At any one time all the whales in a population sing the same song, which differs significantly from songs of other populations. The song of each population evolves continuously, progressively, and so rapidly that nonreversing changes can be measured month to month in a singing season.
Such changes, which affect the songs at all levels, seem to arise through improvisation and imitation rather than through accident or as conveyors of information. The greatest amount of change appears when singing is most pervasive and the effort of each singer is most intense.
Rhymelike structures occur in songs that contain much thematic material, perhaps serving as a mnemonic device in the context of a rapidly changing oral culture. Sexual selection may be the driving evolutionary force behind song changing.
This according to “The progressively changing songs of humpback whales: A window on the creative process in a wild animal” by Katharine Payne, an essay included in The origins of music (Cambridge: MIT Press, 2000). Below, underwater recordings of humpback whale songs.
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