This week I’ve been talking a bit about deafness and human hearing. A human cochlea is tiny, and is located in a bony stucture near in the skull called the bulla of the temporal bone. The temporal bone is oft said to be the hardest bone in the body. Predictably, almost all other mammals share this structure (the bulla), as a protective shield around the inner ear. Humans’ bulla are quite small, about the size of a large marble, but that of a whale’s (and especially, a blue whale) is enornous, reaching sizes of a Nerf football. (See picture below the fold of bulla taken from a 20 ft humpback whale.)
Unlike humans, whose bullae (and middle/inner ear) is inside the skull, whale’s bulla is located inside their jaw. Incoming auditory information enters their head straight-on and travels through their fat-filled jawbone to the bulla. Their bulla is curved, like a seashell, which aids in collecting and amplifying sounds.
Apparently sound brings the tympanic bone (located inside the bulla), and especially its thin tympanic plate, into vibration. The ossicles in the air-filled middle ear cavity form a bridge from the tympanic plate to the periotic bone connecting the vibrating plate to the oval window and the whale’s cochlea. The equivalent of the tympanic plate in humans is the eardrum (the tympanic membrane).
If you’re interested in the evolution of the whale ear (quite fascinating, as it catalogs their travels from sea to land to sea), PZ has written an informative post here. Afarensis also has an article on whale semicircular canals (part of the vestibular system) here.