2. Cetacean Ears
59
the head. Telescoping was accompanied also by a dramatic repositioning of
the ears (Fig. 2.2). Two classical mammalian auditory features, the pinna and
external canal, were effectively decommissioned as the middle and inner
ears migrated out from the skull bed. In most odontocetes, the migration is
complete; no attachments to the skull other than suspensory ligaments
remain (Fig. 2.2A). In mysticetes, the ear forms firm, bony connections to
the skull (Fig. 2.2B), but like the ears of odontocetes, the bulk of the ear is
well outside the skull. Eventually, these parallel processes of externalization and elimination led to a remarkable design for sound reception.
5.1.1 Sound Reception: External Ear Analogues
External pinnae are absent in Cetacea, although vestigial pinnal rings are
found embedded in the subcutaneous fat near the external meatus in some
individuals. The meatal opening, generally less than 3mm in diameter even
in the largest mysticetes, is marked externally by a dimple or depression
in the skin. Some form of residual external auditory canal is present in all
cetaceans; the level of integrity varies by species. In general, odontocete
external canals are plugged with cellular debris and dense cerumen, becoming progressively narrower, and ending in a blind caecum that has no
observable connection with the tympanic membrane or temporal bones. It
is unclear whether any segment of the canal is functional in any odontocete. No true association of the canal with the tympanic membrane or
middle ear has been documented in odontocetes.
Reysenbach de Haan (1956) and Dudok van Heel (1962) were among
the first to propose that soft tissues of the head served as ear canal analogues for sound conduction to the odontocete ear. Reysenbach de Haan
reasoned that since the transmission characteristics of blubber and sea
water are similar, using a canal occluded with mixed and variable substances
is inefficient compared to a regular soft tissue or bone conduction path.
Dudok van Heel concluded the canal was not used for hearing because
behavioral measures of minimum audible angle in bottlenose dolphins,
T truncatus, were more consistent with intercochlear than intermeatal
distances.
At present, the bulk of experimental and anatomical studies indicate specialized fatty tissues in the jaw region are the primary route for conveying
sound to odontocete middle and inner ears. The concept of jaw or pan bone
hearing was first proposed by Norris (1968) who observed that the posterior area of the odontocete mandible has two exceptional properties: a large
cavity that is open medially and houses a fatty cylinder and an ovoid of thin
bone called the pan bone with fat overlying it (Figs. 2.2A and 2.3). Norris
(1969) described the fat body in the mandibular channel as a "lozenge ...
of pellucid fats," noting that it attached to the surface of the tympanic bone.
He also observed that the jaw fats resembled fats in the melon core and
were therefore probably acoustically significant (Norris 1968; see also
59
the head. Telescoping was accompanied also by a dramatic repositioning of
the ears (Fig. 2.2). Two classical mammalian auditory features, the pinna and
external canal, were effectively decommissioned as the middle and inner
ears migrated out from the skull bed. In most odontocetes, the migration is
complete; no attachments to the skull other than suspensory ligaments
remain (Fig. 2.2A). In mysticetes, the ear forms firm, bony connections to
the skull (Fig. 2.2B), but like the ears of odontocetes, the bulk of the ear is
well outside the skull. Eventually, these parallel processes of externalization and elimination led to a remarkable design for sound reception.
5.1.1 Sound Reception: External Ear Analogues
External pinnae are absent in Cetacea, although vestigial pinnal rings are
found embedded in the subcutaneous fat near the external meatus in some
individuals. The meatal opening, generally less than 3mm in diameter even
in the largest mysticetes, is marked externally by a dimple or depression
in the skin. Some form of residual external auditory canal is present in all
cetaceans; the level of integrity varies by species. In general, odontocete
external canals are plugged with cellular debris and dense cerumen, becoming progressively narrower, and ending in a blind caecum that has no
observable connection with the tympanic membrane or temporal bones. It
is unclear whether any segment of the canal is functional in any odontocete. No true association of the canal with the tympanic membrane or
middle ear has been documented in odontocetes.
Reysenbach de Haan (1956) and Dudok van Heel (1962) were among
the first to propose that soft tissues of the head served as ear canal analogues for sound conduction to the odontocete ear. Reysenbach de Haan
reasoned that since the transmission characteristics of blubber and sea
water are similar, using a canal occluded with mixed and variable substances
is inefficient compared to a regular soft tissue or bone conduction path.
Dudok van Heel concluded the canal was not used for hearing because
behavioral measures of minimum audible angle in bottlenose dolphins,
T truncatus, were more consistent with intercochlear than intermeatal
distances.
At present, the bulk of experimental and anatomical studies indicate specialized fatty tissues in the jaw region are the primary route for conveying
sound to odontocete middle and inner ears. The concept of jaw or pan bone
hearing was first proposed by Norris (1968) who observed that the posterior area of the odontocete mandible has two exceptional properties: a large
cavity that is open medially and houses a fatty cylinder and an ovoid of thin
bone called the pan bone with fat overlying it (Figs. 2.2A and 2.3). Norris
(1969) described the fat body in the mandibular channel as a "lozenge ...
of pellucid fats," noting that it attached to the surface of the tympanic bone.
He also observed that the jaw fats resembled fats in the melon core and
were therefore probably acoustically significant (Norris 1968; see also