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Chapter eleven: Acoustics
However, basal thickness are flaccid and the widths are broad, and the laminar supports are either poorly developed or absent, all of which further supports the idea of
infrasonic specialization.
As with odontocetes, auditory fiber diameters, ganglion cell counts, and innervation
densities are generally greater than observed in terrestrial mammals and presumably are
related to the same adaptation of decreasing latencies and enabling the extraction of information from complex acoustic arrivals. The mysticete brain does not demonstrate the same
degree of hypertrophy of brain auditory structures observed in the odontocetes, although
the auditory cortex appears to have a greater cellular volume and other comparable specializations exist (Eriksen and Pakkenberg 2007). With an EQ < 1.0 for all mysticetes, brain
size appears to be uncoupled from body size and deviates less from allometric predictions
than it does in odontocete counterparts (Worthy and Hickie 1986; Boddy et al. 2012), which
is likely driven by the evolution of underwater echolocation in odontocete species.
11.2.1.2 Sound production
The location of sound production by odontocetes, whether it occurred in the nasal passages or the larynx, was once hotly debated. However, nearly all data to date have provided evidence that sound production occurs in the nasal system of odontocetes. It is now
commonly accepted that sound production in odontocetes is primarily achieved through
the use of specialized structures located within the nasal passages and below the blowhole
plug. These structures, termed the phonic lips, are believed to be used in producing the
most typical of dolphin acoustic signals, mainly whistles, burst-pulses, and echolocation
clicks (Cranford 2000). The odontocete nasal passages have been demonstrated to pressurize just prior to signal production (Ridgway et al. 1980; Amundin and Andersen 1983) and
it is the passage of pressurized air across the phonic lips that is responsible for signal production. The phonic lips are associated with a series of air sacs (pre-maxillary, vestibular,
and accessory) that support the pneumatic operation of signal production, although the
exact role they play is not well understood.
Whistles are tonal signals, commonly frequency modulated, that are typically produced at fundamental frequencies below 20 kHz (but which may have harmonic components at higher frequencies). Whistles are believed to be used primarily for communication
and social interaction, but not all species of odontocetes produce whistles (e.g., the sperm
whale [Physeter macrocephalus]). Whistles demonstrate a more omnidirectional propagation from the animal relative to signals like echolocation clicks (although the higher the
frequency of the signal, the more directionality it will display). Based upon the contours
displayed on a spectrogram, whistles can roughly be grouped as constant frequency,
upsweep, downsweep, concave, convex, or sinusoidal (multiple) (see Au and Hastings
2008, for a review). The role of the different signal types in communication is largely
unknown, although the identification of individually distinctive whistles produced by
dolphins and the context in which they are produced have been used as evidence for a
class of signature whistles that dolphins utilize for identification and individual localization (King et al. 2013). The signature whistle hypothesis, however, does have its detractors
(e.g., McCowan and Reiss 2001).
Burst pulses are short duration, broadband signals produced in rapid succession and
which resemble squeaks, creaks, or groans. They are produced by all species of odontocete
studied to date, although there is some deviation and specialization of the theme (e.g., sperm
whale production of codas). Burst pulses are much less studied than whistles and will not
be discussed extensively here. However, it is worth noting that burst pulses may be the primary means of acoustic communication in some species (e.g., harbor porpoises).
Chapter eleven: Acoustics
However, basal thickness are flaccid and the widths are broad, and the laminar supports are either poorly developed or absent, all of which further supports the idea of
infrasonic specialization.
As with odontocetes, auditory fiber diameters, ganglion cell counts, and innervation
densities are generally greater than observed in terrestrial mammals and presumably are
related to the same adaptation of decreasing latencies and enabling the extraction of information from complex acoustic arrivals. The mysticete brain does not demonstrate the same
degree of hypertrophy of brain auditory structures observed in the odontocetes, although
the auditory cortex appears to have a greater cellular volume and other comparable specializations exist (Eriksen and Pakkenberg 2007). With an EQ < 1.0 for all mysticetes, brain
size appears to be uncoupled from body size and deviates less from allometric predictions
than it does in odontocete counterparts (Worthy and Hickie 1986; Boddy et al. 2012), which
is likely driven by the evolution of underwater echolocation in odontocete species.
11.2.1.2 Sound production
The location of sound production by odontocetes, whether it occurred in the nasal passages or the larynx, was once hotly debated. However, nearly all data to date have provided evidence that sound production occurs in the nasal system of odontocetes. It is now
commonly accepted that sound production in odontocetes is primarily achieved through
the use of specialized structures located within the nasal passages and below the blowhole
plug. These structures, termed the phonic lips, are believed to be used in producing the
most typical of dolphin acoustic signals, mainly whistles, burst-pulses, and echolocation
clicks (Cranford 2000). The odontocete nasal passages have been demonstrated to pressurize just prior to signal production (Ridgway et al. 1980; Amundin and Andersen 1983) and
it is the passage of pressurized air across the phonic lips that is responsible for signal production. The phonic lips are associated with a series of air sacs (pre-maxillary, vestibular,
and accessory) that support the pneumatic operation of signal production, although the
exact role they play is not well understood.
Whistles are tonal signals, commonly frequency modulated, that are typically produced at fundamental frequencies below 20 kHz (but which may have harmonic components at higher frequencies). Whistles are believed to be used primarily for communication
and social interaction, but not all species of odontocetes produce whistles (e.g., the sperm
whale [Physeter macrocephalus]). Whistles demonstrate a more omnidirectional propagation from the animal relative to signals like echolocation clicks (although the higher the
frequency of the signal, the more directionality it will display). Based upon the contours
displayed on a spectrogram, whistles can roughly be grouped as constant frequency,
upsweep, downsweep, concave, convex, or sinusoidal (multiple) (see Au and Hastings
2008, for a review). The role of the different signal types in communication is largely
unknown, although the identification of individually distinctive whistles produced by
dolphins and the context in which they are produced have been used as evidence for a
class of signature whistles that dolphins utilize for identification and individual localization (King et al. 2013). The signature whistle hypothesis, however, does have its detractors
(e.g., McCowan and Reiss 2001).
Burst pulses are short duration, broadband signals produced in rapid succession and
which resemble squeaks, creaks, or groans. They are produced by all species of odontocete
studied to date, although there is some deviation and specialization of the theme (e.g., sperm
whale production of codas). Burst pulses are much less studied than whistles and will not
be discussed extensively here. However, it is worth noting that burst pulses may be the primary means of acoustic communication in some species (e.g., harbor porpoises).