Intraskeletal consistency in patterns of vascularity within bat limb bones.

Bats are the only mammals to have achieved powered flight. A key innovation allowing for bats to conquer the skies was a forelimb modified into a flexible wing. The wing bones of bats are exceptionally long and dynamically bend with wingbeats. Bone microarchitectural features supporting these novel performance attributes are still largely unknown. The humeri and femora of bats are typically avascular, except for large-bodied taxa (e.g., pteropodid flying foxes). No thorough investigation of vascular canal regionalization and morphology has been undertaken as historically it has been difficult to reconstruct the 3D architecture of these canals. This study augments our understanding of the vascular networks supporting the bone matrix of a sample of bats (n = 24) of variable body mass, representing three families (Pteropodidae [large-bodied, species = 6], Phyllostomidae [medium-bodied, species = 2], and Molossidae [medium-bodied, species = 1]). We employed Synchrotron Radiation-based micro-Computed Tomography (SRµCT) to allow for a detailed comparison of canal morphology within humeri and femora. Results indicate that across selected bats, canal number per unit volume is similar independent of body size. Differences in canal morphometry based on body size and bone type appear primarily related to a broader distribution of the canal network as cortical volume increases. Heavier bats display a relatively rich vascular network of mostly longitudinally-oriented canals that are localized mainly to the mid-cortical and endosteal bone envelopes. Taken together, our results suggest that relative vascularity of the limb bones of heavier bats forms support for nutrient exchange in a regional pattern.

Generation of propulsive force via vertical undulations in snakes.

Lateral undulation is the most widespread mode of terrestrial vertebrate limbless locomotion, in which posteriorly propagating horizontal waves press against environmental asperities (e.g. grass, rocks) and generate propulsive reaction forces. We hypothesized that snakes can generate propulsion using a similar mechanism of posteriorly propagating vertical waves pressing against suitably oriented environmental asperities. Using an array of horizontally oriented cylinders, one of which was equipped with force sensors, and a motion capture system, we found snakes generated substantial propulsive force and propulsive impulse with minimal contribution from lateral undulation. Additional tests showed that snakes could propel themselves via vertical undulations from a single suitable contact point, and this mechanism was replicated in a robotic model. Vertical undulations can provide snakes with a valuable locomotor tool for taking advantage of vertical asperities in a variety of habitats, potentially in combination with lateral undulation, to fully exploit the 3D structure of the habitat.