Marine Mammal Ears: Functional Anatomy

All marine mammals have special adaptations of the external (closure, wall thickening, wax

plugs) and middle ear (thickened middle ear mucosa, broad Eustachian tubes) consistent with

deep, rapid diving and long-term submersion, but they retain an air-filled middle ear and have the

same basic inner ear configuration as terrestrial species. Each group has distinct adaptations that

correlate with both their hearing capacities and with their relative level of adaptation to water.

Cetaceans

Outer Ear

Pinnae are absent, although vestigial pinnal rings occur in some individuals. External

auditory canals are present in Cetacea, but it is debatable whether they are functional. In

odontocetes, the external canal is exceptionally narrow and plugged with cellular debris and

dense, waxy cerumen. The canal has no observable attachment to the tympanic membrane or the

middle ear. In mysticetes, the canal is narrow along most of its length, but the proximal end

flares, cloaking the "glove finger", a complex, thickened membrane capped by a waxy mound in

adults (Reysenbach de Haan, 1956).

Reysenbach de Haan (1956) and Dudok van Heel (1962) were among the first researchers to

suggest soft tissue paths as an alternative to conventional external canal sound conduction in

odontocetes. Reysenbach de Haan (1956) reasoned that since the transmission characteristics of

blubber and sea water are similar, using a canal occluded with multiple substances would be less

efficient than conduction through body fat, fluid, or bone. Dudok van Heel (1962) found the

minimum audible angle in Tursiops was more consistent with an interbullar critical interaural

distance than with intermeatal distances and concluded the canal was irrelevant. A passive

resonator system involving the teeth of the lower jaw has been suggested for delphinids

(Goodson and Klinowska 1990), but this cannot be considered a general explanation because it

cannot account for echolocation by relatively toothless species; e.g. the Monodontidae (narwhals

and belugas) and Ziphiidae (pelagic beaked whales). Currently, the lower jaw is considered the

primary reception path for ultrasonic signals in odontocetes. Norris (1968, 1980) observed that

the odontocete lower jaw has two exceptional properties: a fatty core and a thin, ovoid "pan

bone" area in the posterior third of the mandible. Norris (1969) speculated this mandibular fat

channel acts as a preferential low impedance path to the middle ear and the pan bone as an

acoustic window to the middle ear region.

Several forms of data support this hypothesis. The fats in the mandible are wax esters with

acoustic impedances close to sea water (Varanasi and Malins 1971). Evoked responses and

cochlear potentials in Stenella and Tursiops were significantly greater for sound stimuli above 20

kHz from transducers placed on or near the mandible (Bullock et al. 1968, McCormick et al.

1970). Measurements with implanted hydrophones in severed Tursiops heads found best

transmission characteristics for sources directed into the pan bone (Norris and Harvey 1974).

Brill et al. (1988) found that encasing the lower jaw in neoprene significantly impaired

performance in echolocation tasks. Some results disagreed, notably those by Popov and Supin

(1990b) and Bullock et al. (1968), who found best thresholds for low to sonic frequencies near

the external meatus. However, recent computerized tomographic and magnetic resonance

imaging of dolphins revealed a second channel of similar fats lateral to the pan bone (Ketten

1994), which may explain the discrepancy in the data since the lateral fatty lobes are near the

meatus in delphinids. No discreet soft tissue channels to the ear have as yet been identified in

mysticetes.

Ear placement

The inner ear is housed in a periotic bone fused at one or more points to the tympanic, or

middle ear bone. This "tympano-periotic" bullar complex is located outside the skull, which

increases the acoustic separation of the middle and inner ears, as discussed earlier in the section

on localization and interaural distances.

Odontocete tympano-periotics are suspended in a spongy mucosa, the peribullar plexus, by

five or more sets of ligaments. This mucosal cushion and the lack of bony connections to the

skull isolate the ear from bony sound conduction and hold the tympanic loosely in line with the

mandibular fatty channels and pan bone.

In mysticetes, extensive bony flanges wedge the periotic against the skull. The tight coupling

of these flanges to the skull suggests both bony and soft tissue sound conduction to the ear occur

in baleen whales.