Meeting Abstract
Most squamates rotate vertebral and sternal ribs to ventilate their lungs, whereas snakes lack sternal ribs and use only vertebral ribs to breathe. Rib rotations are described about a dorsoventral axis (bucket), a craniocaudal axis (caliper), and a mediolateral axis (pump). Our objectives were to use XROMM to quantify rib rotations of Boa constrictor during ventilation and compare their kinematics and musculature to three previously studied squamates: Iguana iguana, Varanus exanthematicus, and Salvator merianae. We compared the relative contribution of each rotational axis to overall rib motion by fitting a linear mixed effects model to the variance-normalized centroids. We found that B. constrictor breathe with predominantly bucket, with moderate caliper and substantial pump, that bucket and pump are opposite in polarity, and that these motion patterns were significantly different from I. iguana and S. merianae. In contrast, we found the motions of B. constrictor were not significantly different from V. exanthematicus and that both use cranial pump rotations during inhalation, opposite the other species. These similarities appear related to derived muscles in B. constrictor and V. exanthematicus: both have accessory costal muscles that run cranially from each rib to the next cranial vertebrae, i.e. levator costae, whereas the other species do not. Our data suggest that snakes evolved such accessory muscles to breathe without a pectoral girdle, a structure that may synergize with intercostals to rotate the ribs of most squamates. These accessory levator costae appear to enable snakes to breathe with spatially disparate regions of their body. This modular ventilation mechanism may have permitted the evolution of constriction and large prey ingestion in snakes and been a prerequisite to their extensive radiation.
