Bipedalism

Bipedalism is a form of terrestrial locomotion where an animal 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.
Several groups of modern species are habitual bipeds whose normal method of locomotion is two-legged. 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. Within mammals, habitual bipedalism has evolved multiple times, with the macropods, kangaroo rats and mice, springhare, hopping mice, pangolins and hominin apes such as australopithecines, including humans, as well as many other extinct groups evolving the trait independently.
A larger number of modern species intermittently or briefly use a bipedal gait. Several lizard species move bipedally when running, usually to escape from threats. Many primate and bear species will adopt a bipedal gait in order to reach food or explore their environment, though there are a few cases where they walk on their hind limbs only. Several arboreal primate species, such as gibbons and indriids, exclusively walk on two legs during the brief periods they spend on the ground. Many animals rear up on their hind legs while fighting or copulating. Some animals commonly stand on their hind legs to reach food, keep watch, threaten a competitor or predator, or pose in courtship, but do not move bipedally.

Etymology

The word is derived from the Latin words bi 'two' and ped- 'foot', as contrasted with quadruped 'four feet'.

Advantages

Limited and exclusive bipedalism can offer a species several advantages. Bipedalism raises the head; this allows a greater field of vision with improved detection of distant dangers or resources, access to deeper water for wading animals and allows the animals to reach higher food sources with their mouths. While upright, non-locomotory limbs become free for other uses, including manipulation, flight, digging, combat or camouflage.
The maximum bipedal speed appears slower than the maximum speed of quadrupedal movement with a flexible backbone – both the ostrich and the red kangaroo can reach speeds of, while the cheetah can exceed. Even though bipedalism is slower at first, over long distances, it has allowed humans to outrun most other animals according to the endurance running hypothesis. Bipedality in kangaroo rats has been hypothesized to improve locomotor performance, which could aid in escaping from predators.

Facultative and obligate bipedalism

Zoologists often label behaviors, including bipedalism, as "facultative" or "obligate". 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.

Movement

There are a number of states of movement commonly associated with bipedalism.

Standing. Staying still on both legs. In most bipeds this is an active process, requiring constant adjustment of balance.
Walking. One foot in front of another, with at least one foot on the ground at any time.
Running. One foot in front of another, with periods where both feet are off the ground.
Jumping/hopping. Moving by a series of jumps with both feet moving together.
Skipping. A form of bipedal locomotion that combines the step and hop.
Bipedal animals

The great majority of living terrestrial vertebrates are quadrupeds, with bipedalism exhibited by only a handful of living groups. Humans, gibbons and large birds walk by raising one foot at a time. On the other hand, most macropods, smaller birds, lemurs and bipedal rodents move by hopping on both legs simultaneously. Tree kangaroos are able to walk or hop, most commonly alternating feet when moving arboreally and hopping on both feet simultaneously when on the ground.

Extant reptiles

Many species of lizards become bipedal during high-speed, sprint locomotion, including the world's fastest lizard, the spiny-tailed iguana.

Early reptiles and lizards

The first known biped is the bolosaurid Eudibamus whose fossils date from 290 million years ago. Its long hind-legs, short forelegs, and distinctive joints all suggest bipedalism. The species became extinct in the early Permian.

Archosaurs (includes crocodilians and dinosaurs)

Birds

All birds are bipeds, as is the case for all theropod dinosaurs. However, hoatzin chicks have claws on their wings which they use for climbing.

Other archosaurs

Bipedalism evolved more than once in archosaurs, the group that includes both dinosaurs and crocodilians. All dinosaurs are thought to be descended from a fully bipedal ancestor, perhaps similar to Eoraptor.
Dinosaurs diverged from their archosaur ancestors approximately 230 million years ago during the Middle to Late Triassic period, roughly 20 million years after the Permian-Triassic extinction event wiped out an estimated 95 percent of all life on Earth. Radiometric dating of fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists suspect Eoraptor resembles the common ancestor of all dinosaurs; if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.
Bipedal movement also re-evolved in a number of other dinosaur lineages such as the iguanodonts. Some extinct members of Pseudosuchia, a sister group to the avemetatarsalians, also evolved bipedal forms – a poposauroid from the Triassic, Effigia okeeffeae, is thought to have been bipedal. Pterosaurs were previously thought to have been bipedal, but recent trackways have all shown quadrupedal locomotion.

Mammals

A number of groups of extant mammals have independently evolved bipedalism as their main form of locomotion for example, humans, ground pangolins, the extinct giant ground sloths, numerous species of jumping rodents and macropods. Humans, as their bipedalism has been extensively studied, are documented in the next section. Macropods are believed to have evolved bipedal hopping only once in their evolution, at some time no later than 45 million years ago.
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, kangaroo rats and mice, hopping mice and springhare move bipedally by hopping. Very few non-primate mammals commonly move bipedally with an alternating leg gait. 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. A two-legged fox was filmed in a Derbyshire garden in 2023, most likely having been born that way.

Primates

Most bipedal animals move with their backs close to horizontal, using a long tail to balance the weight of their bodies. The primate version of bipedalism is unusual because the back is close to upright, and the tail may be absent entirely. Many primates can stand upright on their hind legs without any support.
Chimpanzees, bonobos, gorillas, gibbons and baboons exhibit forms of bipedalism. On the ground sifakas move like all indrids with bipedal sideways hopping movements of the hind legs, holding their forelimbs up for balance. Geladas, although usually quadrupedal, will sometimes move between adjacent feeding patches with a squatting, shuffling bipedal form of locomotion. However, they can only do so for brief amounts, as their bodies are not adapted for constant bipedal locomotion.
Humans are the only primates who are normally biped, due to an extra curve in the spine which shifts the center of gravity more dorsally and thus stabilizes the upright position, as well as shorter arms relative to the legs than is the case for the nonhuman great apes. The evolution of human bipedalism began in primates about four million years ago, or as early as seven million years ago with Sahelanthropus or about 12 million years ago with Danuvius guggenmosi. One hypothesis for human bipedalism is that it evolved as a result of differentially successful survival from carrying food to share with group members, although there are alternative hypotheses.
; Injured individuals
Injured chimpanzees and bonobos have been capable of sustained bipedalism.
Three captive primates, one macaque Natasha and two chimps, Oliver and Poko, were found to move bipedally. Natasha switched to exclusive bipedalism after an illness, while Poko was discovered in captivity in a tall, narrow cage. Oliver reverted to knuckle-walking after developing arthritis. Non-human primates often use bipedal locomotion when carrying food, or while moving through shallow water.