly in scientific literature. Tripedal gaits were also observed by K. Hunt in primates. This is usually observed when the animal is using one limb to grasp a carried object and is thus a non-standard gait. Apart from the parrot conjecture, there are no known species where three legs are standard, although the movement of some macropods such as kangaroos, which can alternate between resting their weight on their muscular tails and their two hind legs, may be an example of tripedal locomotion in animals. There are also the tripod fish. Several species of these fish rest on the ocean bottom on two rays from its two pelvic fins and one ray from its caudal fin.
   Since Darwin's "On the Origins of Certain Instincts" (quoted above) in 1873, path integration has been shown to be important to navigation in animals including ants, rodents and birds. When vision (and hence the use of remembered landmarks) is not available, such as when animals are navigating on a cloudy night, in the open ocean, or in relatively featureless areas such as sandy deserts, path integration must rely on "idiothetic" cues from within the body.
   Section::::Uses of Rectilinear Locomotion.
  
  This method of locomotion is extremely slow (between 1–6 cm per second), but is also almost noiseless and very hard to detect, making it the mode of choice for many species when stalking prey. It is primarily used when the space being traversed is too constricting to allow for other forms of movement. When climbing, snakes will often use rectilinear locomotion in conjunction with concertina movements to exploit terrain features such as interstices in the surfaces they are climbing.
  
  Section::::In Robotics.
   It is found in lowland rain forest, in shrubbery, palms and trees, especially in the vicinity of water. It is almost always found in bushes and trees along streams or along the edges for forest clearings, mostly associated with primary forest, although has also been found in older secondary forest near primary forest.
  
  Section::::Behaviour.
  
  Nocturnal, this species spends the day hidden in thick foliage, tree hollows, or at the base of palm fronds, and always in places where it can anchor itself with its prehensile tail. It tends to rely on ambush instead of actively hunting for prey.
  
  Section::::Feeding.
   Section::::Legged locomotion.: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.
   Zoologists often label behaviors, including bipedalism, as "facultative" (i.e. optional) or "obligate" (the animal has no reasonable alternative). Even this distinction is not completely clear-cut — for example, humans other than infants normally walk and run in biped fashion, but almost all can crawl on hands and knees when necessary. There are even reports of humans who normally walk on all fours with their feet but not their knees on the ground, but these cases are a result of conditions such as Uner Tan syndrome — very rare genetic neurological disorders rather than normal behavior. Even if one ignores exceptions caused by some kind of injury or illness, there are many unclear cases, including the fact that "normal" humans can crawl on hands and knees. This article therefore avoids the terms "facultative" and "obligate", and focuses on the range of styles of locomotion "normally" used by various groups of animals. Normal humans may be considered "obligate" bipeds because the alternatives are very uncomfortable and usually only resorted to when walking is impossible.
   With the exception of the birds, terrestrial vertebrate groups with legs are mostly quadrupedal – the mammals, reptiles, and the amphibians usually move on four legs. There are many quadrupedal gaits.
   Few modern species are habitual bipeds whose normal method of locomotion is two-legged. Within mammals, habitual bipedalism has evolved multiple times, with the macropods, kangaroo rats and mice, springhare, hopping mice, pangolins and hominin apes (australopithecines and humans) as well as various other extinct groups evolving the trait independently. In the Triassic period some groups of archosaurs (a group that includes crocodiles and dinosaurs) developed bipedalism; among the dinosaurs, all the early forms and many later groups were habitual or exclusive bipeds; the birds are members of a clade of exclusively bipedal dinosaurs, the Theropods.
   Section::::Sea turtles.
  
  There are several different kinds of marine animals that demonstrate natal homing. The most commonly known is the sea turtle. Loggerhead sea turtles are thought to show two different types of homing. The first of which comes in the early stages of life. When first heading out to sea, the animals are carried out by tides and currents with little swimming involved. Recent studies now show that the animals demonstrate homing to feeding grounds near their natal birthplace.
   Dead reckoning, in animals usually known as path integration, means the putting together of cues from different sensory sources within the body, without reference to visual or other external landmarks, to estimate position relative to a known starting point continuously while travelling on a path that is not necessarily straight. Seen as a problem in geometry, the task is to compute the vector to a starting point by adding the vectors for each leg of the journey from that point.
   Modern birds, though classified as tetrapods, usually have only two functional legs, which some (e.g., ostrich, emu, kiwi) use as their primary, Bipedal, mode of locomotion. A few modern mammalian species are habitual bipeds, i.e., whose normal method of locomotion is two-legged. These include the macropods, kangaroo rats and mice, springhare, hopping mice, pangolins and homininan apes. Bipedalism is rarely found outside terrestrial animals—though at least two types of octopus walk bipedally on the sea floor using two of their arms, so they can use the remaining arms to camouflage themselves as a mat of algae or floating coconut.
   Section::::Behavior.
  
  Unlike the two-toed sloth, three-toed sloths are agile swimmers. They are still slow in trees. The offspring cling to their mother's bellies for around nine months. They cannot walk on all four limbs, so they must use their front arms and claws to drag themselves across the rainforest floor.
   Section::::Behavior.
  
  The bipedal locomotion of jerboas involves hopping, skipping and running gaits. It is associated with rapid and frequent, difficult-to-predict changes in speed and direction, facilitating predator evasion relative to quadrupedal locomotion. This may explain why evolution of bipedal locomotion is favored in desert dwelling rodents that forage in open habitats.
   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.
   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.
   Section::::Biological barriers to wheeled organisms.:Developmental and anatomical constraints.:Friction.
   Section::::Biological barriers to wheeled organisms.:Developmental and anatomical constraints.:Nutrient and waste transfer.
   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.
  
  Section::::Limbless locomotion.:Lower body surface.
   Bipedal movement is less common among mammals, most of which are quadrupedal. All primates possess some bipedal ability, though most species primarily use quadrupedal locomotion on land. Primates aside, the macropods (kangaroos, wallabies and their relatives), kangaroo rats and mice, hopping mice and springhare move bipedally by hopping. Very few mammals other than primates commonly move bipedally by an alternating gait rather than hopping. Exceptions are the ground pangolin and in some circumstances the tree kangaroo. One black bear, Pedals, became famous locally and on the internet for having a frequent bipedal gait, although this is attributed to injuries on the bear's front paws.
   Crocodilians use a "high walk", with a more erect limb posture that minimizes sideways flexing, to cross long distances. However, as they evolved from upright walkers with limited bipedality, this may simply be a remnant of past behavior rather than a specific adaptation to overcome this difficulty. Todd J. Uriona (University of Utah) hypothesizes that costal ventilation may have aided the upright posture in overcoming the constraint.
  
  Section::::Solutions.:Avoiding the constraint.
   Even though plantigrade locomotion usually distributes more weight toward the end of the limb than digitigrade locomotion, which increases energy expenditure in most systems, studies have shown that the human heel-first gait conserves more energy over long distances than other gaits, which is consistent with the belief that humans are evolutionarily specialized for long-distance movement.
  