A therian mammal with sprawling kinematics? Gait and 3D forelimb X-ray motion analysis in tamanduas.

Therian mammals are known to move their forelimbs in a parasagittal plane, retracting the mobilised scapula during stance phase. Non-cursorial therian mammals often abduct the elbow out of the shoulder-hip parasagittal plane. This is especially prominent in Tamandua (Xenarthra), which suggests they employ aspects of sprawling (e.g. lizard-like) locomotion. Here, we tested whether tamanduas use sprawling forelimb kinematics, i.e. a largely immobile scapula with pronounced lateral spine bending and long-axis rotation of the humerus. We analysed high-speed videos and used X-ray motion analysis of tamanduas walking and balancing on branches of varying inclinations and provide a quantitative characterization of gaits and forelimb kinematics. Tamanduas displayed lateral sequence/lateral couplets on flat ground and horizontal branches, but increased diagonality on steeper inclines and declines, resulting in lateral sequence/diagonal couplets gaits. This result provides further evidence for high diagonality in arboreal species, probably maximising stability in arboreal environments. Further, the results reveal a mosaic of sprawling and parasagittal kinematic characteristics. The abducted elbow results from a constantly internally rotated scapula about its long axis and a retracted humerus. Scapula retraction contributes considerably to stride length. However, lateral rotation in the pectoral region of the spine (range: 21 deg) is higher than reported for other therian mammals. Instead, it is similar to that of skinks and alligators, indicating an aspect generally associated with sprawling locomotion is characteristic for forelimb kinematics of tamanduas. Our study contributes to a growing body of evidence of highly variable non-cursorial therian mammal locomotor kinematics.

The evolution of femoral cross-sectional properties in sciuromorph rodents: Influence of body mass and locomotor ecology.

In several groups of mammals, adaptation to differing functional demands is reflected in long bone cross-sectional properties (CSP), which relate to the resistance to compression and to bending loads in the craniocaudal and mediolateral directions. Members of the Sciuromorpha ("squirrel-like" rodents) display a diversity of locomotor ecologies and span three orders of magnitude in terms of body size. The availability of robust phylogenies is rendering them a suitable group to further substantiate the relationship of long bone CSP with locomotor ecology and body mass while taking the phylogenetic non-independence among species into account. Here, we studied 69 species of Sciuromorpha belonging to three lifestyle categories, "arboreal," "fossorial," and "aerial" (i.e., gliding). We hypothesized locomotor category specific loading regimes that act on femora during predominant or, in terms of gliding, critical locomotor behaviors of each category. High resolution computed tomography scans of the specimens' femora were obtained and cross-sections in 5% increments were analyzed. Cross-sectional area, the craniocaudal second moment of area (SMAcc ), and the mediolateral second moment of area were quantified. Further, a scaling analysis was conducted for each bone cross-section to examine how the CSP scale with body mass. Body mass accounted for variances in CSP with mainly positive allometry. The aerial sciuromorphs showed lower values of CSP compared to the arboreal and fossorial species in the distal epiphysis for all quantified parameters and along the bone for SMAcc . In contrast to previous studies on other mammalian lineages, no differences in CSP were found between the fossorial and the arboreal lifestyles.