8. Psychoacoustic Studies of Dolphins and Whales
345
lated 70% detection threshold corresponded to a delay of 2651ls. Similar
results were reported by Vel'min and Dubrovskiy (1975).
2.3 Hearing Loss in Dolphins
Most of the pure tone sensitivity data described so far are based on
measurements of only one or a very few young animals (see Section 2.1).
Investigations of hearing in other species, particularly, humans, have
found that individuals vary somewhat in their hearing capacities, especially as a function of age. High-frequency hearing tends to become less
sensitive with age. The effects of age are more pronounced among males
(Green 1976). Individual variations and age-related changes in hearing
were examined by Ridgway and Carter (1997) with T truncatus. They
studied hearing capabilities in eight bottlenose dolphins, four males
and four females ranging in age from 7 to 35 years, for tonal signals from
5 to 120 kHz. A unique methodology was used to rapidly collect thresholds.
Instead of pressing a paddle, the dolphins were required to vocalize
when they detected the presence of a tone. The animals positioned themselves on a bite plate 1m below the surface at the start of a testing dive and
remained stationary until signaled by an underwater buzzer to return to
the surface for a fish reward. The bite plate ensured that they kept their
heads in a fixed position relative to the sound source. The animals were
gradually trained to make up to 20 tone-detection responses in a row over
a period of up to 2 min before hearing the buzzer, surfacing, and receiving
the fish reward. Test tones were presented in random order with a variable
delay between 1.1 and 2.1 s. Interspersed at random points among these
tone sequences were nontone periods, the equivalent of catch trials in a
more standard procedure, during which the dolphin was supposed to refrain
from vocalizing.
All dolphins responded at better than 90% correct to tones with frequencies of 5,10,20,40, and 50 kHz, except for one old male, MAU (34 yrs),
whose performance dropped to about 50% for a 50 kHz signal at 111 dB.
Hearing at the higher frequencies was quite variable. One of the male dolphins (7yrs) and three of the females (11,32, and 35yrs) responded with
more than 90% accuracy to all test tones. The other three males (23,26, and
34 yrs) and the remaining female (33 yrs) were impaired to varying degrees
on detecting tones above 60 kHz. One of the dolphins, lAY, had previously
been the subject of another study (Ljungblad et al. 1982) at age 13, at which
time his hearing appeared comparable to that described by Johnson (1967).
It therefore seems likely that the present impairments are a result of either
age or exposure in these older animals. Not only are there apparent individual differences in the hearing capabilities of dolphins, but these capabilities are also age dependent. Ridgway and Carder (1997) also report
on another wild-caught dolphin that appeared to be substantially or
completely deaf.
345
lated 70% detection threshold corresponded to a delay of 2651ls. Similar
results were reported by Vel'min and Dubrovskiy (1975).
2.3 Hearing Loss in Dolphins
Most of the pure tone sensitivity data described so far are based on
measurements of only one or a very few young animals (see Section 2.1).
Investigations of hearing in other species, particularly, humans, have
found that individuals vary somewhat in their hearing capacities, especially as a function of age. High-frequency hearing tends to become less
sensitive with age. The effects of age are more pronounced among males
(Green 1976). Individual variations and age-related changes in hearing
were examined by Ridgway and Carter (1997) with T truncatus. They
studied hearing capabilities in eight bottlenose dolphins, four males
and four females ranging in age from 7 to 35 years, for tonal signals from
5 to 120 kHz. A unique methodology was used to rapidly collect thresholds.
Instead of pressing a paddle, the dolphins were required to vocalize
when they detected the presence of a tone. The animals positioned themselves on a bite plate 1m below the surface at the start of a testing dive and
remained stationary until signaled by an underwater buzzer to return to
the surface for a fish reward. The bite plate ensured that they kept their
heads in a fixed position relative to the sound source. The animals were
gradually trained to make up to 20 tone-detection responses in a row over
a period of up to 2 min before hearing the buzzer, surfacing, and receiving
the fish reward. Test tones were presented in random order with a variable
delay between 1.1 and 2.1 s. Interspersed at random points among these
tone sequences were nontone periods, the equivalent of catch trials in a
more standard procedure, during which the dolphin was supposed to refrain
from vocalizing.
All dolphins responded at better than 90% correct to tones with frequencies of 5,10,20,40, and 50 kHz, except for one old male, MAU (34 yrs),
whose performance dropped to about 50% for a 50 kHz signal at 111 dB.
Hearing at the higher frequencies was quite variable. One of the male dolphins (7yrs) and three of the females (11,32, and 35yrs) responded with
more than 90% accuracy to all test tones. The other three males (23,26, and
34 yrs) and the remaining female (33 yrs) were impaired to varying degrees
on detecting tones above 60 kHz. One of the dolphins, lAY, had previously
been the subject of another study (Ljungblad et al. 1982) at age 13, at which
time his hearing appeared comparable to that described by Johnson (1967).
It therefore seems likely that the present impairments are a result of either
age or exposure in these older animals. Not only are there apparent individual differences in the hearing capabilities of dolphins, but these capabilities are also age dependent. Ridgway and Carder (1997) also report
on another wild-caught dolphin that appeared to be substantially or
completely deaf.