Forelimb position affects facultative bipedal locomotion in lizards

Abstract

Recent work indicates that bipedal posture in lizards is advantageous during obstacle negotiation. However, little is known about how bipedalism occurs beyond a lizard's acceleratory threshold. Furthermore, no study to date has examined the effects of forelimb position on the body center of mass (BCoM) in the context of bipedalism. This study quantified the frequency of bipedalism when sprinting with versus without an obstacle at 0.8 m from the start of a sprint. Forelimb positions were quantified during bipedal running at the start of a sprint and when crossing an obstacle. Two species with contrasting body forms (and thus different BCoM) were studied (Sceloporus woodi and Aspidoscelis sexlineata) to assess potential variation due to body plan and obstacle-crossing behavior. No significant difference in frequency of bipedalism was observed in S. woodi with or without an obstacle. However, A. sexlineata primarily used a bipedal posture when sprinting. Forelimb positions were variable in S. woodi and stereotyped in A. sexlineata. Caudal extension of the forelimbs helped shift the BCoM posteriorly and transition to, or maintain, a bipedal posture in A. sexlineata, but not in S. woodi. The posterior shift in BCoM, aided by more caudally placed forelimbs, helps raise the trunk from the ground, regardless of obstacle presence. The body plan, specifically the length of the trunk and tail, and forelimb position work together with acceleration to shift the BCoM posteriorly to transition to a bipedal posture. Thus, species exhibit morphological and behavioral adjustments to transition to and maintain facultative bipedalism while sprinting.

An obstacle disturbance selection framework: emergent robot steady states under repeated collisions

“…We hypothesize that the disturbances from obstacles can be regarded as opportunities to enhance mobility in complex environments (Figure 1). Biological studies have demonstrated that animals (Kinsey and McBrayer, 2018; Kohlsdorf and Biewener, 2006; McInroe et al, 2016; Wilshin et al, 2017) can coordinate their appendages or body segments (Schiebel et al, 2019; Zhong et al, 2018) to adjust the timing and positions of environment engagement (Gart and Li, 2018; Gart et al, 2018; Li et al, 2015) to achieve effective locomotion. In analogy to the selected leg sequence timing in biological locomotors, Johnson and Koditschek (2013) demonstrated that with a human-programmed leg activation sequence, a hexapedal robot can jump up a vertical cliff by using its front legs to hook on the cliff edge while pushing its rear legs against the vertical surface.…”

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

“…It has been noted in dinosaurs that tail preservation is notoriously poor, very rarely does a caudal vertebral series appear complete and intact (Hone, 2012). In exploring our dataset, not including the tail, we have demonstrated that caudal vertebrae are not essential for indicating a facultatively bipedal mode, though long tails are undoubtedly associated with a bipedal mode in lizards (Snyder, 1962;Kinsey & McBrayer, 2018). Testing the predictive capabilities of the indicators presented here in a wider extant sample, such as hindlimb element length with respect to body size, will help form future research directions before application to the lepidosaur fossil record.…”

“…Regarding the morphology of the forelimb across locomotor modes, any interpretations should be considered in line with studies into forelimb kinematics in facultative bipedality. Recently, it has been noted that forelimb positioning contributes significantly to stabilization of lizard bipedal locomotion (Kinsey & McBrayer, 2018). This is particularly interesting, given our observation of longer ulnae in larger facultative bipeds than their obligately quadrupedal counterparts.…”

“…This locomotor style has been observed in several squamate species (e.g. basilisks, scrub lizards and others (Hsieh, 2003;Kinsey & McBrayer, 2018)) and in other vertebrates, such as rodents and primates (Alexander, 2004).…”