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Marine Mammal Physiology: Requisites for Ocean Living
Echolocation clicks are short duration (tens to hundreds of µs), transient signals,
although the exact type of signal varies by species. For example, the bottlenose dolphin,
which is a delphinid, produces clicks that are broadband and may have half-power frequency bandwidths of greater than 85 kHz (Houser et al. 1999). From a sonar design
perspective, such a signal provides considerable information about the targets that are
ensonified by returning echoic information to the dolphin across a large band of frequencies. Conversely, the harbor porpoise, which is a phocoenid, produces narrower band signals with peak frequencies typically between 120 and 140 kHz (Au et al. 2006; Koblitz et al.
2012). It has been hypothesized that the smaller harbor porpoise produces high frequency,
narrowband signals in order to avoid detection by one of its primary predators, the killer
whale (Orcinus orca) (Koblitz et al. 2012); that is, the signal is produced above the killer
whale’s upper-frequency limit of hearing.
Echolocation clicks are projected through the melon, a specialized body of lipids that
forms the forehead of the odontocete. The lipids are similar in composition to those that
fill the lower jaw and which connect to the auditory bullae. These so-called acoustic fats
form a structure that has a low sound-velocity core with a gradation of higher velocity
fats surrounding it (Norris and Harvey 1974). The melon serves to collimate the projected
echolocation signal and the echolocation beam has been found to be narrow and forward
projected in all species studied (e.g., see Table 1 in Koblitz et al. 2012). Echolocation clicks
are produced in series, or in trains, and for some species they can vary dynamically in
frequency content. The echolocation beamwidth can also be varied to some extent and
steered, much like one’s vision can be changed by movement of the eyes but without
movement of the head (Moore et al. 2008; Madsen et al. 2010). The ability to manipulate the frequency content of click and the beam structure is likely achieved through
manipulation of the melon by underlying muscle in combination with changes in the
shape and air volume of associated air sacs, which would act as reflectors; however, the
exact process remains uncharacterized (Cranford 1992; Moore et al. 2008; Madsen et al.
2010). The melon and the acoustic fats of the lower jaw are also vascularized, yet they are
metabolically inert structures requiring little in the way of oxygen or glucose delivery
(Houser et al. 2004). Since the lipid density is dependent on temperature, and the density of the lipid will affect the bulk modulus, shear modulus, and sound speed through
the acoustic fats, it has been proposed that the vasculature might serve to regulate the
temperature of the acoustic fats and thus stabilize the propagation of sound under conditions of varying temperature (e.g., changes in water depth or seasonal temperature
changes) (Houser et al. 2004).
The mechanisms by which mysticete whales produce sounds are unknown, and unlike
odontocetes where considerable experimental evidence supports the phonic lips as the primary sound production mechanism, it is quite possible that mysticete sound production is
laryngeal in origin. Mysticete whales produce a variety of call types that can roughly be categorized as songs, simple calls, complex calls, and knocks, grunts, pulses or clicks (Clark 1990).
The duration of sounds produced by mysticetes may range from <100 ms (e.g., knocks and
grunts) to several seconds (simple calls) and may contain dominant frequency content as low
as 10 Hz and as high as 10 kHz. Many signals may be strung together in units and phrases
to produce songs, as is probably most well known for the humpback whale (Megaptera novaeangliae). Mysticete vocalizations can be produced at high amplitudes and the source levels of
some vocalizations have been observed to exceed (root-mean-squared) sound pressure levels of 180 dB (re 1 µPa); (decibels relative to a reference pressure of 1 micro-Pascal). There is
considerable species variability in sound production by mysticetes and a summary of the
frequency, duration, and source levels can be found in Au and Hastings (2008).
Marine Mammal Physiology: Requisites for Ocean Living
Echolocation clicks are short duration (tens to hundreds of µs), transient signals,
although the exact type of signal varies by species. For example, the bottlenose dolphin,
which is a delphinid, produces clicks that are broadband and may have half-power frequency bandwidths of greater than 85 kHz (Houser et al. 1999). From a sonar design
perspective, such a signal provides considerable information about the targets that are
ensonified by returning echoic information to the dolphin across a large band of frequencies. Conversely, the harbor porpoise, which is a phocoenid, produces narrower band signals with peak frequencies typically between 120 and 140 kHz (Au et al. 2006; Koblitz et al.
2012). It has been hypothesized that the smaller harbor porpoise produces high frequency,
narrowband signals in order to avoid detection by one of its primary predators, the killer
whale (Orcinus orca) (Koblitz et al. 2012); that is, the signal is produced above the killer
whale’s upper-frequency limit of hearing.
Echolocation clicks are projected through the melon, a specialized body of lipids that
forms the forehead of the odontocete. The lipids are similar in composition to those that
fill the lower jaw and which connect to the auditory bullae. These so-called acoustic fats
form a structure that has a low sound-velocity core with a gradation of higher velocity
fats surrounding it (Norris and Harvey 1974). The melon serves to collimate the projected
echolocation signal and the echolocation beam has been found to be narrow and forward
projected in all species studied (e.g., see Table 1 in Koblitz et al. 2012). Echolocation clicks
are produced in series, or in trains, and for some species they can vary dynamically in
frequency content. The echolocation beamwidth can also be varied to some extent and
steered, much like one’s vision can be changed by movement of the eyes but without
movement of the head (Moore et al. 2008; Madsen et al. 2010). The ability to manipulate the frequency content of click and the beam structure is likely achieved through
manipulation of the melon by underlying muscle in combination with changes in the
shape and air volume of associated air sacs, which would act as reflectors; however, the
exact process remains uncharacterized (Cranford 1992; Moore et al. 2008; Madsen et al.
2010). The melon and the acoustic fats of the lower jaw are also vascularized, yet they are
metabolically inert structures requiring little in the way of oxygen or glucose delivery
(Houser et al. 2004). Since the lipid density is dependent on temperature, and the density of the lipid will affect the bulk modulus, shear modulus, and sound speed through
the acoustic fats, it has been proposed that the vasculature might serve to regulate the
temperature of the acoustic fats and thus stabilize the propagation of sound under conditions of varying temperature (e.g., changes in water depth or seasonal temperature
changes) (Houser et al. 2004).
The mechanisms by which mysticete whales produce sounds are unknown, and unlike
odontocetes where considerable experimental evidence supports the phonic lips as the primary sound production mechanism, it is quite possible that mysticete sound production is
laryngeal in origin. Mysticete whales produce a variety of call types that can roughly be categorized as songs, simple calls, complex calls, and knocks, grunts, pulses or clicks (Clark 1990).
The duration of sounds produced by mysticetes may range from <100 ms (e.g., knocks and
grunts) to several seconds (simple calls) and may contain dominant frequency content as low
as 10 Hz and as high as 10 kHz. Many signals may be strung together in units and phrases
to produce songs, as is probably most well known for the humpback whale (Megaptera novaeangliae). Mysticete vocalizations can be produced at high amplitudes and the source levels of
some vocalizations have been observed to exceed (root-mean-squared) sound pressure levels of 180 dB (re 1 µPa); (decibels relative to a reference pressure of 1 micro-Pascal). There is
considerable species variability in sound production by mysticetes and a summary of the
frequency, duration, and source levels can be found in Au and Hastings (2008).