248
Marine Mammal Physiology: Requisites for Ocean Living
tympanic membrane and the canal is filled with cerumen and cellular debris. These
factors contribute to arguments that the outer ear of the dolphin contributes relatively
little to sound reception (Kellogg 1938; McCormick et al. 1970; Ridgway 2000). In contrast, the external ear canal of the mysticete whale clearly connects to an everted tympanic membrane.
Abundant anatomical, physiological, and behavioral evidence suggests that the acoustic pathway to the odontocete ear primarily is achieved via fatty channels that connect
directly to the auditory bulla. The fats, commonly referred to as acoustic fats, are contained
within the lower jaw of the dolphin and consist of wax esters that have acoustic impedances similar to that of water. A thin, bony region of the lower mandible, termed the pan,
is in connection with mandibular fat bodies both internally and externally. The fat bodies
extend posteriorly from the jaw to connect with the tympanic bone and middle ear, while
another fat body also connects to the tympanic bone. Studies have demonstrated that
sound passes into the jaw fats, and it is believed that the sound energy is directly coupled
to the tympanic bone and middle ear via this path (Brill and Harder 1991). Odontocetes
are particularly sensitive to ultrasonic frequencies (i.e., >25 kHz or so) along the jaw, and
specifically in the region of the pan, suggesting the importance of this pathway in echolocation (see below). Indeed, hearing via this pathway is highly directional, that is, there
is a narrow receiving beam for sound along the forward looking longitudinal axis of the
animal (Au and Moore 1984). A narrow receiving beam is an important component of any
sonar system and enables target localization while minimizing unwanted interferences
and reverberation. At lower frequencies, within the communication range of odontocetes
(<20 kHz), hearing is less directional and the greatest sensitivity to sounds within this frequency range appears to be more lateral with acoustically sensitive regions existing near
the external auditory meatus (Popov et al. 2006). Nevertheless, the lack of a connection
between the external ear canal and the tympanum in odontocetes suggests that the meatus
and ear canal are vestigial and contribute little to sound reception. However, the presence
of the posterolateral fat body adjoined to the tympanic might indicate another peripheral
pathway, possibly tuned to lower frequency sound reception (Ketten 1994, 1997; Popov
et al. 2008). Collectively, these findings are suggestive of different modes of hearing between
echolocation and communication signals within the odontocetes.
The middle and inner ears of the odontocetes are contained within the tympanoperiotic complex or the auditory bulla. These bones are the densest bones in the body of an
odontocete, and as noted previously, are detached from the skull (Figure 11.2). The bullae
are suspended by ligamentous connections and partially surrounded both medially and
dorsally by air, an anatomical arrangement that serves to acoustically isolate the ears. The
isolation and presence of air between the bullae likely contributes to changes in received
sound as it passes from one ear to the other and probably contributes to sound localization
capabilities, that is, the air may impede sound conduction and contribute to spectral differences in the sound received at each ear (Houser et al. 2004; Mulsow et al. 2014).
The middle ear of the odontocete is characterized by ossicles that are more massive
than in most terrestrial mammals and stiffened by ligaments and membranous sheaths.
Characteristic of the ossicular chain is a fixed connection of the malleus to the tympanic
bulla and the presence of a robust stapedial ligament attached to the head of the stapes.
Although the tympanic membrane has a membranous attachment to the malleus, the lack
of a connection between the external auditory canal and the tympanic membrane has led
to much debate over the exact function of the ossicular chain in odontocete sound reception. Prior work in which the tympanic conus, malleus, and external auditory canal were
disrupted had little effect on cochlear potentials (McCormick et al. 1970). This finding
Marine Mammal Physiology: Requisites for Ocean Living
tympanic membrane and the canal is filled with cerumen and cellular debris. These
factors contribute to arguments that the outer ear of the dolphin contributes relatively
little to sound reception (Kellogg 1938; McCormick et al. 1970; Ridgway 2000). In contrast, the external ear canal of the mysticete whale clearly connects to an everted tympanic membrane.
Abundant anatomical, physiological, and behavioral evidence suggests that the acoustic pathway to the odontocete ear primarily is achieved via fatty channels that connect
directly to the auditory bulla. The fats, commonly referred to as acoustic fats, are contained
within the lower jaw of the dolphin and consist of wax esters that have acoustic impedances similar to that of water. A thin, bony region of the lower mandible, termed the pan,
is in connection with mandibular fat bodies both internally and externally. The fat bodies
extend posteriorly from the jaw to connect with the tympanic bone and middle ear, while
another fat body also connects to the tympanic bone. Studies have demonstrated that
sound passes into the jaw fats, and it is believed that the sound energy is directly coupled
to the tympanic bone and middle ear via this path (Brill and Harder 1991). Odontocetes
are particularly sensitive to ultrasonic frequencies (i.e., >25 kHz or so) along the jaw, and
specifically in the region of the pan, suggesting the importance of this pathway in echolocation (see below). Indeed, hearing via this pathway is highly directional, that is, there
is a narrow receiving beam for sound along the forward looking longitudinal axis of the
animal (Au and Moore 1984). A narrow receiving beam is an important component of any
sonar system and enables target localization while minimizing unwanted interferences
and reverberation. At lower frequencies, within the communication range of odontocetes
(<20 kHz), hearing is less directional and the greatest sensitivity to sounds within this frequency range appears to be more lateral with acoustically sensitive regions existing near
the external auditory meatus (Popov et al. 2006). Nevertheless, the lack of a connection
between the external ear canal and the tympanum in odontocetes suggests that the meatus
and ear canal are vestigial and contribute little to sound reception. However, the presence
of the posterolateral fat body adjoined to the tympanic might indicate another peripheral
pathway, possibly tuned to lower frequency sound reception (Ketten 1994, 1997; Popov
et al. 2008). Collectively, these findings are suggestive of different modes of hearing between
echolocation and communication signals within the odontocetes.
The middle and inner ears of the odontocetes are contained within the tympanoperiotic complex or the auditory bulla. These bones are the densest bones in the body of an
odontocete, and as noted previously, are detached from the skull (Figure 11.2). The bullae
are suspended by ligamentous connections and partially surrounded both medially and
dorsally by air, an anatomical arrangement that serves to acoustically isolate the ears. The
isolation and presence of air between the bullae likely contributes to changes in received
sound as it passes from one ear to the other and probably contributes to sound localization
capabilities, that is, the air may impede sound conduction and contribute to spectral differences in the sound received at each ear (Houser et al. 2004; Mulsow et al. 2014).
The middle ear of the odontocete is characterized by ossicles that are more massive
than in most terrestrial mammals and stiffened by ligaments and membranous sheaths.
Characteristic of the ossicular chain is a fixed connection of the malleus to the tympanic
bulla and the presence of a robust stapedial ligament attached to the head of the stapes.
Although the tympanic membrane has a membranous attachment to the malleus, the lack
of a connection between the external auditory canal and the tympanic membrane has led
to much debate over the exact function of the ossicular chain in odontocete sound reception. Prior work in which the tympanic conus, malleus, and external auditory canal were
disrupted had little effect on cochlear potentials (McCormick et al. 1970). This finding