Dragon lizards, such as the frilled-necked lizard Chlamydosaurus kingii and the South American basilisk lizard Basiliscus basiliscus, readily run on their hindlimbs. Though other two-legged groups, such as primates, dinosaurs and birds, use their forelimbs for grasping or flight, this is not the case for lizards – so why don’t they all run on four legs?

Originally, it was thought that bipedalism provided a speed advantage, since the front limbs were no longer getting in the way of the hindlimbs. But, when measured, the speed of two-legged strides proved to be no greater than that of four-legged ones.

Other scientists theorised that running on two legs would save the lizards energy, yet those that travelled on two legs could not keep up with ones that ran on all four.

However, mathematical modelling of small lizards has suggested a new answer: bipedalism in lizards may be an accidental consequence of acceleration. Forward acceleration at the hindlegs pushes the rear body forward, but the inertia of the front of the body wants it to stay put. As a result, the back legs want to run under the body, which causes the front end to rise – just like a motorcycle popping a wheelie.

Main image: Illustration of a the frilled-necked lizard Chlamydosaurus kingii © Getty Images