4. Acoustic Communication in Whales and Dolphins
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among nonhuman terrestrial mammals, but has evolved several times in
marine mammals, marine mammals provide an important comparative perspective on the evolution of vocal learning. There are at least three different viable hypotheses for factors influencing the evolution of vocal learning:
echolocation, sexual selection, and the demands of maintaining a stable
signature in diving animals.
There is some evidence for a link between echolocation and vocal learning among some bats and dolphins. The best evidence for vocal learning in
bats actually involves echolocation calls. The echolocation calls of horseshoe bats (R. ferrumequinum) rise in frequency over the first one to two
years of life and then decrease in frequency with increasing age. The echolocation calls of young bats that have older mothers are lower in frequency
than calls of young bats with younger mothers, suggesting that young horseshoe bats match the call of their mother (Jones and Ransome 1993). The
species of odontocetes that have been demonstrated to imitate a variety of
pulsed and whistlelike sounds, species such as beluga whales and bottlenose
dolphins, are able to echolocate. There are suggestions for both bottlenose
dolphins and beluga whales that vocal learning may playa role in the effective operation of their sonar. Both species have been shown to be able to
shift the peak frequency of their echolocation clicks, either as a result of
differing ambient noise (belugas: Au et al. 1985) or through training (bottlenose dolphins: Moore and Pawloski 1990). This suggests that the requirements of echolocation may have selected for a simple form of vocal learning
in these species.
The selective pressures for the evolution of complex advertisements
appear to have played a role in the evolution of vocal learning in songbirds
and baleen whales. As in the case of oscine songbirds, vocal learning in
humpback whales has only been described for their song, which is a reproductive advertisement display that has evolved by sexual selection. Indirect
evidence also suggests that sexual selection may have been a significant
factor in the possible evolution of vocal learning in some seals that also
produce reproductive advertisement displays.
The use of whistles in individual recognition coupled with problems associated with vocal cues for individual recognition in diving animals suggests
that vocal learning may play a critical role in individual recognition and
for maintaining individual-specific social relationships in whistling odontocetes (Janik and Slater 1997; Tyack and Sayigh 1997). The diving habit of
cetaceans may prevent cetaceans from performing vocal individual recognition in the same way as terrestrial mammals. Slight variations in the vocal
tracts of terrestrial animals lead to predictable differences in the voices of
individuals. Many of the features that distinguish the calls of individual
terrestrial animals appear to be subtle cues resulting from these variations
in the vocal tracts of different individuals. These involuntary characteristics
of voice cannot be as reliable for diving animals, however. The vocal tract
is a gas-filled cavity, and as an animal dives, these gases halve in volume for
211
among nonhuman terrestrial mammals, but has evolved several times in
marine mammals, marine mammals provide an important comparative perspective on the evolution of vocal learning. There are at least three different viable hypotheses for factors influencing the evolution of vocal learning:
echolocation, sexual selection, and the demands of maintaining a stable
signature in diving animals.
There is some evidence for a link between echolocation and vocal learning among some bats and dolphins. The best evidence for vocal learning in
bats actually involves echolocation calls. The echolocation calls of horseshoe bats (R. ferrumequinum) rise in frequency over the first one to two
years of life and then decrease in frequency with increasing age. The echolocation calls of young bats that have older mothers are lower in frequency
than calls of young bats with younger mothers, suggesting that young horseshoe bats match the call of their mother (Jones and Ransome 1993). The
species of odontocetes that have been demonstrated to imitate a variety of
pulsed and whistlelike sounds, species such as beluga whales and bottlenose
dolphins, are able to echolocate. There are suggestions for both bottlenose
dolphins and beluga whales that vocal learning may playa role in the effective operation of their sonar. Both species have been shown to be able to
shift the peak frequency of their echolocation clicks, either as a result of
differing ambient noise (belugas: Au et al. 1985) or through training (bottlenose dolphins: Moore and Pawloski 1990). This suggests that the requirements of echolocation may have selected for a simple form of vocal learning
in these species.
The selective pressures for the evolution of complex advertisements
appear to have played a role in the evolution of vocal learning in songbirds
and baleen whales. As in the case of oscine songbirds, vocal learning in
humpback whales has only been described for their song, which is a reproductive advertisement display that has evolved by sexual selection. Indirect
evidence also suggests that sexual selection may have been a significant
factor in the possible evolution of vocal learning in some seals that also
produce reproductive advertisement displays.
The use of whistles in individual recognition coupled with problems associated with vocal cues for individual recognition in diving animals suggests
that vocal learning may play a critical role in individual recognition and
for maintaining individual-specific social relationships in whistling odontocetes (Janik and Slater 1997; Tyack and Sayigh 1997). The diving habit of
cetaceans may prevent cetaceans from performing vocal individual recognition in the same way as terrestrial mammals. Slight variations in the vocal
tracts of terrestrial animals lead to predictable differences in the voices of
individuals. Many of the features that distinguish the calls of individual
terrestrial animals appear to be subtle cues resulting from these variations
in the vocal tracts of different individuals. These involuntary characteristics
of voice cannot be as reliable for diving animals, however. The vocal tract
is a gas-filled cavity, and as an animal dives, these gases halve in volume for