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D.R. Ketten
tion abilities in essentially toothless species such as the Monodontidae (narwhals and belugas) and Ziphiidae (pelagic beaked whales). It is possible, as
they suggest, that this system operates as an adjunct to some other more
general system, but the hypothesis is problematic because neural links to
auditory centers have not been established.
In mysticetes, no robust theories for sound conduction are currently
available. Whether the external canal is functional is unclear. The external
canal has a wider bore than in odontocetes and connects directly with the
"glove finger," a highly derived, everted tympanic membrane (Fig. 2.4). At
its proximal end the external canal flares, forming a cup around the glove
finger. Active ceruminous glands in this area secrete a conical wax cap over
the tip of the glove finger that accumulates with age (Fraser and Purves
1960). Physically, the glove finger is a long (20 to 50mm), thick-walled
(-1 mm), and broad (-20mm average diameter) membranous tube with a
sealed, blunt outer end, which projects laterally from the middle ear cavity
(Figs. 2.2B, 2.4). Exact dimensions and orientation vary by species. In most
mysticetes, it lies in a postero-Iateral bony channel formed by the squamosal
and exoccipital bones (Fig. 2.2B). According to Fraser and Purves (1960),
in some species a ligament extends from the manubrium of the malleus into
the glove finger lumen, attaching to its inner wall approximately one-third
of the way along the membrane's length.
Because of the complex and robust construction of the glove finger and
the clear connection with the residual external canal, the consensus of
anatomical data is that mysticete external auditory canals are functional, at
least as a source and repository for waxy secretions that abut the tympanic
membrane. The intimate association of the glove finger and its wax cap with
the bony walls and tissues of its squamosal trough strongly suggests sound
reception via bone conduction, but to date there is no clear demonstration
of any coherent volume of soft tissue or fatty structures that are as clearly
connected with mysticete middle ears as the multilobed fat structures found_
in odontocetes except for the wax plug. Also, an imposing squamosal shield
juts outward from the skull, and to varying degrees in each mysticete
species, wraps ventrally over each ear bone (Figs. 2.2B, 2.4). Aside from the
fact that this shield is between the ear and the world, it has no obvious
acoustic element or specialization. However, it is a relatively unique structure, both in terms of shape and association with the whale ear. Like the
wax plug, its uniqueness and association make it worth at least preliminary
consideration. At this stage, all we know clearly about mysticete sound
reception is that the great whales do not have an ear, skull, jaw, and soft
tissue suite that is a larger-scale version of the odontocete head; therefore
sound reception mechanism differ in the two suborders.
Recent observations on low-frequency sound production and reception
in elephants may be relevant to whale hearing. For a little more than a
decade there has been an inconsistency in the literature between behavioral observations and the one available elephant audiogram. Field and zoo
D.R. Ketten
tion abilities in essentially toothless species such as the Monodontidae (narwhals and belugas) and Ziphiidae (pelagic beaked whales). It is possible, as
they suggest, that this system operates as an adjunct to some other more
general system, but the hypothesis is problematic because neural links to
auditory centers have not been established.
In mysticetes, no robust theories for sound conduction are currently
available. Whether the external canal is functional is unclear. The external
canal has a wider bore than in odontocetes and connects directly with the
"glove finger," a highly derived, everted tympanic membrane (Fig. 2.4). At
its proximal end the external canal flares, forming a cup around the glove
finger. Active ceruminous glands in this area secrete a conical wax cap over
the tip of the glove finger that accumulates with age (Fraser and Purves
1960). Physically, the glove finger is a long (20 to 50mm), thick-walled
(-1 mm), and broad (-20mm average diameter) membranous tube with a
sealed, blunt outer end, which projects laterally from the middle ear cavity
(Figs. 2.2B, 2.4). Exact dimensions and orientation vary by species. In most
mysticetes, it lies in a postero-Iateral bony channel formed by the squamosal
and exoccipital bones (Fig. 2.2B). According to Fraser and Purves (1960),
in some species a ligament extends from the manubrium of the malleus into
the glove finger lumen, attaching to its inner wall approximately one-third
of the way along the membrane's length.
Because of the complex and robust construction of the glove finger and
the clear connection with the residual external canal, the consensus of
anatomical data is that mysticete external auditory canals are functional, at
least as a source and repository for waxy secretions that abut the tympanic
membrane. The intimate association of the glove finger and its wax cap with
the bony walls and tissues of its squamosal trough strongly suggests sound
reception via bone conduction, but to date there is no clear demonstration
of any coherent volume of soft tissue or fatty structures that are as clearly
connected with mysticete middle ears as the multilobed fat structures found_
in odontocetes except for the wax plug. Also, an imposing squamosal shield
juts outward from the skull, and to varying degrees in each mysticete
species, wraps ventrally over each ear bone (Figs. 2.2B, 2.4). Aside from the
fact that this shield is between the ear and the world, it has no obvious
acoustic element or specialization. However, it is a relatively unique structure, both in terms of shape and association with the whale ear. Like the
wax plug, its uniqueness and association make it worth at least preliminary
consideration. At this stage, all we know clearly about mysticete sound
reception is that the great whales do not have an ear, skull, jaw, and soft
tissue suite that is a larger-scale version of the odontocete head; therefore
sound reception mechanism differ in the two suborders.
Recent observations on low-frequency sound production and reception
in elephants may be relevant to whale hearing. For a little more than a
decade there has been an inconsistency in the literature between behavioral observations and the one available elephant audiogram. Field and zoo