Breathing with floating ribs: XROMM analysis of lung ventilation in savannah monitor lizards

The structures and functions of the vertebrate lung and trunk are linked through the act of ventilation, but the connections between these structures and functions are poorly understood. We used XROMM to measure rib kinematics during lung ventilation in three savannah monitor lizards, Varanus exanthematicus. All of the dorsal ribs, including the floating ribs, contributed to ventilation; the magnitude and kinematic pattern showed no detectable cranial-to-caudal gradient. The true ribs acted as two rigid bodies connected by flexible cartilage, with the vertebral rib and ventromedial shaft of each sternal rib remaining rigid and the cartilage between them forming a flexible intracostal joint. Rib rotations can be decomposed into bucket handle rotation around a dorsoventral axis, pump handle rotation around a mediolateral axis, and caliper motion around a craniocaudal axis. Dorsal rib motion was dominated by roughly equal contributions of bucket and pump rotation in two individuals and by bucket rotation in the third individual. The recruitment of floating ribs during ventilation in monitors is strikingly different from the situation in iguanas, where only the first few true ribs contribute to breathing. This difference may be related to the design of the pulmonary system and life history traits in these two species. Motion of the floating ribs may maximize ventilation of the caudally and ventrolaterally-positioned compliant saccular chambers in the lungs of varanids, while restriction of ventilation to a few true ribs may maximize crypsis in iguanas.