Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin quattuor for "four", and pes, pedis for "foot"). Quadruped animals are found among both vertebrates and invertebrates.
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 entities that could be described as ‘quadrupedal’ are tetrapods. This last meaning includes certain artificial objects; almost all quadruped organisms are tetrapods (with the exception of some raptorial arthropods adapted for four-footed locomotion, such as the Mantodea).
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 bats, and fins in the case of whales). All of these animals are tetrapods, but none are a quadrupeds. Even snakes, whose limbs have become vestigial or lost entirely, are, nevertheless, tetrapods.
Quadrupedalism is sometimes referred to as being "on all fours", and is observed in crawling, especially by infants. [1]
In the 20th century quadrupedal movement was popularized as a form of physical exercise by Georges Hebert. [2] Kenichi Ito is a Japanese man famous for speed running on four limbs in competitions. [3]
In July 2005, in rural Turkey, scientists discovered five Turkish siblings who had learned to walk naturally on their hands and feet. Unlike chimpanzees, which ambulate on their knuckles, the Ulas family walked on their palms, allowing them to preserve the dexterity of their fingers. [4] [5] [6]
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. [7]
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. [8]
A related concept to quadrupedalism is pronogrady, or having a horizontal posture of the trunk. Although nearly all quadrupedal animals are pronograde, bipedal animals also have that posture, including many living birds and extinct dinosaurs. [9]
Nonhuman apes with orthograde (vertical) backs may walk quadrupedally in what is called knuckle-walking. [10]
Bipedalism is a form of terrestrial locomotion where a tetrapod moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped, meaning 'two feet'. Types of bipedal movement include walking or running and hopping.
Walking is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an "inverted pendulum" gait in which the body vaults over the stiff limb or limbs with each step. This applies regardless of the usable number of limbs—even arthropods, with six, eight, or more limbs, walk. In humans, walking has health benefits including improved mental health and reduced risk of cardiovascular disease and death.
Gait is the pattern of movement of the limbs of animals, including humans, during locomotion over a solid substrate. Most animals use a variety of gaits, selecting gait based on speed, terrain, the need to maneuver, and energetic efficiency. Different animal species may use different gaits due to differences in anatomy that prevent use of certain gaits, or simply due to evolved innate preferences as a result of habitat differences. While various gaits are given specific names, the complexity of biological systems and interacting with the environment make these distinctions "fuzzy" at best. Gaits are typically classified according to footfall patterns, but recent studies often prefer definitions based on mechanics. The term typically does not refer to limb-based propulsion through fluid mediums such as water or air, but rather to propulsion across a solid substrate by generating reactive forces against it.
Ichthyostega is an extinct genus of limbed tetrapodomorphs from the Late Devonian of what is now Greenland. It was among the earliest four-limbed vertebrates ever in the fossil record and was one of the first with weight-bearing adaptations for terrestrial locomotion. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. Although Ichthyostega is often labelled a 'tetrapod' because of its limbs and fingers, it evolved long before true crown group tetrapods and could more accurately be referred to as a stegocephalian or stem tetrapod. Likewise, while undoubtedly of amphibian build and habit, it is not a true member of the group in the narrow sense, as the first modern amphibians appeared in the Triassic Period. Until finds of other early stegocephalians and closely related fishes in the late 20th century, Ichthyostega stood alone as a transitional fossil between fish and tetrapods, combining fish and tetrapod features. Newer research has shown that it had an unusual anatomy, functioning more akin to a seal than a salamander, as previously assumed.
A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial (anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead. In bipedal animals with an upright posture, the term upper limb is often used.
Knuckle-walking is a form of quadrupedal walking in which the forelimbs hold the fingers in a partially flexed posture that allows body weight to press down on the ground through the knuckles. Gorillas and chimpanzees use this style of locomotion, as do anteaters and platypuses.
Euparkeria is an extinct genus of archosauriform reptile from the Triassic of South Africa. Euparkeria is close to the ancestry of Archosauria, the reptile group that includes crocodilians, pterosaurs, and dinosaurs.
Terrestrial locomotion has evolved as animals adapted from aquatic to terrestrial environments. Locomotion on land raises different problems than that in water, with reduced friction being replaced by the increased effects of gravity.
BigDog is a dynamically stable quadruped military robot that was created in 2005 by Boston Dynamics with Foster-Miller, the NASA Jet Propulsion Laboratory, and the Harvard University Concord Field Station. It was funded by DARPA, but the project was shelved after the BigDog was deemed too loud for combat.
The Ulas family of 19 is from rural southern Turkey. Five of the family members walk on all fours with their feet and the palms of their hands in what is called a "bear crawl". Their quadrupedal gait has never been reported in anatomically intact adult humans. The gait is different from the knuckle-walking quadrupedal gait of apes. In 2006, the family was the subject of a documentary: The Family That Walks On All Fours.
A facultative biped is an animal that is capable of walking or running on two legs (bipedal), as a response to exceptional circumstances (facultative), while normally walking or running on four limbs or more. In contrast, obligate bipedalism is where walking or running on two legs is the primary method of locomotion. Facultative bipedalism has been observed in several families of lizards and multiple species of primates, including sifakas, capuchin monkeys, baboons, gibbons, gorillas, bonobos and chimpanzees. Several dinosaur and other prehistoric archosaur species are facultative bipeds, most notably ornithopods and marginocephalians, with some recorded examples within sauropodomorpha. Different facultatively bipedal species employ different types of bipedalism corresponding to the varying reasons they have for engaging in facultative bipedalism. In primates, bipedalism is often associated with food gathering and transport. In lizards, it has been debated whether bipedal locomotion is an advantage for speed and energy conservation or whether it is governed solely by the mechanics of the acceleration and lizard's center of mass. Facultative bipedalism is often divided into high-speed (lizards) and low-speed (gibbons), but some species cannot be easily categorized into one of these two. Facultative bipedalism has also been observed in cockroaches and some desert rodents.
Poposaurus is an extinct genus of pseudosuchian archosaur from the Late Triassic of the southwestern United States. It belongs to the clade Poposauroidea, an unusual group of Triassic pseudosuchians that includes sail-backed, beaked, and aquatic forms. Fossils have been found in Wyoming, Utah, Arizona, and Texas. Except for the skull, most parts of the skeleton are known. The type species, P. gracilis, was described and named by Maurice Goldsmith Mehl in 1915. A second species, P. langstoni, was originally the type species of the genus Lythrosuchus. Since it was first described, Poposaurus has been variously classified as a dinosaur, a phytosaur, and a "rauisuchian".
Tripedalism is locomotion by the use of three limbs. It has been said that parrots (Psittaciformes) display tripedalism during climbing gaits, which was tested and proven in a 2022 paper on the subject, making parrots the only creatures to truly use tripedal forms of locomotion. 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 climbing in parrots, there are no known animal behaviours where the same three extremities are routinely used to contact environmental supports, 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 and hop on all three, 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 their two pelvic fins and one ray from their caudal fin.
A six-legged walking robot should not be confused with a Stewart platform, a kind of parallel manipulator used in robotics applications.
Uner Tan syndrome (UTS) is a syndrome that was discovered by the Turkish evolutionary biologist Üner Tan. People affected by UTS walk with a quadrupedal locomotion and often have severe learning disabilities. Tan postulated that this is an example of "reverse evolution" (atavism). The proposed syndrome was featured in the 2006 BBC2 documentary The Family That Walks On All Fours.
A limb is a jointed, muscled appendage of a tetrapod vertebrate animal used for weight-bearing, terrestrial locomotion and physical interaction with other objects. The distalmost portion of a limb is known as its extremity. The limbs' bony endoskeleton, known as the appendicular skeleton, is homologous among all tetrapods, who use their limbs for walking, running and jumping, swimming, climbing, grasping, touching and striking.
A hindlimb or back limb is one of the paired articulated appendages (limbs) attached on the caudal (posterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term hindleg or back leg is often used instead. In bipedal animals with an upright posture, the term lower limb is often used.
A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element capable of changing length and rotating about an omnidirectional "hip" joint.
Crawling or quadrupedal movement is a method of human locomotion that makes use of all four limbs. It is one of the earliest gaits learned by human infants, and has similar features to four-limbed movement in other primates and in non-primate quadrupeds.
Arm swing in human bipedal walking is a natural motion wherein each arm swings with the motion of the opposing leg. Swinging arms in an opposing direction with respect to the lower limb reduces the angular momentum of the body, balancing the rotational motion produced during walking. Although such pendulum-like motion of arms is not essential for walking, recent studies point that arm swing improves the stability and energy efficiency in human locomotion. Those positive effects of arm swing have been utilized in sports, especially in racewalking and sprinting.