8. Psychoacoustic Studies of Dolphins and Whales
337
imately 80kHz (see also Kellogg 1953). The authors reasoned that animals
capable of hearing at such frequency ranges were likely able to produce
ultrasounds as well, and proposed: "like the bat, (the porpoise) may orient
itself with respect to objects in its environment by echolocation." This early
reference to echolocation by dolphins followed from the seminal work of
Griffin on the modern discovery of echolocation by bats (Griffin and
Galambos 1941; Griffin 1950; for an accessible review see Griffin 1958).
The high-frequency range of hearing for T. truncatus was also examined
by Schevill and Lawrence (1953). Working with a single adult subject, they
reported sensitivity to signals just above 120 kHz, and observed that the
animal's hearing was highly directional. They also speculated on the possible presence of echolocation.
Further evidence related to the high-frequency echolocation proposal
described above came from an early sound-production study by Kellogg et
al. (1953). Sounds from T. truncatus were received through a specially constructed hydrophone and analyzed in the time and frequency domains. Two
distinct signal types were reported: whistles and clicks. Whistles were
described as "birdlike" and melodic, and generally had peak energy levels
below 20 kHz. More common than whistles were rapid series of clicks, at
repetition rates from 5 to 100 clicks per second and higher. The energy in
these clicks extended up to 120 kHz. The authors suggested these signals
would be suitable signals for echo-ranging by a pulse-modulation method.
In summary, early research confirmed that the cetacean species that were
studied responded to sounds over a wide frequency range, up to 120 kHz in
one case, and produced two general types of sounds: tonal lower-frequency
whistles, and short duration clicks that contained energy as high as 120 kHz.
The high-frequency echolocation hypothesis was supported by an experimental demonstration of navigation by a dolphin that was essentially blindfolded by placing suction cups over its eyes (Norris et al. 1961) and
confirmed by a blindfolded dolphin discriminating the difference between
the sizes of spheres (Turner and Norris 1966). The echolocation capabilities
of the bottlenose dolphin (Nachtigall and Moore 1988; Au 1993) generated
great interest in the auditory capabilities of the species and other cetaceans.
2. Auditory Sensitivity
In all vertebrate species that have been studied, the animal's sensitivity to
sound varies as a function of frequency. Most species show low sensitivity
at very low frequencies and at very high frequencies (the meaning of very
low and very high is different for different species) in a kind of U-shaped
pattern. An audiogram shows the minimum detectable sound intensity as a
function of frequency.
Dolphins and small whales generally hear very well at frequencies that
humans consider ultrasonic. Of the nine odontocete species tested so far
337
imately 80kHz (see also Kellogg 1953). The authors reasoned that animals
capable of hearing at such frequency ranges were likely able to produce
ultrasounds as well, and proposed: "like the bat, (the porpoise) may orient
itself with respect to objects in its environment by echolocation." This early
reference to echolocation by dolphins followed from the seminal work of
Griffin on the modern discovery of echolocation by bats (Griffin and
Galambos 1941; Griffin 1950; for an accessible review see Griffin 1958).
The high-frequency range of hearing for T. truncatus was also examined
by Schevill and Lawrence (1953). Working with a single adult subject, they
reported sensitivity to signals just above 120 kHz, and observed that the
animal's hearing was highly directional. They also speculated on the possible presence of echolocation.
Further evidence related to the high-frequency echolocation proposal
described above came from an early sound-production study by Kellogg et
al. (1953). Sounds from T. truncatus were received through a specially constructed hydrophone and analyzed in the time and frequency domains. Two
distinct signal types were reported: whistles and clicks. Whistles were
described as "birdlike" and melodic, and generally had peak energy levels
below 20 kHz. More common than whistles were rapid series of clicks, at
repetition rates from 5 to 100 clicks per second and higher. The energy in
these clicks extended up to 120 kHz. The authors suggested these signals
would be suitable signals for echo-ranging by a pulse-modulation method.
In summary, early research confirmed that the cetacean species that were
studied responded to sounds over a wide frequency range, up to 120 kHz in
one case, and produced two general types of sounds: tonal lower-frequency
whistles, and short duration clicks that contained energy as high as 120 kHz.
The high-frequency echolocation hypothesis was supported by an experimental demonstration of navigation by a dolphin that was essentially blindfolded by placing suction cups over its eyes (Norris et al. 1961) and
confirmed by a blindfolded dolphin discriminating the difference between
the sizes of spheres (Turner and Norris 1966). The echolocation capabilities
of the bottlenose dolphin (Nachtigall and Moore 1988; Au 1993) generated
great interest in the auditory capabilities of the species and other cetaceans.
2. Auditory Sensitivity
In all vertebrate species that have been studied, the animal's sensitivity to
sound varies as a function of frequency. Most species show low sensitivity
at very low frequencies and at very high frequencies (the meaning of very
low and very high is different for different species) in a kind of U-shaped
pattern. An audiogram shows the minimum detectable sound intensity as a
function of frequency.
Dolphins and small whales generally hear very well at frequencies that
humans consider ultrasonic. Of the nine odontocete species tested so far