Facultative Biped - Cause

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, 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).