78
D.R. Ketten
No behavior in bowhead and Northern right whales has been reported that
appears to relate to greater vestibular effort than in any other mysticete.
Nevertheless, the similarity of bullar shape as well as inner ear structures
in these two species suggests that whale ears may have suites of explicitly
functional (as opposed to morphometric) characters that are worth analyzing for systematic affinities. The second group that had well-developed
vestibular elements was the Ziphiidae. The ziphiids noted earlier had exceptionally large, bulbous vestibules, and distinct, moderately sized semicircular canals (Ketten 1998b). Unfortunately, so little is known about the
behavior of most ziphiid species that these observations only raise more
questions than they answer at this time.
5.3.2 The Cochlea
Slepecky (1996) provides a comprehensive overview of mammalian
cochlear structure and the functional role of specific elements. Because different names are occasionally employed for common mammalian ear structures in the animal versus human literature, some basic concepts and terms
will be reviewed briefly. Only notable differences between cetacean ears
and a prototypical mammalian cochlea will be discussed in depth.
The cetacean cochlea has the same fundamental organization as other
mammalian inner ears. It is a fluid-filled, gnomic spiral with a decreasing
radius and uniform rise divided by membranes into three chambers or
scalae (Figs. 2.4,2.6): scala media (cochlear duct), scala tympani, and scala
vestibuli. The scalae appear to be three parallel tubes but are actually two.
An outer U-shaped tube formed by scala tympani and scala vestibuli surrounds the cochlear duct, which is an epithelial walled space that houses
the organ of Corti. The cochlear duct, or scala media, is bounded by the
basilar and vestibular (Reissner's) membranes. The coiled scalae lie inside
the periotic like a spiral staircase. The core of the stair is the modiolus, a
bony tunnel housing the fibers of the auditory branch of the eighth nerve.
The tread of the staircase is the basilar membrane, a graded resonator that
responds as a series of bandpass filters. The organ of Corti, a complex set
of cells that transduces mechanical stimuli into neural responses, is spread
atop the basilar membrane. The diameter of the spiral is greatest at the base
(basal turn) and narrows gradually towards the apex (apical turn). Turn
number varies from 1.5 to 4.5, depending upon the species (Figs. 2.4, 2.7).
The orientation of the cochlear canal in the periotic is probably not an
acoustically significant feature in any mammal, but because it is unusual in
cetaceans and its significance has not been analyzed, it is mentioned here.
The emigration of the tympano-periotic complex carried with it, literally,
the cochlear spiral. In land mammals, including humans, the cochlear or
modiolar axis is oriented anteriorly; that is, the base of the cochlea is posterior and slightly superior to the apex. The main axis of the spiral is oriented parallel to the ground. In most mammals this puts the base of the
D.R. Ketten
No behavior in bowhead and Northern right whales has been reported that
appears to relate to greater vestibular effort than in any other mysticete.
Nevertheless, the similarity of bullar shape as well as inner ear structures
in these two species suggests that whale ears may have suites of explicitly
functional (as opposed to morphometric) characters that are worth analyzing for systematic affinities. The second group that had well-developed
vestibular elements was the Ziphiidae. The ziphiids noted earlier had exceptionally large, bulbous vestibules, and distinct, moderately sized semicircular canals (Ketten 1998b). Unfortunately, so little is known about the
behavior of most ziphiid species that these observations only raise more
questions than they answer at this time.
5.3.2 The Cochlea
Slepecky (1996) provides a comprehensive overview of mammalian
cochlear structure and the functional role of specific elements. Because different names are occasionally employed for common mammalian ear structures in the animal versus human literature, some basic concepts and terms
will be reviewed briefly. Only notable differences between cetacean ears
and a prototypical mammalian cochlea will be discussed in depth.
The cetacean cochlea has the same fundamental organization as other
mammalian inner ears. It is a fluid-filled, gnomic spiral with a decreasing
radius and uniform rise divided by membranes into three chambers or
scalae (Figs. 2.4,2.6): scala media (cochlear duct), scala tympani, and scala
vestibuli. The scalae appear to be three parallel tubes but are actually two.
An outer U-shaped tube formed by scala tympani and scala vestibuli surrounds the cochlear duct, which is an epithelial walled space that houses
the organ of Corti. The cochlear duct, or scala media, is bounded by the
basilar and vestibular (Reissner's) membranes. The coiled scalae lie inside
the periotic like a spiral staircase. The core of the stair is the modiolus, a
bony tunnel housing the fibers of the auditory branch of the eighth nerve.
The tread of the staircase is the basilar membrane, a graded resonator that
responds as a series of bandpass filters. The organ of Corti, a complex set
of cells that transduces mechanical stimuli into neural responses, is spread
atop the basilar membrane. The diameter of the spiral is greatest at the base
(basal turn) and narrows gradually towards the apex (apical turn). Turn
number varies from 1.5 to 4.5, depending upon the species (Figs. 2.4, 2.7).
The orientation of the cochlear canal in the periotic is probably not an
acoustically significant feature in any mammal, but because it is unusual in
cetaceans and its significance has not been analyzed, it is mentioned here.
The emigration of the tympano-periotic complex carried with it, literally,
the cochlear spiral. In land mammals, including humans, the cochlear or
modiolar axis is oriented anteriorly; that is, the base of the cochlea is posterior and slightly superior to the apex. The main axis of the spiral is oriented parallel to the ground. In most mammals this puts the base of the