Biology:Facultative biped

A facultative biped is an animal that is capable of walking or running on two legs, often for only a limited period, in spite of normally walking or running on four limbs or more. It differs from obligate bipedalism in that facultative bipeds use four limbs as their primary method of locomotion, only occasionally becoming bipedal, often for a specific purpose. Primates and lizards are the most common animals that engage in facultative bipedalism–this behavior has been observed in several families of lizards, including Agamidae, Crotaphytidae, Iguanidae and Phrynosomatidae, and multiple species of primates, including sifakas, capuchin monkeys, baboons, gibbons, and chimpanzees. There are multiple different types of bipedal motion, with different facultative bipeds using different types. This corresponds to the different reasons various species have for engaging in facultative bipedalism. In primates, bipedalism is often associated with food gathering and transport. In lizards, 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 has been debated. Facultative bipedalism is often divided into high-speed (lizards) and low-speed (gibbons), but some species can not be easily categorized into one of these two. Facultative bipedalism has also been observed in cockroaches and some desert rodents.

Types of Bipedal Locomotion

Within the category of bipedal locomotion, there are a 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. When both feet are off the ground at the same time, called the aerial phase, this is running. Skipping is when there is an aerial phase, but the two feet hit the ground immediately after each other, and the trailing foot changes after each step. Galloping is similar to skipping, but the trail foot does not change after each step. This is not an exhaustive list of the forms of bipedalism, but most bipedal species utilize one or more of these techniques.

Facultative Bipedal Species

Facultative bipedalism occurs in primates, cockroaches, desert rodents, and lizards; specific lizard families known as facultative bipeds are the Agamidae, Crotaphytidae, Iguanidae, and Phrynosomatidae. Facultative bipedalism evolved in the common ancestor of most major dinosaur groups, and it arose independently within lizards and mammals.

Primates

Monkeys

Baboons

Olive baboons are described as a quadrupel primate, but bipedalism is observed occasionally and spotaneously in captivity and in the wild. Bipedal walking is rarely used, but most often occurs in instances where the infant loses its grip on the mother while she's walking quadrupedally 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 the wild, locomotor behavior of these baboons vary as a result of their need to find food and to avoid predators.

Apes

Chimpanzees

Chimpanzees exhibit bipedalism most often when carrying valuable resources (such asfood gathering/transporting purposes) because chimps can carry more than twice as much when walking bipedally as opposed to walking quadrupedally. Bipedalism is practiced both on the ground and aloft when feeding from fruit trees. Foraging in short trees with both feet on the ground allows for indivuduals to reach higher into trees

Australopithecus Anatomy

The pelvis and lower body morphology in australopithicines indicate bipedalism: the lubar 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. The pelvis is also broad, which requires greater energy to be used during walking. Australopithecines also have short hindlimbs for their weight and height, which also shows a higher energy expenditure. This indicates that this species practiced bipedal locomotion more infrequently, which meant that the costs did not impact them.

Functions

In lizards, facultative bipedalism occurs as a result of rapid acceleration caused by the location of the lizards’ hind legs which induces a friction from the ground to produce a reaction force on the rear legs, effectively creating a turning moment about the lizards center of mass and allowing it to lift off the ground over short distances as a mechanism to evade oncoming predators.

In primates, bipedal movements consist of an irregular, shuffling gait, accomplished by rotating the hip and making short steps, which are constrained by wide pelvis shapes and short hind limbs; primates, such as gelada baboons, use bipedalism to free up their hands for feeding or fighting.