Facultative bipedalism

Animal capable of walking or running on two legs in response to exceptional circumstances
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.

Within the category of bipedal locomotion, there are four main techniques: walking, running, skipping, and galloping. Walking is when the footfalls have an evenly spaced gait and one foot is always on the ground. orangutans, gorillas, and gibbons. Humans are obligate bipeds, not facultative bipeds. and baboons. Among strepsirrhines, it is found in sifakas

The sifaka (Propithecus), which is a type of lemur native to the island of Madagascar, is one of the primary examples of facultative bipedalism. While moving through the trees, they locomote using a vertical clinging and leaping strategy. On the ground, they can walk on their two hind legs as a way to conserve energy. Immature baboons seem to be more bipedal than adults. These bipedal postures and locomotion in infants, although infrequent, seem to clearly distinguish them from adult baboons in terms of maturity level. In the wild, locomotor behavior of these baboons vary as a result of their need to find food and to avoid predators.

Apes in closed forest habitats (habitats enclosed by trees) are considered to be more bipedal than chimpanzees and baboons, both when they are standing stationary or moving bipedally. Gibbons (of the genus Hylobates) are low speed obligate bipeds when on the ground but travel quadrupedally in other contexts.

Although no longer extant, Australopithecines exhibited facultative bipedalism. Their pelvis and lower body morphology are indicative of bipedalism: the lumbar vertebrae curve inward, the pelvis has a human like shape, and the feet have well developed transverse and longitudinal arches that indicate walking. However, other features indicate reduced locomotor competence, or an increase in stress caused by walking bipedally. The pelvis is broad, which requires greater energy to be used during walking. Australopithecines also have short hind limbs for their weight and height, which also shows a higher energy expenditure when walking bipedally. This indicates that this species practiced bipedal locomotion, but did so more infrequently than previously thought. At the times they did practice bipedalism, the benefits outweighed the potential costs that would be imposed on them. When modeled, an exact number of steps and rate of acceleration leads to an exact shift in the center of mass that allows the elevation of the front limbs: too fast and the center of mass moves too far back and the lizard falls over backward, too slow and the front limbs never elevate. However, this model does not account for the fact that lizards may adjust their movements using their forelimbs and tail to increase the range of acceleration in which bipedal locomotion is possible. Although the advantage of facultative bipedalism in lizards remains unclear, increased speed or acceleration is possible, and facultative bipedalism promotes phenotypic diversity which may lead to adaptive radiation as species adapt to fill different niches. In primates, bipedal locomotion may allow them to carry more resources at one time, which could confer an advantage especially if the resources are rare. In other primates, various arboreal adaptations may have affected the evolution of bipedalism as well. Longer forelimbs would be more advantageous when moving through trees that are spaced further apart,