Rib motions don’t completely hinge on joint design: costal joint anatomy and ventilatory kinematics in a teiid lizard, Salvator merianae.
Almost all amniotic animals, such as birds, reptiles, and mammals, actively move air in and out of their lungs to breath. But what drives these motions? In most, it is the rotation of the ribs, a ventilation method known as costal aspiration; but to what extent does this rotation really matter? Many amniotes, ourselves included, exhibit double-headed ribs, known as bicapitate, that have two separate articulation points. The separate articulation points of each rib are oriented similar to a hinge and rotation about these points contributes to the expansion and contraction of the chest cavity.
How then is this breathing affected when an animal only has single-headed ribs? Squamate reptiles, i.e., lizards and snakes, present an interesting study case, as they retain amniotic costal aspiration to ventilate their lungs, but their joint morphology and point of articulation is now singular, i.e. unicapitate. While the axis of rotation in bicapitate ribs has been well characterized in other amniotes, the rotations of squamates unicapitate ribs is less well known. How then does rib motion in squamates relate to this seemingly less constrained rib head shape? To answer this question, a team of researchers set out to investigate the relationship between rib powered ventilation and rib head shape in the Argentine black and white tegu lizard (Salvator merianae). Using an exciting technology known as XROMM, X-ray Reconstruction of Moving Morphology, the researchers were able to visualize how the lizard’s skeletal structures moved in real time. They then were able to create 3D animations of the vertebrae, ribs and sternum during lung ventilations; this allowed for a better understanding of how the movements the ribs relate to their joints.
These researchers then looked at how the motions of ribs and ventilation of the lungs are affected by different joint shapes and how that may relate to different lifestyles and modes of locomotion. To understand this relationship between form and function the team compared their findings from the active foraging, terrestrial Argentine white and black tegu to a previous study which looked at the ventilatory kinematics of the arboreal, sedentary green iguana (Iguana iguana). While both lizards occupy similar ecosystems, their functional roles within those habitats vary greatly. Iguanas are herbivores, i.e. their food cannot run away from them, whereas tegus are omnivorous and are constantly searching for their next meal. As such, tegus are much quicker and consistently active while hunting prey. By comparing these two closely related squamates, with variable lifestyles and locomotor tactics, the researchers were better able to interpret how these activities throughout daily life effect motions associated with breathing.
Not only do these insights allow us a deeper understanding of what is happening during ventilation and locomotion in living species, with relation to their ecology, but this technology also provides us with a better context of application to determine what might have been going on in extinct species of amniotes.
By Kayla Hall
I am a PhD student studying evolutionary morphology, ecology and locomotion of fishes at the University of Washington. (Website https://kseahall.jimdosite.com)