Graduate Student University of Missouri Columbia, Missouri, United States
Abstract Body : Birds evolved a diverse array of feeding behaviors during their adaptive radiation. Anseriform birds in particular evolved a variety of skull morphologies and complex jaw muscles with numerous muscle bellies in association with their derived filter-feeding behavior. Mallards (Anas platyrhynchos) are omnivorous dabbling ducks with similar feeding habits between adults and juveniles, yet they undergo significant ontogenetic shape change, making them useful subjects in which to study ontogenetic change in muscle architecture. Here we used diceCT and 3D soft-tissue reconstruction to compare the anatomy, architecture and function of the jaw muscles of a neonate and adult mallard duck. We expect there to be significant changes in the positions and pennation of the different jaw muscles during duck ontogeny.
Individual fascial plains of the external jaw muscles (mAME) could not be easily distinguished in the neonate, but differences in fiber direction and attachment sites for separate bellies was visible. In the adult, the bellies of mAME differentiated into smaller, more architecturally complex units. The pennation of the superficial adductors (mAMES) greatly increased between the two life stages. The deeper mAME muscles shifted rostrally as the suborbital process extended during growth. Adductor posterior (mAMP) was rostrocaudally elongate and medially concave in the neonate mallard, but it became more dorsoventrally oriented in the adult and lost much of its medial concavity. Pterygoideus ventralis (mPTv) showed substantial change in morphology between the neonate and adult mallards. In the neonate, the muscle was oriented at a 30-degree angle caudoventrally to rostrodorsally and was round in cross-section. In the adult, the muscle adopted a more rostrocaudal orientation and its cross-section became mediolaterally compressed. Depressor mandibulae (mDM) differentiated into two distinct bellies with different pennations and orientations in the adult, and also expanded caudally.
Most, but not all, of the muscles demonstrated change in pennation and morphology with ontogeny. These new data will better reveal ecomorphogical patterns in anseriform cranial evolution and cranial evolution across the avian family tree.
