4. Acoustic Communication in Whales and Dolphins
209
duction frequency and intensity could co-evolve leading to an even greater
advantage to the largest animals. The standard tool for testing for adaptive
specialization across changes in body size is allometric analysis, which
examines the correlation of the trait in question with body size.
Rendell et al. (1999) have performed an allometric analysis of frequency
of tonal calls of cetaceans against body length. There is a strong correlation
in odontocetes between body length and maximum frequency (R
2 = 0.95;
Ding et al. 1995) or mean frequency (R
2 = 0.94; Rendell et al. 1999) of tonal
calls from odontocetes. This suggests that most of the variation in frequency
of tonal calls in odontocetes may arise as a correlated consequence of variation in body size. By contrast, in mysticetes, the linear regression between
body size and mean call frequency was weak (R
2 = 0.64; Rendell et al. 1999)
and not significant for tonal calls of baleen whales. However, if one removed
the two largest baleen whales, fin and blue whales, there was a clear trend
of decreasing call frequency with increasing body size. Fin and especially
blue whales are clear outliers, with calls much lower in frequency than
expected. This suggests that the very low frequency calls of fin and blue
whales do in fact reflect specializations for low-frequency signalling. We
must caution that the analysis of Rendell et al. (1999) rests by necessity
upon spotty published accounts of the frequency range of calls from different mysticete species. More complete analyses of the call repertoires of
mysticetes using comparable methods of frequency analysis are urgently
needed for this kind of allometric analysis.
Several authors have also suggested that the peak frequency of echolocation clicks is inversely correlated with body size in odontocetes. Watkins
(1980b) suggested that the following series of odontocete species in order
of increasing size (T. truncatus, P. crassidens, Globicephala, and 0. orca)
have clicks with emphases at decreasing frequencies. Thomas et al. (1988)
confirm that P. crassidens produce clicks with frequency emphases higher
in frequency than 0. orca and lower in frequency than the smaller odontocetes such as T. truncatus, the phocoenid porpoises and Amazon river
dolphin (Inia geoffrensis). The sperm whale, which is much larger than any
of these species, produces clicks with emphases at the lowest frequencies,
near 2 to 4kHz. Evans (1973) suggested that the frequency of odontocete
clicks may correlate with prey size. Evans' argument assumes that odontocete echolocation is used primarily to detect prey, that the size of the prey
correlates with the size of the predator, and that the acoustic properties of
prey vary simply as a function of size. We know little about how cetaceans
use echolocation in the wild, and we need much more detailed studies relating echolocation signals to the acoustic properties of natural targets. Even
when a P. crassidens was just echolocating upon a simple sphere, its echolocation clicks were highly variable, showing high- and low-frequency components with spectral peaks ranging from 10 to 56 kHz. In the section on
acoustic properties of fish, we pointed out that the assumption relating
resonant frequency to size may be oversimplistic for targets with gas-filled
209
duction frequency and intensity could co-evolve leading to an even greater
advantage to the largest animals. The standard tool for testing for adaptive
specialization across changes in body size is allometric analysis, which
examines the correlation of the trait in question with body size.
Rendell et al. (1999) have performed an allometric analysis of frequency
of tonal calls of cetaceans against body length. There is a strong correlation
in odontocetes between body length and maximum frequency (R
2 = 0.95;
Ding et al. 1995) or mean frequency (R
2 = 0.94; Rendell et al. 1999) of tonal
calls from odontocetes. This suggests that most of the variation in frequency
of tonal calls in odontocetes may arise as a correlated consequence of variation in body size. By contrast, in mysticetes, the linear regression between
body size and mean call frequency was weak (R
2 = 0.64; Rendell et al. 1999)
and not significant for tonal calls of baleen whales. However, if one removed
the two largest baleen whales, fin and blue whales, there was a clear trend
of decreasing call frequency with increasing body size. Fin and especially
blue whales are clear outliers, with calls much lower in frequency than
expected. This suggests that the very low frequency calls of fin and blue
whales do in fact reflect specializations for low-frequency signalling. We
must caution that the analysis of Rendell et al. (1999) rests by necessity
upon spotty published accounts of the frequency range of calls from different mysticete species. More complete analyses of the call repertoires of
mysticetes using comparable methods of frequency analysis are urgently
needed for this kind of allometric analysis.
Several authors have also suggested that the peak frequency of echolocation clicks is inversely correlated with body size in odontocetes. Watkins
(1980b) suggested that the following series of odontocete species in order
of increasing size (T. truncatus, P. crassidens, Globicephala, and 0. orca)
have clicks with emphases at decreasing frequencies. Thomas et al. (1988)
confirm that P. crassidens produce clicks with frequency emphases higher
in frequency than 0. orca and lower in frequency than the smaller odontocetes such as T. truncatus, the phocoenid porpoises and Amazon river
dolphin (Inia geoffrensis). The sperm whale, which is much larger than any
of these species, produces clicks with emphases at the lowest frequencies,
near 2 to 4kHz. Evans (1973) suggested that the frequency of odontocete
clicks may correlate with prey size. Evans' argument assumes that odontocete echolocation is used primarily to detect prey, that the size of the prey
correlates with the size of the predator, and that the acoustic properties of
prey vary simply as a function of size. We know little about how cetaceans
use echolocation in the wild, and we need much more detailed studies relating echolocation signals to the acoustic properties of natural targets. Even
when a P. crassidens was just echolocating upon a simple sphere, its echolocation clicks were highly variable, showing high- and low-frequency components with spectral peaks ranging from 10 to 56 kHz. In the section on
acoustic properties of fish, we pointed out that the assumption relating
resonant frequency to size may be oversimplistic for targets with gas-filled