ABSTRACT: Blue whale Balaenoptera musculus songs can be divided into at least 10 types worldwide, each type retaining the same units and similar phrasing over decades, unlike humpback whale song which changes substantially from year to year. Historical acoustic recordings dating back as far as the 1960s were examined, measuring the tonal frequencies of 1000s of blue whale songs. Within a given year, individuals match the song frequency (related to ‘pitch’ in musical nomenclature) to within less than 3%. The best documented song type, that observed offshore of California, USA, now is sung at a frequency 31% lower than it was in the 1960s. Data available for 7 of the world’s 10 known song types show they are all shifting downward in frequency, though at different rates. Any behavioral, ecological, oceanographic or anthropogenic change hypothesis seeking to explain the observed shifts should account for the worldwide occurrence of a nearly linear downward shift in the tonal frequencies of blue whale song. Hypotheses examined consider sexual selection, increasing ocean noise, increasing whale body size post whaling, global warming, interference from other animal sounds and post whaling increases in abundance. None of the commonly suggested hypotheses were found to provide a full explanation; however, increasing population size post whaling provides an intriguing and testable hypothesis that recovery is altering the sexually selected tradeoff for singing males between song amplitude (the ability to be heard at a greater distance) and song frequency (the ability to produce songs of lower pitch).
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