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. 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.

Species

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.

Cause

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).