16
w.w.L. Au
4.4 Properties of Odontocetes' Sounds
Sound emissions by odontocetes can be classified into two broad categories
of frequency-varying continuous tonal sounds referred to as whistles and
broadband clicks (Evans 1967) including burst pulse sounds. Tyack and
Clark (Chapter 4) discuss the communication and acoustic behavior of dolphins and whales and Herzing (Chapter 5) relates acoustic and social
behavior of dolphins in the wild.
Whistles and burst sounds can be categorized as social sounds. Whistles
appear to be used for intraspecific communications (Herman and Tavolga
1980). Most of the research on social sounds has been performed on whistles and very little work has been done on burst pulse sounds. The main
reason for this is that whistles are mainly in the sonic or audible range and
are therefore easily recorded by off-the-shelf audio tape and digital audio
tape (DAT) recorders. However, whistles can also have higher-order harmonics that extend considerably beyond the ultrasonic frequency range
(Lammers and Au 1996). An example of such a whistle signal with higherorder harmonics that extends close to 80kHz is shown in Figure 1.8 for a
spinner dolphin (Stene/La Longirostris). These harmonics are an integral part
of a whistle and their presence is certainly detected by other dolphins.
4.4.1 Social Sounds: Whistles
Dolphins emit many different types of whistles in captivity and in the wild.
The traditional method that is still being used by many is to establish different categories and to determine the repertoire of whistle sounds by
visual inspection of spectrogram contours. This technique is very subjective
and has produced highly variable results among researchers. Researchers
have been plagued with nomenclature difficulties and lack of standardization. Some of the calls can be very complex and can include combinations
of different individual structural units. In some of the earlier studies, considerable emphasis was placed on the linguistic possibilities of these sounds
so some investigators made very fine distinctions between similar contours.
The findings of many of the earlier publications are included in an excellent review by Herman and Tavolga (1980) and some of these articles will
be briefly mentioned here. Caldwell and Caldwell (1965) obtained five different contours from over 1,400 whistles from four common dolphins,
whereas Dreher and Evans (1964) identified 17 different contours among
nine captive T. truncatus, 16 in a group of five T. gil/i, 19 in a group of wild
common dolphins, and seven in a pod of about 12 short-finned pilot whales
(Globicephala macrorhynchus). Dreher and Evans (1964) also reported 23
different contours for three juvenile T. truncatus, although they stated that
some of the contours classified as being unique may have been minor variations of one another.
Subjectivity comes into play when one is trying to decide if a particular
contour is really unique among all the other contours or just has some
w.w.L. Au
4.4 Properties of Odontocetes' Sounds
Sound emissions by odontocetes can be classified into two broad categories
of frequency-varying continuous tonal sounds referred to as whistles and
broadband clicks (Evans 1967) including burst pulse sounds. Tyack and
Clark (Chapter 4) discuss the communication and acoustic behavior of dolphins and whales and Herzing (Chapter 5) relates acoustic and social
behavior of dolphins in the wild.
Whistles and burst sounds can be categorized as social sounds. Whistles
appear to be used for intraspecific communications (Herman and Tavolga
1980). Most of the research on social sounds has been performed on whistles and very little work has been done on burst pulse sounds. The main
reason for this is that whistles are mainly in the sonic or audible range and
are therefore easily recorded by off-the-shelf audio tape and digital audio
tape (DAT) recorders. However, whistles can also have higher-order harmonics that extend considerably beyond the ultrasonic frequency range
(Lammers and Au 1996). An example of such a whistle signal with higherorder harmonics that extends close to 80kHz is shown in Figure 1.8 for a
spinner dolphin (Stene/La Longirostris). These harmonics are an integral part
of a whistle and their presence is certainly detected by other dolphins.
4.4.1 Social Sounds: Whistles
Dolphins emit many different types of whistles in captivity and in the wild.
The traditional method that is still being used by many is to establish different categories and to determine the repertoire of whistle sounds by
visual inspection of spectrogram contours. This technique is very subjective
and has produced highly variable results among researchers. Researchers
have been plagued with nomenclature difficulties and lack of standardization. Some of the calls can be very complex and can include combinations
of different individual structural units. In some of the earlier studies, considerable emphasis was placed on the linguistic possibilities of these sounds
so some investigators made very fine distinctions between similar contours.
The findings of many of the earlier publications are included in an excellent review by Herman and Tavolga (1980) and some of these articles will
be briefly mentioned here. Caldwell and Caldwell (1965) obtained five different contours from over 1,400 whistles from four common dolphins,
whereas Dreher and Evans (1964) identified 17 different contours among
nine captive T. truncatus, 16 in a group of five T. gil/i, 19 in a group of wild
common dolphins, and seven in a pod of about 12 short-finned pilot whales
(Globicephala macrorhynchus). Dreher and Evans (1964) also reported 23
different contours for three juvenile T. truncatus, although they stated that
some of the contours classified as being unique may have been minor variations of one another.
Subjectivity comes into play when one is trying to decide if a particular
contour is really unique among all the other contours or just has some