Scaling of the cetacean middle ear

Abstract

Functionally interesting dimensions of the tympano-periotic complex were measured and compared in 18 odontocete and six mysticete species, ranging from small porpoises to the blue whale. We determined (i) the masses of the tympanic and periotic bones ( T and P) and of the ossicles malleus, incus, and stapes ( M, I and S), (ii) the volume occupied by the tympanic bone ( V), (iii) the areas of the tympanic plate and oval window ( A 1 and A 2), (iv) the thickness of the tympanic plate ( D), and (v) the densities of the ossicles ( d M, d I, and d S). In most cases, roughly isometric scaling was found in both toothed and baleen whales. P is isometric to T, and the tympanic bone is structurally isometric in all species studied, although not within mysticetes as a group, shown by the isometric relations of V to T, of T 2/3 to A 1, and of D to √ A 1. The essentially isometric scaling of the tympanic bone provides a basis for the functional models described by Hemilä et al. (1999). The relation of S to M+ I is also isometric, but the relation of M+ I+ S to T is negatively allometric, as is the relation of A 2 to A 1, both with slopes close to 2/3. The possible functional implication of this allometry is unknown. The mean ossicular density is 2.64 g/cm 3 for odontocetes, and 2.35 g/cm 3 for mysticetes. The highly mineralized and convex tympanic plate provides cetaceans with a uniquely large and stiff sound collecting area.