Anatomical features of the Baikal seal’s middle ear structure in postnatal ontogenesis

The aim of the study is to investigate the anatomical features of all middle ear structures in Baikal seals during postnatal ontogenesis. The study was conducted at the Department of Animal Morphology and Veterinary Sanitation of the Irkutsk State Agrarian University. The heads of Baikal seals of various ages were used for the study. Anatomical, histological, morphometric, and statistical methods were used. Anatomical features were identified in the structure of the osseous base of the middle ear, the microstructure of its mucous membrane, the auditory ossicles, and the topography of the facial nerve. Increased morphometric parameters of the osseous base of the Baikal seal's auditory organ were observed in all age groups compared to newborns. A large number of branched alveolar glands were found in the submucosa of the mucous membrane, with the glands disappearing in a large part of the submucosa adjacent to the tympanic membrane. In the tympanic cavity of the Baikal seal, like all mammals, there are four auditory ossicles: the malleus, the incus, the lenticular ossicle, and the stapes. In newborns and yearlings, the lenticular ossicle is composed of cartilaginous tissue and ossifies with age. Compared to newborns, the absolute mass of the ossicles in all age groups tends to increase, while some linear parameters increase, and others remain unchanged (the head and handle of the malleus). The facial nerve traverses a short distance within the tympanic cavity, giving off the greater petrosal nerve, the stapedius nerve, and the chorda tympani. The identified anatomical features of the Baikal seal's middle ear are adapted to the perception of sounds both on land and in fresh water.