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First published online June 13, 2008
Journal of Experimental Biology 211, 2058-2065 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018044

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Why go bipedal? Locomotion and morphology in Australian agamid lizards

Christofer J. Clemente^1,*, Philip C. Withers², Graham Thompson³ and David Lloyd⁴

¹ Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
² Zoology, School of Animal Biology, University of Western Australia, Western Australia, Australia
³ Centre for Ecosystem Management, Edith Cowan University, Western Australia, Australia
⁴ School of Sport science, Exercise and Health, University of Western Australia, Western Australia, Australia

^* Author for correspondence (e-mail: cc498{at}cam.ac.uk)

Accepted 17 April 2008

Bipedal locomotion by lizards has previously been considered to provide a locomotory advantage. We examined this premise for a group of quadrupedal Australian agamid lizards, which vary in the extent to which they will become bipedal. The percentage of strides that each species ran bipedally, recorded using high speed video cameras, was positively related to body size and the proximity of the body centre of mass to the hip, and negatively related to running endurance. Speed was not higher for bipedal strides, compared with quadrupedal strides, in any of the four species, but acceleration during bipedal strides was significantly higher in three of four species. Furthermore, a distinct threshold between quadrupedal and bipedal strides, was more evident for acceleration than speed, with a threshold in acceleration above which strides became bipedal. We calculated these thresholds using probit analysis, and compared these to the predicted threshold based on the model of Aerts et al. Although there was a general agreement in order, the acceleration thresholds for lizards were often lower than that predicted by the model. We suggest that bipedalism, in Australian agamid lizards, may have evolved as a simple consequence of acceleration, and does not confer any locomotory advantage for increasing speed or endurance. However, both behavioural and threshold data suggest that some lizards actively attempt to run bipedally, implying some unknown advantage to bipedal locomotion.

Key words: bipedal, sprint speed, acceleration, endurance, locomotion, agamids, dragon lizards

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