Meeting Abstract
The beaks of birds display remarkable morphological, functional and mechanical diversity. However, behind the upper and lower beak of most birds are eight to nine additional mobile bones arranged in parallel sets of four- and five-bar linkages, forming a coupled mechanical system consisting of two levers and a linkage. These linkage bones enable rotation of the upper beak and serve as attachment sites for muscles that adduct the lower beak. While the geometry of these linkage bones likely has implications for beak function, little is known about how these geometries vary across birds and whether particular geometries correspond to particular beak shapes. In this study, we use recently published methods for collecting 3D shape data and predicting linkage kinematics to compare the morphology and leverages of the upper beak, lower beak and linkage bones from at least one representative of each bird order. We compute leverage of the upper and lower beak by measuring the effective mechanical advantage (EMA) for different outlever lengths along the tomium and inlever lengths at force input points. We compute leverage of the linkage bones by estimating kinematic transmission for force inputs to the quadrate, pterygoid and palatine. We then test whether mechanical changes in these components evolve congruently toward the same functional extreme or whether they evolve independently. We find that posterior head width and differences in the articulation of the jugal and palatine with the upper beak determine the major axis of variation in linkage morphology and that a single lever or linkage can yield widely varying leverage values, depending on the assumed sources of input and output force in the system. This work was funded by NSF grants DGE-1144082, DGE-0903637 and IOS-142549.
