XROMM analysis of rib kinematics during lung ventilation in the green iguana, Iguana iguana

The three-dimensional rotations of ribs during breathing are typically described as bucket-handle rotation about a dorsoventrally oriented axis, pump-handle rotation about a mediolateral axis, and caliper rotation about a rostrocaudal axis. In amniotes with double-headed ribs, rib motion is constrained primarily to one degree-of-freedom (DOF) rotation about an axis connecting the two rib articulations. However, in Squamata, the ribs are single headed and the hemispherical costovertebral joints permit rotations with three DOF. In this study, we used X-ray reconstruction of moving morphology (XROMM) to quantify rib rotation during deep breathing in four green iguanas. We found that rib rotation was strongly dominated by bucket-handle rotation, thus exhibiting nearly hinge-like motion, despite the potential for more complex motions. The vertebral and sternal segments of each rib did not deform measurably during breathing, but they did move relative to each other at a thin, cartilaginous intracostal joint. While standing still and breathing deeply, four individual iguanas showed variability in their rib postures, with two breathing around a highly inflated posture, and two breathing around a posture with the ribs folded halfway back. Bucket-handle rotations showed clear rostrocaudal gradients, with rotation increasing from the third cervical to the first or second dorsal rib, and then decreasing again caudally, a pattern that is consistent with the intercostal muscles in the rostral intercostal spaces being the primary drivers of inspiration. The constrained, primarily bucket-handle rotations observed here during breathing do not help to explain the evolution of permissive, hemispherical costovertebral joints in squamates from the more constrained, double-headed rib articulations of other amniotes.