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The anatomy of the Walrus head (Odobenus rosmarus): 4. The ears and their function in aerial and underwater hearing
Kastelein, R.A.; Dubbeldam, J.L.; de Bakker, M.A.G.; Gerrits, N.M. (1996). The anatomy of the Walrus head (Odobenus rosmarus): 4. The ears and their function in aerial and underwater hearing. Aquat. Mamm. 22(2): 95-125
In: Aquatic Mammals. European Association for Aquatic Mammals: Harderwijk. ISSN 0167-5427, more
Peer reviewed article  

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Keyword
    Marine

Authors  Top 
  • Kastelein, R.A.
  • Dubbeldam, J.L.
  • de Bakker, M.A.G.
  • Gerrits, N.M.

Abstract
    Walrus ears have special features which are not found in the ears of most terrestrial carnivores. These are: the lack of pinnae, the long, tubular outer ear of which the lateral side is covered with fat and skin, the ability to open and close the external meatal orifice by auricular muscles, the lining of the cartilaginous and bony parts of the outer ear canal by vascularized tissue, the copious amount of earwax, the large middle ear cavity, the lining of the middle ear cavity by vascularized tissue, the elastic fibres, collagen tissue and cartilaginous rods in the wall of the Eustachian tube, and the dense bones surrounding the base of the outer ear and the entire middle and inner ears. When the ambient pressure increases during diving, the pressure increases in the entire body including the organs and the blood vessels. The pressure in non-collapsible spaces with strong casings can be regulated in two ways: (1) by being in contact with collapsible spaces or (2) by being lined with vascularized mucosa which can contain a varying amount of blood. In the walrus, pressure equilibration between the outer and middle ear likely occurs in both ways. The middle ear cavity is in contact with the respiratory tract via the Eustachian tube and the bony part of the outer ear canal is in contact with the cartilaginous part of the outer ear canal. The middle ear cavity and the outer ear canal are lined with vascularized mucosa. In air, sound waves probably reach the walrus’ tympanic membrane in the same way as they do in terrestrial mammals. Under water, due to large impedance differences between water and the cranial bones, sounds may be reflected off the bones which surround the middle ear. The impedance difference between water and the soft tissues is less, so sound travels most easily through these tissues (cartilage, skin, fat and muscles) to the outer ear canal. The cartilaginous tubular outer ear with its vascular lining is probably the main pathway by which sound is conducted to the tympanic membrane under water. The vascularized mucosa lining the middle ear may alter the resonance of the middle ear cavity, and when inflated during diving, rigidify the ossicle chain and improve the conduction of high frequency sounds.

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