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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 tympanic cavity. Along the way to the adaptation to the water lifestyle in semi-aqueous and aqueous 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. Morphofunctional 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 aqueous species appear at early stages of development in spite of intrauterine growth without the direct influence of environmental conditions.

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In both phylogenesis and ontogenesis, the inner ear is formed first as a core, which, phylogenetically, is the most ancient part of the peripheral region of the auditory system. Along with the development of the inner ear, other links of different evolutionary ages (the auricle is a phylogenetically young link) begin to form in the peripheral auditory system.

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The peripheral part of acoustic analyzer was studied during pre- and postnatal development of mammals. The main trends of structural evolution of the outer, middle, and inner ear were followed in representatives of different ecological groups during postnatal development. The use of ecologomorphological approach made it possible to establish that specific structural features of the hearing organ in different mammals are determined by adaptation to specific acoustic properties of the environment. It was shown that morphofunctional adaptations directed at optimization of acoustic sensitivity in different environmental conditions were leading in the hearing organ evolution. Comparative-embryological studies of the peripheral part of acoustic system made it possible to determine the stages of formation of individual structures and establish general patterns of prenatal development of the organs of hearing and equilibrium in different mammals.

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We have conducted for the first time a comparative study of the prenatal development of the vestibular system in representatives of pinnipeds (Phocidae, Otariidae, Odobenidae) and cetaceans (Odontoceti, Mysticeti) in comparison with terrestrial mammals. This allowed us to perform a detailed study of the structural organization of the inner ear in species with different ecological specialization, to find out adaptive aspects and stages of the formation of cochlear and vestibular structures in each of the studied species, and to correlate it with the properties of the environment. Studies on early embryogenesis of the labyrinth in cetaceans and pinnipeds, which represent a special direction in the evolution of placentates, give a significant contribution to the solution of problem about the evolutionary origin of the labyrinth in mammals studied. Sensory systems clearly demonstrate the scope of evolutionary and adaptive transformations, which appear in mammals during the transition from the terrestrial to the aquatic mode of life. At the same time, these results may provide help in understanding the general trends of the development of structure and function of the inner ear in mammals as a whole.

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This paper is the first comparative study into prenatal ontogenesis of the peripheral auditory system in mammals. A wide range of ecologically different species was studied, including terrestrial, semi-aquatic and aquatic forms with auditory systems functioning at low, medium, and high frequencies. Structural peculiarities of the development of auditory organ were determined for each studied species in relation to its operation frequency and acoustic properties of the outer medium. The origin and individual stages of morphological adaptations found in semi-aquatic, aquatic, and echolocating species are described. The data obtained allowed to reveal general principles of development of the peripheral auditory system in mammals with different ecological specialization.

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