Possible Directions of Further Development of F.Ya. Dzerzhinsky’s Program in Research on the Jaw Apparatus of Birds

The works by F.Ya. Dzerzhinsky and his disciples have rendered the jaw apparatus of birds a conventional and thoroughly characterized research object for functional morphology in Russia. The author, who communicated with Dzerzhinsky for many years, attempts to reconsider his scientific heritage in a critical manner and to use it as a foundation for outlining the most promising directions of further research. The analysis shows that underestimation of the problem of muscle mass economy in the jaw apparatus, despite the critical importance of any excess ballast for birds, was a serious drawback of Dzerzhinsky’s method. A new model based on the separation of the kinematics of bill opening and food object compression provides a solution for the problem of economical use of muscle mass in the avian jaw apparatus. The length of the major jaw adductor muscles remains virtually unchanged when the bill is opened, so that these muscles act as inextensible links of the closed multilink kinematic chain of the kinetic skull. Length change upon mouth opening being unnecessary, these muscles can be short-fibered, that is, lightweight but strong. The large force of these muscles is only used by the bird when the object is fixed in the bill and the jaw joint locking mechanism is switched on; a different kinematic chain used for object compression only is formed in this case. If the nonparallel arrangement of the pterygoid muscle and the jugal bar is also taken into account, the avian capacity for using the full force potential of the jaw adductor muscles without careful adjustment to each food object and the specific conditions of food object capturing becomes easy to understand. The main elements of the proposed model, which were found out by Dzerzhinsky himself, acquire a new meaning in light of the concept of economical use of the muscle mass. This enables us to make a major generalization concerning the adaptive significance of the cranial kinesis in birds as a means of reducing the jaw adductors’ mass without decreasing their force. In addition to the new model of jaw mechanics, other promising directions are proposed for the development of Dzerzhinsky’s work based on the expansion of the set of methods. The analysis of the aponeurotic framework of jaw muscles from the standpoint of optimizing the spatial packaging of muscle fibers may be a promising topic for research based on a combination of computer tomography and computational modeling.