32
W.W.L.Au
acoustic properties of the different baleen whales is given in Table 1.1. Calls
and songs are most likely used for some sort of communication, however,
at this time the specific meanings of these sounds are not known. It is
extremely difficult to study the context and functions of baleen whale vocalization. The sounds of mysticete whales have also been summarized nicely
by Richardson et al. (1995). Instead of discussing the characteristics of
various sounds, the properties of calls are summarized in Table 1.1. The
calls of mysticete whales are mainly in the low-frequency range, from about
an infrasonic frequency of 12.5 Hz for B. musculus to about 3.5 kHz for
Balaenoptera borealis. There are a variety of different types of calls, from
frequency modulation tones to moans, grunts, and discrete pulses. How
these sounds are used by whales is still an open question since it is often
very difficult to observe behavior associated with different calls.
6. Auditory Nervous System
Dolphins possess a highly developed and intricate auditory system, perhaps
the most developed of all auditory systems in the animal kingdom, considering their large frequency range of hearing and their ability to perceive
very short signals on the order of tens of microseconds. Much of our knowledge of the auditory capabilities of dolphins has come from behavioral
and psychophysical studies as discussed by Nachtigall et al. (Chapter 8).
However, electrophysiological and anatomical studies have also been
instrumental in providing knowledge as to the special characteristics of the
dolphin auditory system. The hearing capabilities of animals can be assessed
by measuring the evoked potential response of the auditory nervous system along the auditory pathway from the inner ear to the central auditory nervous system. An auditory evoked potential (AEP) is an electrical response of the nervous system produced by an external acoustic stimulus that can be measured as voltage by measuring electrodes. Dolphin
(Chapter 7) discusses electrophysiological measures of auditory processing
and Ridgway (Chapter 6) discusses the auditory central nervous system in
dolphins.
The auditory system of dolphins seems to be adapted for processing
complex acoustic systems. Although the number of inner hair cells in the
cochlea of T. tursiops is nearly the same as in humans and the number of
outer hair cells is only slightly larger, the number of ganglion cells is considerable larger in dolphins than in humans (Wever et al. 1971a, 1971b,
1972). The bottlenose dolphin, T. truncatus, has about 95,000 ganglion cells
as compared to about 31,000 for humans. The number of ganglion cells to
hair cells per unit length of basilar membrane is much higher in dolphins
than in other mammals (Wever et al. 1971b, 1972). This high ratio of ganglion cells to hair cells may aid in the representation of high-frequency
acoustic information and of fine details of cochlear events to the higher
W.W.L.Au
acoustic properties of the different baleen whales is given in Table 1.1. Calls
and songs are most likely used for some sort of communication, however,
at this time the specific meanings of these sounds are not known. It is
extremely difficult to study the context and functions of baleen whale vocalization. The sounds of mysticete whales have also been summarized nicely
by Richardson et al. (1995). Instead of discussing the characteristics of
various sounds, the properties of calls are summarized in Table 1.1. The
calls of mysticete whales are mainly in the low-frequency range, from about
an infrasonic frequency of 12.5 Hz for B. musculus to about 3.5 kHz for
Balaenoptera borealis. There are a variety of different types of calls, from
frequency modulation tones to moans, grunts, and discrete pulses. How
these sounds are used by whales is still an open question since it is often
very difficult to observe behavior associated with different calls.
6. Auditory Nervous System
Dolphins possess a highly developed and intricate auditory system, perhaps
the most developed of all auditory systems in the animal kingdom, considering their large frequency range of hearing and their ability to perceive
very short signals on the order of tens of microseconds. Much of our knowledge of the auditory capabilities of dolphins has come from behavioral
and psychophysical studies as discussed by Nachtigall et al. (Chapter 8).
However, electrophysiological and anatomical studies have also been
instrumental in providing knowledge as to the special characteristics of the
dolphin auditory system. The hearing capabilities of animals can be assessed
by measuring the evoked potential response of the auditory nervous system along the auditory pathway from the inner ear to the central auditory nervous system. An auditory evoked potential (AEP) is an electrical response of the nervous system produced by an external acoustic stimulus that can be measured as voltage by measuring electrodes. Dolphin
(Chapter 7) discusses electrophysiological measures of auditory processing
and Ridgway (Chapter 6) discusses the auditory central nervous system in
dolphins.
The auditory system of dolphins seems to be adapted for processing
complex acoustic systems. Although the number of inner hair cells in the
cochlea of T. tursiops is nearly the same as in humans and the number of
outer hair cells is only slightly larger, the number of ganglion cells is considerable larger in dolphins than in humans (Wever et al. 1971a, 1971b,
1972). The bottlenose dolphin, T. truncatus, has about 95,000 ganglion cells
as compared to about 31,000 for humans. The number of ganglion cells to
hair cells per unit length of basilar membrane is much higher in dolphins
than in other mammals (Wever et al. 1971b, 1972). This high ratio of ganglion cells to hair cells may aid in the representation of high-frequency
acoustic information and of fine details of cochlear events to the higher