54
D.R. Ketten
as the threshold level of the target in noise (in dB) minus the masker level
(in dB). Critical bands tend to be a constant function of critical ratios
throughout an animal's functional hearing range (Fay 1992). Odontocetes
are better than most mammals at detecting signals in noise and have more
critical bands with smaller critical ratios than other mammals. Odontocete
critical bandwidths are not a constant factor of the critical ratio at different frequencies. The classic example is T. truncatus with 40 critical bands
that vary from ten times the critical ratio at 30kHz to eight times the critical ratio at 120kHz (Johnson 1968; see also Nachtigall, Lemonds, and Roitblat, Chapter 8 for review). This ability may be related to having longer
basilar membranes than many land mammals (Table 2.1) or better resolution at high frequencies or a combined effect.
3.2 Mysticete Acoustic Categories
Currently, there are no direct measures of hearing for any mysticete. Vocalization data imply mysticetes are predominately low sonic range animals
«5 kHz), and it is likely that several species hear well at infrasonic frequencies. Recent data from deep ocean stationary arrays suggest mysticetes,
like odontocetes, have three, distinct sound production groups (EddsWalton 1997) that parallel three temporal bone morphometric categories
among mysticetes, but cross taxonomic lines (Ketten 1992; Ketten personal
observation). Habitat and functional relationships for these potential
acousto-morphometric groupings are not yet clear. For this discussion, all
mysticetes are categorized conservatively as Type M. In general, mysticete
vocalizations are significantly lower in frequency than those of odontocetes,
with peak spectra between 0.012 and 3 kHz. Most mysticete signals are characterized as low-frequency moans (0.4 to 40s, fundamental <200 Hz); simple
calls (impulsive, peak <1 kHz); complex calls (broadband pulsatile AM or
FM signals); and complex "songs" with varied phrasing and spectra. Infrasonic signals between 10 and 20 Hz are well documented in at least two
species, the blue whale (8. musculus, Edds 1982) and the fin whale, (Balaenoptera physalus) (Edds 1988; Watkins et al. 1987). Suggestions that lowfrequency mysticete signals are used for oceans basin scale communication
or a low-frequency form of echolocation, such as topological imaging, are
compelling but have not been definitively demonstrated.
Comparisons of hearing curves and sounds produced by odontocetes
indicate that, like most mammals, they have good sensitivity near the frequencies or harmonics of frequencies they emit. It is reasonable to expect
this is true for mysticetes as well. Based on the frequency ranges and peak
spectra of sounds employed by both odontocetes and mysticetes, ears of
cetaceans should fall into distinct morphometric categories that span infrato high ultrasonic adaptations and are consistent with sound Types I, II, or
M. The next question is, then, what structural correlates are there and what
can they tell us about cetacean hearing?
D.R. Ketten
as the threshold level of the target in noise (in dB) minus the masker level
(in dB). Critical bands tend to be a constant function of critical ratios
throughout an animal's functional hearing range (Fay 1992). Odontocetes
are better than most mammals at detecting signals in noise and have more
critical bands with smaller critical ratios than other mammals. Odontocete
critical bandwidths are not a constant factor of the critical ratio at different frequencies. The classic example is T. truncatus with 40 critical bands
that vary from ten times the critical ratio at 30kHz to eight times the critical ratio at 120kHz (Johnson 1968; see also Nachtigall, Lemonds, and Roitblat, Chapter 8 for review). This ability may be related to having longer
basilar membranes than many land mammals (Table 2.1) or better resolution at high frequencies or a combined effect.
3.2 Mysticete Acoustic Categories
Currently, there are no direct measures of hearing for any mysticete. Vocalization data imply mysticetes are predominately low sonic range animals
«5 kHz), and it is likely that several species hear well at infrasonic frequencies. Recent data from deep ocean stationary arrays suggest mysticetes,
like odontocetes, have three, distinct sound production groups (EddsWalton 1997) that parallel three temporal bone morphometric categories
among mysticetes, but cross taxonomic lines (Ketten 1992; Ketten personal
observation). Habitat and functional relationships for these potential
acousto-morphometric groupings are not yet clear. For this discussion, all
mysticetes are categorized conservatively as Type M. In general, mysticete
vocalizations are significantly lower in frequency than those of odontocetes,
with peak spectra between 0.012 and 3 kHz. Most mysticete signals are characterized as low-frequency moans (0.4 to 40s, fundamental <200 Hz); simple
calls (impulsive, peak <1 kHz); complex calls (broadband pulsatile AM or
FM signals); and complex "songs" with varied phrasing and spectra. Infrasonic signals between 10 and 20 Hz are well documented in at least two
species, the blue whale (8. musculus, Edds 1982) and the fin whale, (Balaenoptera physalus) (Edds 1988; Watkins et al. 1987). Suggestions that lowfrequency mysticete signals are used for oceans basin scale communication
or a low-frequency form of echolocation, such as topological imaging, are
compelling but have not been definitively demonstrated.
Comparisons of hearing curves and sounds produced by odontocetes
indicate that, like most mammals, they have good sensitivity near the frequencies or harmonics of frequencies they emit. It is reasonable to expect
this is true for mysticetes as well. Based on the frequency ranges and peak
spectra of sounds employed by both odontocetes and mysticetes, ears of
cetaceans should fall into distinct morphometric categories that span infrato high ultrasonic adaptations and are consistent with sound Types I, II, or
M. The next question is, then, what structural correlates are there and what
can they tell us about cetacean hearing?