4. Acoustic Communication in Whales and Dolphins
179
have peak frequencies of 40 to 60kHz and source levels of about 170dB
(Evans 1973). When dolphins echolocate at sea in a less reverberant environment, they tend to produce clicks with peak frequencies of 120 to 130
kHz and source levels over 200 dB relJ.1Pa at 1m(Au et aI.1974). Dolphins
have been shown to control the correlated source level or peak frequency
of their clicks as a function of ambient noise (Au et al. 1985) or as a function of the echolocation task (Moore and Pawloski 1990).
The best known tonal calls of dolphins are called whistles. Whistles
usually last from several tenths of a second to several seconds and range
in frequency from about 2 to 25kHz (Caldwell et al. 1990). Dolphins have
also been reported to produce shorter, highly frequency modulated chirps
lasting less than lOOms (Caldwell and Caldwell 1970), and low-frequency
tonal calls with fundamentals less than 1kHz (Schultz et al. 1995). It is not
certain whether the apparent breakpoints in duration of chirps versus
whistles near lOOms or the differences in frequency above or below 1 kHz
actually reflect a gap in duration or frequencies of tonal calls. It is even
less certain whether they represent different categories of vocalization to
dolphins.
When bottlenose dolphins are isolated, they often produce an individually distinctive whistle called a signature whistle. Signature whistles have
been recorded from over 100 captive (Caldwell et al. 1990) and over 100
wild dolphins (Tyack and Sayigh 1997). Signature whistles have also been
reported for common dolphins, Delphinus delphis (Caldwell and Caldwell
1968), the Pacific whitesided dolphin, Lagenorhynchus obliquidens (Caldwell and Caldwell 1971) and the spotted dolphin, Stenella plagiodon
(Caldwell et al. 1973).
McCowan (1995) used a new method to categorize whistles of captive
dolphins and McCowan and Reiss (1995, 1997) used these results to challenge the signature whistle hypothesis. They recorded whistles from small
social groups of captive dolphins, using bubblestreams to identify which
dolphin produced which whistle. McCowan and Reiss (1995,1997) reported
a large repertoire of whistles within small groups of captive dolphins,
sharing of whistle types across social groups, and they found one common
whistle type, an upsweep similar to the chirp reported by Caldwell and
Caldwell (1970), which was not individually distinctive.
There are several reasons why McCowan and Reiss (1995, 1997) may
have results that differ from work on signature whistles. The Caldwells also
identified upsweeps that were common among dolphins, but they classified
them as chirps rather than whistles. In addition, the contexts in which
McCowan and Reiss identified whistles, involving bubblestreams, are likely
to introduce different sampling biases than the isolation context most often
used by the Caldwells.
Perhaps the most important reason for these differing interpretations of
dolphin whistles stems from differences in the categorization of whistles.
Janik (1999) compared categorizations performed by humans and three
179
have peak frequencies of 40 to 60kHz and source levels of about 170dB
(Evans 1973). When dolphins echolocate at sea in a less reverberant environment, they tend to produce clicks with peak frequencies of 120 to 130
kHz and source levels over 200 dB relJ.1Pa at 1m(Au et aI.1974). Dolphins
have been shown to control the correlated source level or peak frequency
of their clicks as a function of ambient noise (Au et al. 1985) or as a function of the echolocation task (Moore and Pawloski 1990).
The best known tonal calls of dolphins are called whistles. Whistles
usually last from several tenths of a second to several seconds and range
in frequency from about 2 to 25kHz (Caldwell et al. 1990). Dolphins have
also been reported to produce shorter, highly frequency modulated chirps
lasting less than lOOms (Caldwell and Caldwell 1970), and low-frequency
tonal calls with fundamentals less than 1kHz (Schultz et al. 1995). It is not
certain whether the apparent breakpoints in duration of chirps versus
whistles near lOOms or the differences in frequency above or below 1 kHz
actually reflect a gap in duration or frequencies of tonal calls. It is even
less certain whether they represent different categories of vocalization to
dolphins.
When bottlenose dolphins are isolated, they often produce an individually distinctive whistle called a signature whistle. Signature whistles have
been recorded from over 100 captive (Caldwell et al. 1990) and over 100
wild dolphins (Tyack and Sayigh 1997). Signature whistles have also been
reported for common dolphins, Delphinus delphis (Caldwell and Caldwell
1968), the Pacific whitesided dolphin, Lagenorhynchus obliquidens (Caldwell and Caldwell 1971) and the spotted dolphin, Stenella plagiodon
(Caldwell et al. 1973).
McCowan (1995) used a new method to categorize whistles of captive
dolphins and McCowan and Reiss (1995, 1997) used these results to challenge the signature whistle hypothesis. They recorded whistles from small
social groups of captive dolphins, using bubblestreams to identify which
dolphin produced which whistle. McCowan and Reiss (1995,1997) reported
a large repertoire of whistles within small groups of captive dolphins,
sharing of whistle types across social groups, and they found one common
whistle type, an upsweep similar to the chirp reported by Caldwell and
Caldwell (1970), which was not individually distinctive.
There are several reasons why McCowan and Reiss (1995, 1997) may
have results that differ from work on signature whistles. The Caldwells also
identified upsweeps that were common among dolphins, but they classified
them as chirps rather than whistles. In addition, the contexts in which
McCowan and Reiss identified whistles, involving bubblestreams, are likely
to introduce different sampling biases than the isolation context most often
used by the Caldwells.
Perhaps the most important reason for these differing interpretations of
dolphin whistles stems from differences in the categorization of whistles.
Janik (1999) compared categorizations performed by humans and three