Facultative bipedalism
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.
[8] Skipping involves an aerial phase, but the two feet hit the ground immediately after each other, and the trailing foot changes after each step.
[8] Facultative bipedalism occurs in some species of antbears, cockroaches, jerboa, kangaroo rats, primates, and lizards.
[18] The sifaka (Propithecus), which is a type of lemur native to the island of Madagascar, is one of the primary examples of facultative bipedalism.
[8] Sifakas can locomote bipedally in two separate ways: walking, with an evenly spaced gait and no aerial phase; or galloping, switching the trailing and leading foot every 5-7 steps.
It is thought that the reduced energetic costs of a pendulum-like gait (such as in humans) are what led to the evolution of obligate bipedalism.
These bipedal postures and locomotion in infants, although infrequent, seem to clearly distinguish them from adult baboons in terms of maturity level.
In specific circumstances, such as ground conditions, some ape feet perform better than human feet in terms of bipedal standing, as they have a larger RPL (ratio of the power arm to the load arm) and reduce the muscle force when the foot contacts the ground.
[16] Because they usually move through trees, their anatomy has become specialized for vertical clinging and leaping, which uses hip and knee joint extensions that are similar to those used in bipedal motion.
[12][13][14] They also use three back muscles (the multifidus, longissimus thoracis, and iliocostalis lumborum) that are key to bipedal motion in chimpanzees as well as humans.
However, other features indicate reduced locomotor competence, or an increase in stress caused by walking bipedally.
Australopithecines also have short hind limbs for their weight and height, which also shows a higher energy expenditure when walking bipedally.
Many families of lizards, including Agamidae, Teiidae, Crotaphytidae, Iguanidae, and Phrynosomatidae, have been observed to engage in facultative bipedalism.
[23][24] 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.
Advantages could include faster speeds to evade predators, or less energy consumption, and could explain why this behavior has evolved.
[3] Alternatively, while the origin of the behavior may have been solely the physical motion and acceleration, traveling bipedally may have conferred an advantage, such as easier maneuvering, that was then exploited.
[12] The evolutionary explanation for the development of this behavior is often linked to load-carrying in chimpanzees, bonobos, macaques, capuchin monkeys, and baboons.
[16] The ability to carry more materials can be either a selective pressure or a significant advantage, especially in uncertain environments where commodities must be collected when found.
Great apes that engage in male-male fights have an advantage when standing on their hind legs, as this allows them to use their forelimbs to strike their opponent.
