2. Cetacean Ears
69
Moore et al. (1995) demonstrated that T. truncatus has an IATD on the
order of 7Jls, which is better than the average human value (10 Jls) and well
below that of most land mammals tested. If 1M distances are used for land
mammals and pinnipeds in air and IC distances are used for cetaceans and
underwater pinniped data, marine mammal and land mammal data for
IATD versus high-frequency limits have essentially the same regression
(Fig. 2.5).
Intensity differences can be detected monaurally or binaurally, but
binaural cues are most important for localizing high frequencies. In land
mammals, intensity discrimination thresholds (IDT) are independent of
frequency, decrease with increasing sound levels, and are generally better
in larger animals (Fay 1992; Heffner and Heffner 1992). Humans and
macaques commonly detect intensity differences of 0.5 to 2 dB throughout
their functional hearing range; gerbils and chinchillas, 2.5 to 8 dB. Presumably ITDs in marine mammals depend upon the same reception paths as
IATDs. Behavioral and evoked potential data show intensity differences
are detectable by odontocetes at levels equal to those of land mammals and
that the detection thresholds, like those of land mammals, decline with
increasing sound level. Binaural behavioral studies and evoked potential
recordings for T. truncatus indicate an approximate IDT limit of 1 to 2 dB
(Bullock et al. 1968; Moore et al. 1995). In P phocoena, IDTs range 0.5 to
3dB (Popov et al. 1986). Thresholds in I. geoffensis range from 3 to 5dB
(Supin and Popov 1993), but, again, because of small sample size and
methodological differences, it is unclear whether these numbers represent
true species differences. Fay (1992) points out that the IDT data for land
mammals do not fit Weber's law, which would predict a flat curve for IDT.
In summary, the most salient structural features of cetacean temporal
bones are that they are extracranial and massive regardless of size, shape,
or hearing type. The conclusion is that the fundamental construction of
whale and dolphin ears is driven by physical aquatic parameters, particularly high atmospheric pressures, as well as the physical constraints of
underwater acoustics. The dimensions of cetacean tympano-periotic bullae,
which are stunning in some species, are not acoustically driven but are correlated with animal size like those of land mammals. This does not mean,
however, that size, mass, and density of whale tympano-periotic complexes
have no acoustic consequences, which brings us to the middle ear.
5.2 Cetacean Middle Ears
Cetacean middle ears are complex, and their function is still poorly understood. In all species, the ossicles are well developed, complexly shaped, and
massive. Both the middle ear cavity and the ossicles show species- and
animal-size-dependent variations. Intense, conflicting opinions abound
about whether cetacean middle ears are functional (e.g., Fraser and Purves
1954; McCormick et al. 1970; Fleischer 1978; Ridgway and Carder 1997),
69
Moore et al. (1995) demonstrated that T. truncatus has an IATD on the
order of 7Jls, which is better than the average human value (10 Jls) and well
below that of most land mammals tested. If 1M distances are used for land
mammals and pinnipeds in air and IC distances are used for cetaceans and
underwater pinniped data, marine mammal and land mammal data for
IATD versus high-frequency limits have essentially the same regression
(Fig. 2.5).
Intensity differences can be detected monaurally or binaurally, but
binaural cues are most important for localizing high frequencies. In land
mammals, intensity discrimination thresholds (IDT) are independent of
frequency, decrease with increasing sound levels, and are generally better
in larger animals (Fay 1992; Heffner and Heffner 1992). Humans and
macaques commonly detect intensity differences of 0.5 to 2 dB throughout
their functional hearing range; gerbils and chinchillas, 2.5 to 8 dB. Presumably ITDs in marine mammals depend upon the same reception paths as
IATDs. Behavioral and evoked potential data show intensity differences
are detectable by odontocetes at levels equal to those of land mammals and
that the detection thresholds, like those of land mammals, decline with
increasing sound level. Binaural behavioral studies and evoked potential
recordings for T. truncatus indicate an approximate IDT limit of 1 to 2 dB
(Bullock et al. 1968; Moore et al. 1995). In P phocoena, IDTs range 0.5 to
3dB (Popov et al. 1986). Thresholds in I. geoffensis range from 3 to 5dB
(Supin and Popov 1993), but, again, because of small sample size and
methodological differences, it is unclear whether these numbers represent
true species differences. Fay (1992) points out that the IDT data for land
mammals do not fit Weber's law, which would predict a flat curve for IDT.
In summary, the most salient structural features of cetacean temporal
bones are that they are extracranial and massive regardless of size, shape,
or hearing type. The conclusion is that the fundamental construction of
whale and dolphin ears is driven by physical aquatic parameters, particularly high atmospheric pressures, as well as the physical constraints of
underwater acoustics. The dimensions of cetacean tympano-periotic bullae,
which are stunning in some species, are not acoustically driven but are correlated with animal size like those of land mammals. This does not mean,
however, that size, mass, and density of whale tympano-periotic complexes
have no acoustic consequences, which brings us to the middle ear.
5.2 Cetacean Middle Ears
Cetacean middle ears are complex, and their function is still poorly understood. In all species, the ossicles are well developed, complexly shaped, and
massive. Both the middle ear cavity and the ossicles show species- and
animal-size-dependent variations. Intense, conflicting opinions abound
about whether cetacean middle ears are functional (e.g., Fraser and Purves
1954; McCormick et al. 1970; Fleischer 1978; Ridgway and Carder 1997),