Humpback whales (megaptera novaeangliae) emit long, complex patterned vocalizations, or "songs". A number of discrete populations of humpback whales exist which, at any point in time, can be characterized by a unique song shared by all singing population members. The songs of the various populations which have been studied all have in common a complex hierarchical structure.

The generally accepted analysis of whalesong grammar identifies 5 levels in this structural hierarchy:

The duration of songs, particularly the number of phrases per theme and themes per song, varies, even between songs in the same song session sung by the same individual. However, the structure and sequence of this grammar is so universally adhered to that the few observed deviations have been labelled as aberrant.

Over a series of years, the characteristic song of each humpback population changes extensively and irreversibly. Just as the songs themselves are highly structured, song evolution appears to be a progressive process.