Locomotor loading mechanics in the hindlimbs of tegu lizards (Tupinambis merianae): comparative and evolutionary implications

Abstract

SUMMARYSkeletal elements are usually able to withstand several times their usual load before they yield, and this ratio is known as the bone's safety factor. Limited studies on amphibians and non-avian reptiles have shown that they have much higher limb bone safety factors than birds and mammals. It has been hypothesized that this difference is related to the difference in posture between upright birds and mammals and sprawling ectotherms; however, limb bone loading data from a wider range of sprawling species are needed in order to determine whether the higher safety factors seen in amphibians and non-avian reptiles are ancestral or derived conditions. Tegus (family Teiidae) are an ideal lineage with which to expand sampling of limb bone loading mechanics for sprawling taxa, particularly for lizards, because they are from a different clade than previously sampled iguanas and exhibit different foraging and locomotor habits (actively foraging carnivore versus burst-activity herbivore). We evaluated the mechanics of locomotor loading for the femur of the Argentine black and white tegu (Tupinambus merianae) using threedimensional measurements of the ground reaction force and hindlimb kinematics, in vivo bone strains and femoral mechanical properties. Peak bending stresses experienced by the femur were low (tensile: 10.4±1.1MPa; compressive: -17.4±0.9MPa) and comparable to those in other reptiles, with moderate shear stresses and strains also present. Analyses of peak femoral stresses and strains led to estimated safety factor ranges of 8.8-18.6 in bending and 7.8-17.5 in torsion, both substantially higher than typical for birds and mammals but similar to other sprawling tetrapods. These results broaden the range of reptilian and amphibian taxa in which high femoral safety factors have been evaluated and further indicate a trend for the independent evolution of lower limb bone safety factors in endothermic taxa.

Pelvic girdle mobility of cryptodire and pleurodire turtles during walking and swimming

“…LAR, long-axis rotation; var., variance. Sheffield et al, 2011;Blob et al, 2014). In this context, our data on pelvic and femoral movement in turtles are noteworthy in two regards.…”

Broad similarities in shoulder muscle architecture and organization across two amniotes: implications for reconstructing non-mammalian synapsids

“…We collected anatomical data from six wild adult Argentine black and white tegus (S. merianae) and six wild adult Virginia opossums (D. virginiana) (Tables S1 and S2). While varanid and iguanian lizards have traditionally been used as models of plesiomorphic amniote posture and locomotion (Jenkins & Goslow, 1983;Padian & Olsen, 1984;Ritter, 1996;Blob & Biewener, 1999;Blob, 2000;Farlow & Pianka, 2000;Clemente et al, 2011;Dick & Clemente, 2016), tegus were chosen here to represent a more shallowly-nested clade of terrestrial generalists (Sheffield et al, 2011;Simões et al, 2018) of growing importance as laboratory animals (Bennett & John-Alder, 1984;Montero et al, 2004;Toledo et al, 2008;Sheffield et al, 2011). In comparison, among extant mammals, didelphid opossums are a well-established plesiomorphic model for therian development, anatomy, and locomotion (Broom, 1899;Jenkins, 1971b;Hiiemae & Crompton, 1985;Klima, 1985;Parchman, Reilly & Biknevicius, 2003;Sánchez-Villagra & Maier, 2003;Gosnell et al, 2011;Hübler et al, 2013;Diogo et al, 2016;Bhullar et al, 2019).…”

In search of the pitching momentum that enables some lizards to sustain bipedal running at constant speeds

“…In that case, the ground-reaction force vector may still be anterior of the COM, and only nose-up pitching moments would be generated. Strong asymmetry in hindlimb vertical ground-reaction force towards the instant of initial contact has been measured in quadrupedal running tegu lizards [11], but such measurements are currently not available in the literature for bipedal running in lizards. Yet, through mathematical modelling, we can explore the necessity of asymmetry in vertical ground-reaction force profiles in explaining how steady bipedal running can work in lizards.…”

“…Yet, this model was never tested for the wind-milling, splayed hindlimbs of lizards. In fact, force platform measurements of a single hindlimb during quadrupedal running in the tegu Tupinambus merianae did show a strongly asymmetrical profile: about 69 per cent of the total vertical impulse by the right hindlimb was generated during the first half of foot-contact time [11]. The degree of asymmetry of this measurement falls in between the two synthetic asymmetrical profiles used in our simulations (figure 4), which have 64 per cent and 75 per cent of the vertical impulse during the first half of foot-contact time.…”