Meeting Abstract
Rib motions are important for both ventilation and locomotion in squamates. Squamate vertebral ribs are unicapitate, permitting three degrees of rotation between the ribs and vertebral column. In this study, X-ray Reconstruction of Moving Morphology (XROMM) was used to quantify rib rotations in 3 individuals of Varanus exanthematicus during ventilation. These rotations are composed of bucket-handle rotation about a dorsoventral axis, pump-handle rotation about a mediolateral axis, and caliper motion about a rostrocaudal axis. Our data suggest a functionally tripartite rib arrangement in V. exanthematicus: the vertebral ribs did not deform relative to the main shaft of each sternal rib, but the thin dorsolateral segment of each sternal rib moved relative to both the vertebral rib and the main shaft of the sternal rib during each breath. In deep breathing in standing and prone lizards, vertebral rib motion consisted mainly of equal parts bucket and pump handle motions. In the sternal ribs, by contrast, all three rotations contributed to ventilation with bucket handle rotation the most dominant and caliper the least dominant motion. Compared to deep breaths, shallow breaths consisted of significantly less vertebral rib bucket-handle rotation. The vertebral ribs also exhibited a greater degree of bucket handle rotation in prone breaths compared to standing. These differences may help to explain the evolution of unrestrictive costal joint anatomies in Squamata, with this joint design permitting variations in ventilatory and locomotor motions under different conditions and postures. We found most of the ribs of V. exanthematicus to move during ventilation, unlike in iguanas. These differences in ventilatory kinematics may reflect differences in locomotor strategies or lung design between Iguana and Varanus.
