158
PL. Tyack and c.w. Clark
specializations similar to those of the echolocating bats. In addition, just as
bats and dolphins have evolved specializations for hearing based upon the
functional requirements of echolocation, so too the functional requirements
of communication, orientation, and exploration of the underwater world
may have influenced auditory processes in whales and dolphins (Tyack
1997). So little is known about auditory processing in these animals that
there are few places where we can reach final conclusions. Rather, in this
chapter we use information from vocal behavior and behavioral ecology to
suggest some of the most interesting phenomena for further study.
It appears that animals such as sperm whales have evolved specialized
organs for sound production (Norris and Harvey 1972), but there is considerable debate about mechanisms for sound production in cetaceans.
Most cetacean biologists in the past several decades have argued that odontocete cetaceans produce sound when air flows past tissue in the nasal passages in the skull (nasal plugs: Norris et al. 1971 and Dormer 1979; monkey
lips/dorsal bursae: Cranford et al. 1996 and Cranford, Chapter 3). On the
other hand, most cetaceans possess a well-developed larynx. Reidenberg
and Laitmann (1988) argue that the cetacean larynx has vocal folds that
might be capable of vibrating to produce sound when air passes over them.
Even though the word "vocalization" refers to the vocal folds, it has taken
on a more general meaning among students of animal communication.
When we use the term "vocalization" to refer to a cetacean sound in this
chapter, we do not mean to imply that the sound is produced in the larynx
as opposed to the nasal system. We will not discuss vocal production mechanisms, which is taken up in Chapter 3, other than to note that the debate
among cetologists about whether the larynx or nasal system is the source
of sound has limited more sophisticated models of sound production,
most of which involve not just a sound source but also study the spatial and
spectral filtering effects of the vocal tract.
The acoustic adaptations of marine mammals are fascinating in their own
right, and they are important for comparative analyses. In addition, understanding how marine mammals hear and use sound has become important
for the development of rational policies to protect them from adverse
effects of human-made underwater noise (Richardson et al. 1995). Over the
past century, the propulsion noise of motorized shipping has profoundly
changed the acoustic environment of the world's oceans, elevating average
deep-sea ambient noise by 10 to 100 times in the 20 to 500 Hz region
(Fig. 4.1) (Urick 1983). This could reduce the effective range of communication, depending upon details of auditory processing and the abilities of
animals to compensate with louder vocalizations. During the past century
we humans have also started to learn how to use underwater sound to
communicate and explore the oceans. As we converge on solutions similar
to those evolved by cetaceans tens of millions of years ago, our signal may
be their noise, and we may interfere with their activities, or vice versa. Our
understanding of marine mammal bioacoustics is critical for finding the
PL. Tyack and c.w. Clark
specializations similar to those of the echolocating bats. In addition, just as
bats and dolphins have evolved specializations for hearing based upon the
functional requirements of echolocation, so too the functional requirements
of communication, orientation, and exploration of the underwater world
may have influenced auditory processes in whales and dolphins (Tyack
1997). So little is known about auditory processing in these animals that
there are few places where we can reach final conclusions. Rather, in this
chapter we use information from vocal behavior and behavioral ecology to
suggest some of the most interesting phenomena for further study.
It appears that animals such as sperm whales have evolved specialized
organs for sound production (Norris and Harvey 1972), but there is considerable debate about mechanisms for sound production in cetaceans.
Most cetacean biologists in the past several decades have argued that odontocete cetaceans produce sound when air flows past tissue in the nasal passages in the skull (nasal plugs: Norris et al. 1971 and Dormer 1979; monkey
lips/dorsal bursae: Cranford et al. 1996 and Cranford, Chapter 3). On the
other hand, most cetaceans possess a well-developed larynx. Reidenberg
and Laitmann (1988) argue that the cetacean larynx has vocal folds that
might be capable of vibrating to produce sound when air passes over them.
Even though the word "vocalization" refers to the vocal folds, it has taken
on a more general meaning among students of animal communication.
When we use the term "vocalization" to refer to a cetacean sound in this
chapter, we do not mean to imply that the sound is produced in the larynx
as opposed to the nasal system. We will not discuss vocal production mechanisms, which is taken up in Chapter 3, other than to note that the debate
among cetologists about whether the larynx or nasal system is the source
of sound has limited more sophisticated models of sound production,
most of which involve not just a sound source but also study the spatial and
spectral filtering effects of the vocal tract.
The acoustic adaptations of marine mammals are fascinating in their own
right, and they are important for comparative analyses. In addition, understanding how marine mammals hear and use sound has become important
for the development of rational policies to protect them from adverse
effects of human-made underwater noise (Richardson et al. 1995). Over the
past century, the propulsion noise of motorized shipping has profoundly
changed the acoustic environment of the world's oceans, elevating average
deep-sea ambient noise by 10 to 100 times in the 20 to 500 Hz region
(Fig. 4.1) (Urick 1983). This could reduce the effective range of communication, depending upon details of auditory processing and the abilities of
animals to compensate with louder vocalizations. During the past century
we humans have also started to learn how to use underwater sound to
communicate and explore the oceans. As we converge on solutions similar
to those evolved by cetaceans tens of millions of years ago, our signal may
be their noise, and we may interfere with their activities, or vice versa. Our
understanding of marine mammal bioacoustics is critical for finding the