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P.L. Tyack and C. W. Clark
4. Comparing Fundamental Frequency of Tonal
Cetacean Vocalizations to Body Size and
Hearing Ability
We have argued that several cetaceans have specializations for low- or highfrequency vocalizations. Some variation in the fundamental frequency of
vocalizations might result not from selection for the vocalizations themselves, but rather from some other character that affects the vocalization
frequency. For example, the vocal tract of most terrestrial mammals acts as
a filter that affects the frequency structure of the source signal. The vocal
tract consists of cavities whose size is bounded by the skull, and skull size
is correlated with body size. If one thinks of resonance in the vocal tract as
analogous to an organ pipe, then one can readily imagine how frequency
characteristics of calls might vary as a function of body size. Whatever the
reason(s) for which baleen whales were initially selected for large body size,
the low frequencies of their calls might simply be a consequence of the associated growth of their vocal tracts rather than a result of a specific selection pressure for changing the vocalization frequency. However, if lower
vocal fundamental was a secondary result of increased body size, an additional benefit could result from this change that confers greater communication area to larger animals.
The preceding discussion shows that simply demonstrating differences in
call frequency is not sufficient to demonstrate selection for specialized features. Cetaceans show great variation in body size, with lengths varying from
1 to 30m. If the frequency characteristics of calls correlate strongly with
body size, and if one believes that call frequency is a critical trait, then one
might argue that selection pressures for specialized frequencies of calling
might have driven the evolution of body size. However, there are many
other more likely candidates for the evolution of body size in cetaceans. For
example, many marine mammalogists have argued that the large body size
of baleen whales was initially selected for because it allowed for greater
storage of fat and more efficient energetics for swimming long distances,
thus increasing the ability to exploit high-latitude seasonal food resources
in the summer and tropical breeding habitat in the winter. It is possible that
this scenario may have preadapted some baleen whales for subsequent specialization for low-frequency communication. Payne and Webb (1971)
argued that if greater communication area results in increased mating
success for large baleen whales then there would be selection for lower call
frequency and greater source intensity. Some baleen whales such as humpback and gray whales congregate in traditional breeding areas only tens of
kilometers across, but fin and blue whales are thought to disperse into the
open ocean during the breeding season. In these species, selection could
have favored specialized vocal production mechanisms capable of producing lower-frequency vocalizations. At this point, body size and vocal pro-
P.L. Tyack and C. W. Clark
4. Comparing Fundamental Frequency of Tonal
Cetacean Vocalizations to Body Size and
Hearing Ability
We have argued that several cetaceans have specializations for low- or highfrequency vocalizations. Some variation in the fundamental frequency of
vocalizations might result not from selection for the vocalizations themselves, but rather from some other character that affects the vocalization
frequency. For example, the vocal tract of most terrestrial mammals acts as
a filter that affects the frequency structure of the source signal. The vocal
tract consists of cavities whose size is bounded by the skull, and skull size
is correlated with body size. If one thinks of resonance in the vocal tract as
analogous to an organ pipe, then one can readily imagine how frequency
characteristics of calls might vary as a function of body size. Whatever the
reason(s) for which baleen whales were initially selected for large body size,
the low frequencies of their calls might simply be a consequence of the associated growth of their vocal tracts rather than a result of a specific selection pressure for changing the vocalization frequency. However, if lower
vocal fundamental was a secondary result of increased body size, an additional benefit could result from this change that confers greater communication area to larger animals.
The preceding discussion shows that simply demonstrating differences in
call frequency is not sufficient to demonstrate selection for specialized features. Cetaceans show great variation in body size, with lengths varying from
1 to 30m. If the frequency characteristics of calls correlate strongly with
body size, and if one believes that call frequency is a critical trait, then one
might argue that selection pressures for specialized frequencies of calling
might have driven the evolution of body size. However, there are many
other more likely candidates for the evolution of body size in cetaceans. For
example, many marine mammalogists have argued that the large body size
of baleen whales was initially selected for because it allowed for greater
storage of fat and more efficient energetics for swimming long distances,
thus increasing the ability to exploit high-latitude seasonal food resources
in the summer and tropical breeding habitat in the winter. It is possible that
this scenario may have preadapted some baleen whales for subsequent specialization for low-frequency communication. Payne and Webb (1971)
argued that if greater communication area results in increased mating
success for large baleen whales then there would be selection for lower call
frequency and greater source intensity. Some baleen whales such as humpback and gray whales congregate in traditional breeding areas only tens of
kilometers across, but fin and blue whales are thought to disperse into the
open ocean during the breeding season. In these species, selection could
have favored specialized vocal production mechanisms capable of producing lower-frequency vocalizations. At this point, body size and vocal pro-