Why do some dragon lizards run bipedally?

Dragon lizards, such as the Frilled-necked Lizard (Chlamydosaurus kingii), Bicycle Dragon (Ctenophorus cristatus), Lozenge-marked Dragon (C. scutulatus) and Gilbert’s Dragon (Amphibolurus gilberti) will readily run on their hind limbs. Do they do this is because they can run faster, it is more efficient or it is just more fun?

Frilled-necked lizard (Chlamydosaurus kingii)

Clemente et al (2008) reported that many of Western Australia’s dragon lizards were able to run bipedally, but some would do it more often than others. Lizards that ran bipedally appeared not to run for as long as those that ran quadrupedally, indicating that bipedalism incurred an extra energetic cost. So lizards were not running on two legs to save energy. Also the average and maximal bipedal running speeds were not significantly different to the average and maximal quadrupedal running speeds; so lizards were not doing it to run faster either. However, average acceleration was significantly greater for bipedal strides than quadrupedal strides, suggesting that acceleration is important for bipedalism.

It appeared as if there was a threshold for acceleration that initiated the change to bipedalism. Forward acceleration at the rear legs pushes the rear body forward, but the inertia of the front of the body wants it to stay put. As a result the hind legs want to run under the body, which causes the front of the body to lift up, just like a motorcycle popping a wheelie. This suggests that bipedalism in lizards could have evolved accidentally! However, this acceleration threshold differed among species probably due to changes in body shape, but some species could shift to a bipedal stance before we might predict them to do so, from this simple model. These species actively tuck their fore limbs against their body, and lift their tails up and this would assist with moving the centre of mass toward the hip, and therefore cause them to run bipedally earlier. The significance of this is that it suggests that some lizards actively try to run bipedally for longer than they would by accident Whether there is an advantage to running on two legs over four, is a subject for further research.

I have seen Frilled-necked Lizards move slowly bipedally and they will certainly remain stationary in a bipedal position. So this is not posture based on acceleration. Varanus gouldii and V. panoptes will also both stand stationary on their hind limbs, and it appears as if this is done using the hind limbs and their tail as a tripod. The mechanics of these body postures is an area for further investigation.

Varanus gouldii	Varanus panoptes

Have a look at these two great videos of kinematics in Australian reptiles – bipedal dragon and Varanus panoptes. For more great biomechanics research check out Dr Chris Clemente’s work.

Reference: Clemente, C. J., P. C. Withers, G. Thompson, and D. Lloyd. 2008. Why go bipedal? Locomotion and morphology in Australian agamid lizards. Journal of Experimental Biology 211:2058-2065