338
P.E. Nachtigall et al.
(Richardson et al. 1997), most of them hear very well in the general area
between 20 and 90kHz, with the possible exception of the killer whale
(Orcinus orca) (Hall and Johnson 1972). The one audiogram published for
this species may be based on an animal with high-frequency hearing loss
(see Section 2.3). Another unpublished study reported suggestive evidence
that this species also hears frequencies above 100kHz (mentioned in Au
1993). The reliance on measurements obtained from one or at most a very
few individuals highlights one of the difficulties with psychophysical investigations using cetaceans. Behavioral psychophysical measures, though very
reliable and essential, are expensive and time consuming. The animal must
be trained very well and the behavior must be stable and reliable before an
audiogram can be measured. Because of these necessary limiting conditions, data have frequently been gathered on a single animal and these data
have been assumed to represent the species. This assumption is sometimes
questionable.
2.1 Pure- Tone Sensitivity
Much of what we know about the hearing of odontocetes began with the
audiogram obtained for T. truncatus by C. Scott Johnson (1966). It should
be noted that this audiogram is generally accepted as a standard, with other
studies of T. truncatus and other marine mammal species routinely compared with Johnson's results. It is important, then, to know the details
of the experimental design employed in this study. Any methodological
differences that may exist with other studies will be reported with those
studies, otherwise assume that similar procedures were followed. The
subject of this experiment, an approximately eight- to nine-year-old male
bottlenose dolphin, T. truncatus, was trained to respond to individual pure
tone acoustic stimuli between 75 Hz and 150 kHz presented to the subject
for a period of 3s. The subject was trained on a go/no-go staircase procedure to measure thresholds at each frequency. False alarm responses were
followed by a 90-s "time-out," which probably induced the animal to be very
conservative, potentially elevating its thresholds. The results of this study
are shown in the audiogram in Figure 8.1. The lowest thresholds occurred
near 50kHz at a level around 45dB re IIlPa, with 10dB-down bandwidth
extending from 15 to 110kHz. Below the frequency region of maximum
sensitivity, thresholds increased continuously up to a level of 137 dB at
75 Hz. Above 50 kHz, thresholds increased slowly up to a level of 55 dB at
100 kHz, then increased rapidly above this to about 135 dB at 150 kHz.
The auditory sensitivity of a harbor porpoise, Phocoena phocoena, was
measured by Andersen (1970). Like Johnson's study, this study also
employed a go/no-go procedure, but this time using the method of constants. The subject was not rewarded for correct no-go responses, and
instead was trained to wait until a tone was heard. Following false alarms,
the subject was given a 3-min time-out. Despite these differences in tech-
P.E. Nachtigall et al.
(Richardson et al. 1997), most of them hear very well in the general area
between 20 and 90kHz, with the possible exception of the killer whale
(Orcinus orca) (Hall and Johnson 1972). The one audiogram published for
this species may be based on an animal with high-frequency hearing loss
(see Section 2.3). Another unpublished study reported suggestive evidence
that this species also hears frequencies above 100kHz (mentioned in Au
1993). The reliance on measurements obtained from one or at most a very
few individuals highlights one of the difficulties with psychophysical investigations using cetaceans. Behavioral psychophysical measures, though very
reliable and essential, are expensive and time consuming. The animal must
be trained very well and the behavior must be stable and reliable before an
audiogram can be measured. Because of these necessary limiting conditions, data have frequently been gathered on a single animal and these data
have been assumed to represent the species. This assumption is sometimes
questionable.
2.1 Pure- Tone Sensitivity
Much of what we know about the hearing of odontocetes began with the
audiogram obtained for T. truncatus by C. Scott Johnson (1966). It should
be noted that this audiogram is generally accepted as a standard, with other
studies of T. truncatus and other marine mammal species routinely compared with Johnson's results. It is important, then, to know the details
of the experimental design employed in this study. Any methodological
differences that may exist with other studies will be reported with those
studies, otherwise assume that similar procedures were followed. The
subject of this experiment, an approximately eight- to nine-year-old male
bottlenose dolphin, T. truncatus, was trained to respond to individual pure
tone acoustic stimuli between 75 Hz and 150 kHz presented to the subject
for a period of 3s. The subject was trained on a go/no-go staircase procedure to measure thresholds at each frequency. False alarm responses were
followed by a 90-s "time-out," which probably induced the animal to be very
conservative, potentially elevating its thresholds. The results of this study
are shown in the audiogram in Figure 8.1. The lowest thresholds occurred
near 50kHz at a level around 45dB re IIlPa, with 10dB-down bandwidth
extending from 15 to 110kHz. Below the frequency region of maximum
sensitivity, thresholds increased continuously up to a level of 137 dB at
75 Hz. Above 50 kHz, thresholds increased slowly up to a level of 55 dB at
100 kHz, then increased rapidly above this to about 135 dB at 150 kHz.
The auditory sensitivity of a harbor porpoise, Phocoena phocoena, was
measured by Andersen (1970). Like Johnson's study, this study also
employed a go/no-go procedure, but this time using the method of constants. The subject was not rewarded for correct no-go responses, and
instead was trained to wait until a tone was heard. Following false alarms,
the subject was given a 3-min time-out. Despite these differences in tech-