Scaling of the cetacean middle ear

Functionally interesting dimensions of the tympano-periotic complex were me asured and compared in 18 odontocete and six mysticete species, ranging fro m small porpoises to the blue whale. We determined (i) the masses of the ty mpanic 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), (i ii) the areas of the tympanic plate and oval window (A(1) and A(2)), (iv) t he thickness of the tympanic plate (D), and (v) the densities of the ossicl es (d(M), d(I), and d(S)). In most cases, roughly isometric scaling was fou nd 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 root A(1). The essentially isometric scaling of the t ympanic bone provides a basis for the functional models described by Hemila et al. (1999). The relation of S to M+I is also isometric, but the relatio n 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 th is allometry is unknown. The mean ossicular density is 2.64 g/cm(3) for odo ntocetes, and 2.35 g/cm(3) for mysticetes. The highly mineralized and conve x tympanic plate provides cetaceans with a uniquely large and stiff sound c ollecting area. (C) 1999 Elsevier Science B.V. All rights reserved.