The middle ear in the ontogenesis of mammals

The middle ear in mammals is characterized by structural variations and a broad spectrum of adaptive transformations related to peculiarities of species ecology, but it preserves the general basic principle of structure in most mammals. In species remote from a phylogenetic point of view but close in ecologic specialization, features of parallelism are observed concerning the development of separate elements of auditory ossicles as well as the way of their interconnection and attachment to the tympanum. Along the way to the adaptation to the water lifestyle in semi-aquatic and aquatic species, new additional structures, not intrinsic to initial terrestrial forms, have been formed. The use of ecological and morphological approaches to research the peripheral division of the auditory system of mammals with different ecological specialization in the ontogenesis permitted us to reveal that peculiarities of its structure in different groups of mammals are preconditioned by the animals’ adaptation to specific acoustic properties of their environment. Morphological and functional adaptations of the peripheral auditory system aimed at optimizing auditory sensitivity in the environments differing in physical properties are of great importance in evolution. Adaptive specific features in the structure of the middle ear in aquatic species appear at early stages of development in spite of intrauterine growth without the direct influence of environmental conditions.