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. Well-known examples include many lizards such as the Basilisk lizard , and even some cockroaches when running at top speed. 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 their arms raised for balance.
In order to be considered a true facultative biped, an animal must be capable of sustained movement over many strides while bipedal—simply adopting a static bipedal posture while resting or looking around is not sufficient.
Facultative bipedality is most common in lizards, but also occurs in primates , bears , insects , crabs and even octopuses . It is commonly suggested that many extinct basal archosaurs were facultative bipeds, as well as hadrosaurs .
In many cases, facultative bipedality is a function of speed. Many lizard species, as well as cockroaches and crabs, will switch to a bipedal gait at very high speeds. Reasons for this are unclear: it may be that a bipedal gait allows greater stride length by precluding the forelimbs from interfering with the rhythm and movements of the hind limbs; or it may be that at high speeds the forces in the muscles operating the hind limbs are such that cause the animal's "upper" body to rise—similar to turning a "wheelie " in bicycling.
Low-speed bipedality is less common, as is most commonly associated with threat displays (bears , goannas , frilled lizards ), camouflage (octopus), or possessing an anatomy that is highly specialized for arboreal locomotion and makes terrestrial locomotion difficult (gibbons ).
* v * t * e
Animal locomotion on land