Why go bipedal? : Locomotion and morphology in Australian agamid lizards

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