Song structure and spatial dynamics of humpback whales ( Megaptera novaeangliae ) on the breeding grounds
Schneider, Jennifer Nicole
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Humpback whales ( Megaptera novaeangliae ) sing songs that contain a wide variety of sounds arranged in a hierarchal pattern at amplitudes that can be heard for several kilometers underwater. While the structure of these songs has been well defined and several hypotheses have been put forth as to the function of singing, little is known about the information contained in song. However, different functions necessitate that different types of information be available to receivers. The purpose of this dissertation is to examine the acoustic structure of song to determine whether two types of information, fitness and location, are available to receivers. The availability of fitness cues was assessed in relation to size cues and performance quality. An examination of the acoustic structure of song elements revealed regular patterns of emphasized (termed formants) and deemphasized frequencies consistent with those created when sound travels through the vocal tract and is used to judge size in terrestrial animals. However, these patterns were not consistent between units of the same song and were masked by changes in the sound due to propagation after only tens of meters, indicating that formants do not convey size information. An examination of humpback whale song in light of fitness cues reported in the literature revealed that consistency of unit and phrase production within a song session may serve as a fitness cue, but more research is needed. All other cues that were evaluated were considered unlikely. The availability of cues for locating song in azimuth and ranging a singer's distance were assessed using broadcast studies. In addition, distance cues were assessed using recordings of song from a live whale. Several potential cues for localization in azimuth were identified including interaural timing differences and interaural spectral differences. Possible sources of identified spectral differences are sound shadowing and mechanical coupling between hydrophones, analogous to shadowing produced by a whale's skull and bone conduction. Possible cues available for ranging include absolute and frequency dependent amplitude cues and echoes. Frequency dependent cues differed between units, indicting that sound may be specialized for accurate ranging at specific distances, depending on the structure of the sound and the singers'/receivers' environments. Finally, a preliminary study revealed that the amount of time whales spend producing some categories of pulsed, high-frequency, and non-modulated medium-frequency sounds changes predictably between the beginning, middle, and end of the breeding season. The hypothesized functions of whale song were evaluated in terms of these findings.