Body and tail-assisted pitch control facilitates bipedal locomotion in Australian agamid lizards

Abstract

<jats:p>Certain lizards are known to run bipedally. Modelling studies suggest bipedalism in lizards may be a consequence of a caudal shift in the body centre of mass, combined with quick bursts of acceleration, causing a torque moment at the hip lifting the front of the body. However, some lizards appear to run bipedally sooner and for longer than expected from these models, suggesting positive selection for bipedal locomotion. While differences in morphology may contribute to bipedal locomotion, changes in kinematic variables may also contribute to extended bipedal sequences, such as changes to the body orientation, tail lifting and changes to the ground reaction force profile. We examined these mechanisms among eight Australian agamid lizards. Our analysis revealed that angular acceleration of the trunk about the hip, and of the tail about the hip were both important predictors of extended bipedal running, along with increased temporal asymmetry of the ground reaction force profile. These results highlight important dynamic movements during locomotion, which may not only stabilize bipedal strides, but also to de-stabilize quadrupedal strides in agamid lizards, in order to temporarily switch to, and extend a bipedal sequence.</jats:p>

The relationship between body shape, body size and locomotor mode in extant lepidosaurs

“…Functionally, this facilitates larger muscle moment arms, longer stride lengths and greater ground clearance, all of which contribute to more powerful strides and greater support for the body on two limbs. Values echoing these factors are consistently found in the literature (Snyder, 1954(Snyder, , 1962Christian, Horn & Preuschoft, 1994;Hsieh, 2003;Clemente & Wu, 2018), contributing to work on musculoskeletal modelling of the facultative locomotor mode in these animals (Aerts et al, 2003). Interestingly, the gradient of SVL to hindlimb segment length remains similar across locomotor modes, indicating a largely mechanical relationship between body size and locomotor demands in this group, that is the size of long bones is constrained by locomotor mode (differences in intercept between locomotor modes), as opposed to body size (signified by differences in gradient between the two modes).…”

“…This is particularly interesting, given our observation of longer ulnae in larger facultative bipeds than their obligately quadrupedal counterparts. Additionally, the forelimb ground reaction forces of facultative bipeds are significantly different to those of obligate quadrupeds (Clemente & Wu, 2018). Interpreting the drivers of these shifts is difficult, and not explored in more detail here, though whether these shifts are by-products of acquiring a bipedal mode, or active changes to enable this behaviour, is intriguing.…”

“…Body measurements were taken using the Ruler tool, which allows measurements to be taken in rendered 3D space. Linear measurements were based on a comparative literature survey of facultative bipedality in reptiles (Snyder, 1952(Snyder, , 1962Irschick & Jayne, 1999a, 1999bAerts et al, 2003;Alexander, 2004;Kubo & Kubo, 2012;Clemente & Wu, 2018). We use precaudal length as an osteological correlate of snout-vent length (SVL; the standard measure of body size in herpetology) as our metric for body size.…”

“…Modelling work suggests that the postural shift is linked to acceleration (a change in speed in a given direction) rather than to velocity (speed in a given direction) (Aerts et al, 2003). Recent studies have further refined the identification of the trigger for bipedality to angular acceleration about the hip joint, rather than simply acceleration of the whole animal (Clemente & Wu, 2018). Paradoxically, bipedal locomotion appears to be neither faster nor more energetically efficient in lizards expressing this behaviour, raising more profound questions focused upon why this behaviour has evolved (Clemente et al, 2008).…”

“…Due to the iterative nature of scientific observation, our understanding of squamate behaviour has improved over time. Modern field reports and experimental studies on a wider variety of animals promote confidence in correct identification of locomotor mode than that of years gone by (Greene et al, 1978;Irschick & Jayne, 1999b;Clemente & Wu, 2018). For example, the green iguana (Iguana iguana) was identified as an obligate quadruped by Snyder (1962).…”