Quadrupedalism is a form oflocomotion in which animals have four legs that are used tobear weight and move around. Ananimal ormachine that usually maintains a four-legged posture and moves using all four legs is said to be aquadruped (fromLatinquattuor for "four", andpes,pedis for "foot"). Quadruped animals are found among bothvertebrates andinvertebrates.
Although the words 'quadruped' and 'tetrapod' are both derived from terms meaning 'four-footed', they have distinct meanings. Atetrapod is any member of thetaxonomic unitTetrapoda (which is defined by descent from a specific four-limbed ancestor), whereas a quadruped actually uses four limbs for locomotion. Not all tetrapods are quadrupeds and not all quadrupedal animals are tetrapods; some arthropods are adapted for four-footed locomotion, such as theraptorialMantodea, or mantises, and theNymphalidae, or brush-footed butterflies—the largest butterfly family, with ~6000 species, including the well-knownmonarch (see photo).
The distinction between quadrupeds and tetrapods is important inevolutionary biology, particularly in the context of tetrapods whose limbs have adapted to other roles (e.g., arms and hands in the case of humans, wings in the case of birds and bats, and fins in the case of whales). All of these animals are tetrapods, but not all are quadrupeds. Even snakes, whose limbs have becomevestigial or lost entirely, are, nevertheless, tetrapods.
Quadrupedalism is sometimes referred to as being "on all fours", and is observed incrawling, especially by infants.[1]
In the 20th century quadrupedal movement was popularized as a form of physical exercise byGeorges Hebert.[2]Kenichi Ito is a Japanese man famous for speed running on four limbs in competitions.[3]
In July 2005, in ruralTurkey, scientists discovered fiveTurkish siblings who habitually walked on both their hands and feet. Unlikechimpanzees, which ambulateon their knuckles, theUlas family walked on their palms, allowing them to preserve the dexterity of their fingers.[4][5][6]
BigDog is a dynamically stable quadrupedrobot created in 2005 byBoston Dynamics withFoster-Miller, theNASAJet Propulsion Laboratory, and theHarvard University Concord Field Station.[7] Its successor was Spot.
Also by NASA JPL, in collaboration withUniversity of California, Santa Barbara Robotics Lab, is RoboSimian, with emphasis on stability and deliberation. It has been demonstrated at theDARPA Robotics Challenge.[8]
A related concept to quadrupedalism is pronogrady, or having a horizontal posture of thetrunk. Although nearly all quadrupedal animals are pronograde, bipedal animals also have that posture, including many living birds and extinct dinosaurs.[9]
Nonhuman apes withorthograde (vertical) backs may walk quadrupedally in what is calledknuckle-walking.[10]