As all the anatomical conditions are not present to permit any movement between the cranial segments, the bill aperture and closure by the rotation on the quadrato mandibular joint is, in Ramphastos , the only action studied in relation to muscles organization and bone architecture by torques analysis. From a mechanical advantage viewpoint, the beginning of the closure seems to be performed by velocity muscles, while the action is ended by power muscles. The stresses issued by the muscles actions are balanced by bone trabeculae and other muscles, and may be considered as responsible of the mechanical stiffness of the whole bill system.