Body and tail-assisted pitch control facilitates bipedal locomotion in Australian agamid lizards - PubMed (original) (raw)
Body and tail-assisted pitch control facilitates bipedal locomotion in Australian agamid lizards
Christofer J Clemente et al. J R Soc Interface. 2018.
Abstract
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.
Keywords: bio-inspiration; destabilization; exaptation; ground reaction force.
© 2018 The Author(s).
Conflict of interest statement
We declare we have no competing interests.
Figures
Figure 1.
Linear discriminate (LD) loads between gaits, with boxplots of top three loads for positive (mean body–shoulder: meanBS, mean head–body: meanHB, hip height at end stance: HHES), and negative (difference in upper forelimb angle: difUFL, mean angular speed of the forelimb: meanspeedUFL, mean angular speed of the body–shoulder: meanspeedBS) loadings for LD1. (a–g) Data are presented as individual data points for bipedal (green dots, n = 26), transitional (purple dots, n = 23), and quadrupedal (orange dots, n = 26) strides for each animal. Boxplots show 25th and 75th percentile, and the centreline is the 50th percentile, lines represent the max/min values (within the 1.5 × IQR) and dots represent outliers (< Q1 − 1.5 × IQR || > Q3 + 1.5 × IQR). Letters represents significance between gaits. (h) Schematic diagram of kinematic changes during bipedal strides with grey arrow representing direction of changes in angle during bipedal stride. Full names of abbreviations and descriptions of direction of angle change provided in electronic supplementary material, table S1. Summary statistics in electronic supplementary material, tables S2 and S3. (Online version in colour.)
Figure 2.
Summary of change in body kinematics during acceleration (m s–2) of bipedal (a–c) and transitional (d–f) strides. Data presented as individual data points for bipedal (•, n = 25), and transitional (•, n = 23) strides for each animal. Gradient colours represent the percentage of bipedal strides from Clemente & Withers [4] and this study for the individuals of each species. Regression lines presents model prediction, and vertical dash lines represents no acceleration. Summary statistics in electronic supplementary material, tables S4 and S5. (Online version in colour.)
Figure 3.
Asymmetry of force measurements for each gait between forefoot and hindfoot. For (a) the percentage of the stance phase at which half of the total vertical impulse had been produced (force 1), and (b) percentage of force produced at midstance (force 2). Boxplots as for figure 1. Data presented as individual data points each stride for each animal (quadrupedal: orange dots, forelimb n = 18, hindlimb n = 20, transitional: purple dots, forelimb n = 2, hindlimb n = 4, bipedal: green dots, n = 15). Top bell curve graphs represents shift in force symmetry for force 1 (left), and force 2 (right). Within the panels, the top sets present forelimb GRF profiles, while the bottom sets represent hindlimb GRF profiles. Letters represent significance between gaits. Vertical dashed line indicates predicted response for a symmetrical vertical GRF pattern. Summary statistics in electronic supplementary material, table S6. (Online version in colour.)
Figure 4.
The relative importance of 30 different kinematic variables in predicting the differences between estimated and empirically derived acceleration thresholds. Data presented are for 75 observations from eight species, including 26 bipedal strides, 26 quadrupedal strides, and 23 transitional strides. Each kinematic variable is compared to the difference between predicted and estimated acceleration thresholds (a). Shows example data of the difference between the model prediction and estimates for empirical thresholds (electronic supplementary material, table S1), and results are shown stacked for each kinematic variable (b). The significant relationship between this difference and two example kinematic variables is shown in (c), for the mean head to body angle (meanHB), and the mean acceleration of the body–shoulder segment about the hip (meanaccelBS). (Online version in colour.)
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