1. Overview
35
of a given body of waters. It is not uncommon for these sounds to travel
tens of miles and for conspecifics and other animals to hear vocalizing
whales over large distances. Odontocetes, on the other hand, emit higherfrequency sounds that are greater than about 5 kHz. Their sounds can have
frequency components that go up to more than 100 kHz. The range of odontocetes sounds is generally limited to hundreds of meters.
We have a relatively good understanding of the sensitivity and frequency
range of hearing for small odontocetes. Unfortunately, this cannot be said
for baleen whales and the two largest odontocetes (P catodon and B.
bairdii). Our knowledge of hearing in baleen whales is rudimentary at best.
From playback experiments and from the sounds that the whales make,
we can infer gross frequency ranges over which baleen whales can hear.
However, the absolute sensitivity at any frequency is not known. The mechanisms of whales' hearing are also unknown. Are sound paths for hearing
in dolphins similar for baleen whales? Without an understanding of their
absolute sensitivity, it is difficult to estimate the effects of anthropogenic
sounds on their auditory system. So far, all we can do is relate to the intensity of sounds that whales expose conspecifics and other animals to when
calling or singing. If neighboring whales seem not to be disturbed or
affected by the level of sounds being projected by another whale, perhaps,
we could use a similar criterion for anthropogenic noise. Perhaps the challenge at this time is to obtain good calibrated recordings of whale calls and
sounds, accounting accurately for propagation losses.
The question of site and mechanism of sound generation in dolphins is
slowly being resolved through the work of investigators like Ridgway et al.
(1980), Cranford (Chapter 3), Aroyan (1996), and others. Once the sites of
production of whistles, burst pulses, echolocation clicks, and other sounds
are pinpointed, then investigators can turn to the generation mechanism
and investigate how specific sounds are generated. All the evidence points
to a site in the nasal passage, with some evidence implicating the museau
de singe or monkey lips. There are two monkey lips and it may be possible
that one is used mainly for whistles and the other for burst pulses and clicks.
It may also be possible that the lips work together in some fashion to generate a particular acoustic signal. The question of sound generation by
baleen whales is another matter. The site of sound generation and the mechanisms involved are not known. Aroyan et al. (Chapter 10) considers this
problem and offer a specific model for B. musculus. It will be through such
models that perhaps investigators may gain insight as to what types of
measurements and observations they need to perform in order to address
this issue.
The significance of sounds emitted by dolphins is another issue that needs
more attention. We know that dolphins and whales can be very vociferous,
yet our understanding of the function and meanings of these sounds is very
poor. Part of the reason for this state of uncertainty is the difficulty in
observing the behavior of an aquatic animal while producing different types
35
of a given body of waters. It is not uncommon for these sounds to travel
tens of miles and for conspecifics and other animals to hear vocalizing
whales over large distances. Odontocetes, on the other hand, emit higherfrequency sounds that are greater than about 5 kHz. Their sounds can have
frequency components that go up to more than 100 kHz. The range of odontocetes sounds is generally limited to hundreds of meters.
We have a relatively good understanding of the sensitivity and frequency
range of hearing for small odontocetes. Unfortunately, this cannot be said
for baleen whales and the two largest odontocetes (P catodon and B.
bairdii). Our knowledge of hearing in baleen whales is rudimentary at best.
From playback experiments and from the sounds that the whales make,
we can infer gross frequency ranges over which baleen whales can hear.
However, the absolute sensitivity at any frequency is not known. The mechanisms of whales' hearing are also unknown. Are sound paths for hearing
in dolphins similar for baleen whales? Without an understanding of their
absolute sensitivity, it is difficult to estimate the effects of anthropogenic
sounds on their auditory system. So far, all we can do is relate to the intensity of sounds that whales expose conspecifics and other animals to when
calling or singing. If neighboring whales seem not to be disturbed or
affected by the level of sounds being projected by another whale, perhaps,
we could use a similar criterion for anthropogenic noise. Perhaps the challenge at this time is to obtain good calibrated recordings of whale calls and
sounds, accounting accurately for propagation losses.
The question of site and mechanism of sound generation in dolphins is
slowly being resolved through the work of investigators like Ridgway et al.
(1980), Cranford (Chapter 3), Aroyan (1996), and others. Once the sites of
production of whistles, burst pulses, echolocation clicks, and other sounds
are pinpointed, then investigators can turn to the generation mechanism
and investigate how specific sounds are generated. All the evidence points
to a site in the nasal passage, with some evidence implicating the museau
de singe or monkey lips. There are two monkey lips and it may be possible
that one is used mainly for whistles and the other for burst pulses and clicks.
It may also be possible that the lips work together in some fashion to generate a particular acoustic signal. The question of sound generation by
baleen whales is another matter. The site of sound generation and the mechanisms involved are not known. Aroyan et al. (Chapter 10) considers this
problem and offer a specific model for B. musculus. It will be through such
models that perhaps investigators may gain insight as to what types of
measurements and observations they need to perform in order to address
this issue.
The significance of sounds emitted by dolphins is another issue that needs
more attention. We know that dolphins and whales can be very vociferous,
yet our understanding of the function and meanings of these sounds is very
poor. Part of the reason for this state of uncertainty is the difficulty in
observing the behavior of an aquatic animal while producing different types