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Quadrupedalism or pronograde posture is a form of terrestrial locomotion in animals using four limbs or legs. An animal or machine that usually moves in a quadrupedal manner is known as a quadruped, meaning "four feet" (from the Latin quattuor for "four" and pes for "foot"). The majority of quadrupeds are vertebrate animals, including mammals such as cattle, dogs and cats, and reptiles such as lizards.
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Few other animals are quadrupedal, though a few birds like the shoebill sometimes use their wings to right itself after lunging at prey.
Quadrupeds vs. tetrapods
Although the words quadruped and tetrapod are both derived from terms meaning "four-footed", they have distinct meanings. A tetrapod is any member of the taxonomic unit Tetrapoda (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 quadrupeds are tetrapods.
The distinction between quadrupeds and tetrapods is important in evolutionary biology, particularly in the context of tetrapods whose limbs have adapted to other roles (e.g. hands in the case of humans, wings in the case of birds, and fins in the case of whales). All of these animals are tetrapods, but none is a quadruped. Even snakes, whose limbs have become vestigial or lost entirely, are nevertheless tetrapods.
Most quadrupedal animals are tetrapods but there are a few exceptions. For instance, among the insects, the praying mantis is a quadruped.
In humans
In July 2005, in rural Turkey, scientists discovered five Kurdish siblings who had learned to walk naturally on their hands and feet. Unlike chimpanzees, which ambulate on their knuckles, the Kurdish siblings walked on their palms, allowing them to preserve the dexterity of their fingers.
Many people, especially practitioners of Parkour and Freerunning and Georges Hébert's Natural Method, find benefit in quadrupedal movements to build full body strength. Kenichi Ito is a Japanese man famous for speed running on four limbs. Quadrupedalism is sometimes referred to as being on all fours, and is observed in crawling especially by infants.
Quadrupedal robots
BigDog is a dynamically stable quadruped robot created in 2005 by Boston Dynamics with Foster-Miller, the NASA Jet Propulsion Laboratory, and the Harvard University Concord Field Station.
Also by NASA JPL, in collaboration with University of California, Santa Barbara Robotics Lab, is RoboSimian, with emphasis on stability and deliberation. It has been demonstrated at the DARPA Robotics Challenge.