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. The switch to facultative bipedalism often occurs when an animal begins to run at high speeds, notably in many lizards, such as the basilisk lizard, and in some cockroaches. Low-speed facultative bipedality is less common; the gibbon, a primate with an anatomy highly specialized for arboreal locomotion, can walk bipedally in trees or on the ground with its arms raised for balance.
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.
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.
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