Bending Properties of the Jugal Bone in Mallard Ducks and its Significance for Cranial Biomechanics

Abstract

Few species evolved skulls as derived as ducks and other Anseriform birds, which employ rapid, dexterous movements of the palate and jaws to feed. Despite their exceptional behavior and morphology, little is known about the structure and biomechanics of their jaw muscles, cranial joints, and linkages that facilitate cranial kinesis. In particular, the jugal bone is capable of bending considerably as it links the quadrate and the upper beak. Here we employ a series of methods including microCT imaging, morphometrics, 3D modeling and histology to discover how and why the jugal is capable of such astounding flexibility. Although a narrow suture connects the caudal limb of the jugal with the quadratojugal, flexibility instead appears to occur at narrow regions of the bone. This gross morphology is further complemented by changes to the skeletal tissues that compose the element. Moreover, the orientation of jaw muscle resultants and loading of the skull indicate the jugal experiences a range of loading during different instances of the gape cycle. These findings illustrate the complicated series of adaptations in morphology, histology and biomechanics ducks and other vertebrates employ to engage in derived forms of cranial kinesis and feeding.In birds, cranial kinesis implies the ability to move the upper bill relative to the braincase, contributing to the evolution of the flexible skulls found in birds. The mechanism for elevation of the upper beak consists of a forward movement of the quadrate, exerting force through the jugal and the pterygoid that connect the quadrate to the upper jaw, and elevate the upper beak by bending of the nasofrontal hinge joint and possibly a flexion zone in the jugal. The jugal bone plays a significant role in Mallard feeding as it links the posterior part of the skull with the flexible anterior part of the skull. Interestingly the jugal is quite flexible and when removed from the skull; it is capable of bending 90 degrees while still returning to its original conformation. For the first time, we are studying the biomechanics of birds to determine how their skulls work; while primarily investigating the cause of the flexibility in the jugal bone. We have studied a sample of duck skulls through CT data, 3D modeling, and measurement of cross‐sectional properties of the element. From this data we have found areas of the jugal bone that afford greater consideration in determining the flexibility throughout the jugal bone. This project will create new data about the jugal bone's flexibility, which will contribute to the study of biomechanics of bones and cranial kinesis.