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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. 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, without the forelimbs interfering with the swinging and placement of the hind limbs, or it may simply be that at high speeds, the forces in the muscles which retract and extend the hind limbs are so great that animal's body rises into the air, similar to a "wheelie" in bikes.
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).
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