New Insights into Form‐Function Relationships of Feeding Systems from XROMM and Fluoromicrometry

Studies of form‐function relationships in vertebrate feeding systems have a long and distinguished history of inferring function from anatomical structure, and a shorter but fruitful history of measuring function directly in living animals with sophisticated laboratory methods such as electromyography and cineradiography. Two relatively new methods, X‐ray Reconstruction of Moving Morphology (XROMM) and fluoromicrometry, show great promise for revealing the three‐dimensional (3‐D) form‐function relationships of cranial musculoskeletal structures in feeding systems. XROMM produces highly precise (±100 microns) 3‐D animations of 3‐D mesh bone models making it possible to visualize and measure both bone shape and bone motion in vivo . XROMM has been applied to measure 3‐D jaw kinematics and tooth occlusion in mammalian mastication, 3‐D pharyngeal jaw mechanics in fishes, and 3‐D jaw kinematics and cranial kinesis in squamates, birds and fishes. The power of XROMM for studying tooth occlusion is that the mesh model from the CT scan includes all the detail of tooth morphology. In miniature pigs feeding on pig chow, we used XROMM to show that the yaw of the mandible produces a grinding motion between upper and lower premolars that includes a mesiodistal component, in addition to the expected buccolingual component, resulting in an oblique power stroke. In ongoing work on short‐tailed opossums, we are tracing the changes in tooth‐tooth and tooth‐food interaction within a chewing bout. Detecting long‐axis rotation (LAR) is a particular strength of XROMM because the method reconstructs the full 6 degree‐of‐freedom movements of the skeletal elements. In suction feeding sharks, we found small LAR of the Meckel's cartilages during jaw depression, as well as large LAR of the hyomandibula and ceratohyal contributing to buccal cavity expansion. Unlike mammals, most vertebrates have many mobile skeletal elements within the skull and substantial intracranial kinesis. In ducks, we used XROMM to show that 3D motions of the quadrate include mediolateral motions associated with lateral spreading of the hemimandibles and rostrocaudal motions associated with upper bill elevation and depression. In ray‐finned fishes we are using XROMM to animate the skull and hyoid bones during suction feeding. These bones then define the outer shell of the oropharyngeal cavity, and we fit a deformable polygon within the space to yield a dynamic digital endocast of volume at each time step. Combined with pressure measured in the buccal cavity, instantaneous suction power can be calculated throughout the suction feeding strike. Combining with radio‐opaque markers implanted in muscles and tracked to measure muscle strain (i.e. fluoromicrometry), we found that the axial muscles of largemouth bass and sunfish are providing essentially all the muscle power required for suction feeding, with the cranial muscles contributing a negligible amount of power. In the future, feeding studies that may particularly benefit from XROMM and fluoromicrometry are tooth‐food interactions during mastication, food transport and swallowing, and the form‐function relationships of feeding muscles with complex architecture, such as the mammalian muscles of mastication and the multi‐part adductor mandibulae of fishes. Support or Funding Information NSF DBI‐1661129 and IOS‐1655756 This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .