58
D.R. Ketten
frequency relationship. While the scaling of gross features of odontocete
ears to their body mass is isomorphic with that of land mammals (Fig. 2.1),
the scaling of acoustically functional elements, particularly of the inner ear
is entirely different. Put another way, cetaceans and land mammals, despite
their overt differences in shape, have a similar bauplan for gross construction of the ear, but cetaceans evolved a radically different functional
acoustic morphometry, effectively a zeeohrplan, that permits underwater
ultrasonic hearing in a megascale ear (Ketten 1984,1992). The next section
details the salient features of this alternative aquatic ear.
5. Cetacean Ears
Hearing capacities are the result of the integrated activity of the ear's three
fundamental divisions: (1) the outer ear captures sound, (2) the middle ear
selectively transfers acoustical power to the inner ear, and (3) the inner ear
performs a spectral analysis and transforms the middle ear's mechanical
input into neural impulses. In the context of this chapter, the primary question about the outer ear is: How is water-borne sound captured? For the
middle ear, the significant issue is: Does an impedance matching function
remain? For the inner ear, it is: How do whale ears achieve exceptional
frequency representation?
5.1 The Outer Ear
The outer ear is subdivided conventionally into a pinna or ear flap, a funnelshaped concha, and the ear canal or auditory tube. All three elements are
important for the collection and transmission of sound power to the middle
and inner ear. External pinnae are important aids also to localization, acting
as asymmetric funnels that selectively admit sounds along the pinnal axis
(Heffner and Heffner 1992; Rosowski 1994). Clearly these are important
functions for a mammalian ear, yet whales and dolphins appear to have
abandoned at least two and possibly all three outer ear elements.
The evolutionary head remodeling process of telescoping mentioned
in the introduction is covered thoroughly in other chapters (Cranford,
Chapter 3; Aroyan et aI., Chapter 10), but some points bear repeating here
because of their impact on the peripheral auditory system, particularly on
the outer ear. Telescoping had a profound effect on sound reception and
ear position. As the rostrum elongated, the cranial vault foreshortened and
the nares were pulled rearward to a dorsal position behind the eyes. At the
same time, the maxillary bones of the upper jaw were transposed back to
the vertex of the skull, overlapping the compressed frontal bones. Telescoping may have been driven essentially by nonauditory influences, such
as respiration with only a small portion of the head exposed, but it also produced a multilayer skull that seriously impedes sound transmission through
D.R. Ketten
frequency relationship. While the scaling of gross features of odontocete
ears to their body mass is isomorphic with that of land mammals (Fig. 2.1),
the scaling of acoustically functional elements, particularly of the inner ear
is entirely different. Put another way, cetaceans and land mammals, despite
their overt differences in shape, have a similar bauplan for gross construction of the ear, but cetaceans evolved a radically different functional
acoustic morphometry, effectively a zeeohrplan, that permits underwater
ultrasonic hearing in a megascale ear (Ketten 1984,1992). The next section
details the salient features of this alternative aquatic ear.
5. Cetacean Ears
Hearing capacities are the result of the integrated activity of the ear's three
fundamental divisions: (1) the outer ear captures sound, (2) the middle ear
selectively transfers acoustical power to the inner ear, and (3) the inner ear
performs a spectral analysis and transforms the middle ear's mechanical
input into neural impulses. In the context of this chapter, the primary question about the outer ear is: How is water-borne sound captured? For the
middle ear, the significant issue is: Does an impedance matching function
remain? For the inner ear, it is: How do whale ears achieve exceptional
frequency representation?
5.1 The Outer Ear
The outer ear is subdivided conventionally into a pinna or ear flap, a funnelshaped concha, and the ear canal or auditory tube. All three elements are
important for the collection and transmission of sound power to the middle
and inner ear. External pinnae are important aids also to localization, acting
as asymmetric funnels that selectively admit sounds along the pinnal axis
(Heffner and Heffner 1992; Rosowski 1994). Clearly these are important
functions for a mammalian ear, yet whales and dolphins appear to have
abandoned at least two and possibly all three outer ear elements.
The evolutionary head remodeling process of telescoping mentioned
in the introduction is covered thoroughly in other chapters (Cranford,
Chapter 3; Aroyan et aI., Chapter 10), but some points bear repeating here
because of their impact on the peripheral auditory system, particularly on
the outer ear. Telescoping had a profound effect on sound reception and
ear position. As the rostrum elongated, the cranial vault foreshortened and
the nares were pulled rearward to a dorsal position behind the eyes. At the
same time, the maxillary bones of the upper jaw were transposed back to
the vertex of the skull, overlapping the compressed frontal bones. Telescoping may have been driven essentially by nonauditory influences, such
as respiration with only a small portion of the head exposed, but it also produced a multilayer skull that seriously impedes sound transmission through