Ventilatory rib kinematics in the savannah monitor lizard

Robert L. Cieri, Sabine Moritz, and Elizabeth L. Brainerd

Figure 1. Costal joint motion and axial anatomy in Varanus exanthematicus.In this study, marker-based XROMM (Movie 1) was used to quantify the three dimensional rib rotations in three individuals of Varanus exanthematicus, which are typically described as bucket-handle rotation about a dorsoventral axis, pump-handle rotation about a mediolateral axis, and caliper motion about a rostrocaudal axis (Figure 1).

Figure 2. Typical 6DOF motion at the costovertebral joint of the first vertebral rib. Left: costovertebral joint coordinate system. Right: rotations of the first vertebral rib around its articulation with the vertebral column. The red trace depicts X axis, pump-handle motion, green Y axis caliper rotation, and the blue trace Z axis bucket-handle motion.Figure 3. Typical 6DOF motion at the sternocostal joint of the first sternal rib. Left: sternocostal joint coordinate system. Right: rotations of the first sternal rib around its own articulation with the sternum. The red trace depicts X axis, pump-handle motion, green Y axis caliper rotation, and the blue trace Z axis bucket-handle motion.Figure 4. Differences in Rib Motion – Deep vs. Shallow Breaths Rotations of the first vertebral rib around its articulation with the vertebral column during five, deep (solid lines) and shallow (dashed lines) breaths from one individual. The red trace depicts X axis, pump-handle motion, green Y axis caliper rotation, and the blue trace Z axis bucket-handle motion. Compared to deep breaths, shallow breaths consist of significantly less bucket handle rotation (p<0.01, Welch two-sample T-test), but similar amounts of pump handle and caliper motions.During deep breathing in standing and prone lizards, we found vertebral rib motion to mainly consist of equal parts bucket and pump handle motions with caliper motion contributing little to the ventilatory motion (Figure 2). In the sternal ribs, by contrast, all three rotations contributed to ventilation, with bucket handle the most dominant and caliper the least dominant motion (Figure 3). Compared to deep breaths, shallow breaths consisted of significantly less bucket-handle rotation in the vertebral ribs (Figure 4). Compared to standing breaths, the sternum and vertebral column center around a more acute angle during prone breaths, and the vertebral ribs exhibit a greater degree of bucket handle rotation in prone breaths compared to standing.

Movie 1: Biplane videofluoroscopy of breathing in Varanus exanthematicus. Small segments of insect pins attached to radio-opaque beads have been implanted in the ribs for marker-based XROMM.

These differences in kinematics between breaths during different postures may help to explain the evolution of unrestrictive costal joint anatomies in Squamata, as the joint design must permit variations in ventilatory and locomotor motions under different conditions and postures. We found most of the ribs of V. exanthematicus to move during ventilation (Movie 2), unlike in iguanas. This difference in ventilatory strategy may reflect differences in endurance, locomotor strategies, or lung designs between Iguana and Varanus.

Movie 2: XROMM animation of ribs during deep breathing in Varanus exanthematicus. Only the first three pairs of ribs connect the sternum in V. exanthematicus, but ribs all down the body contribute to ventilation.