Background
Songs
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:
- At the lowest level are primitive phonological units, "the shortest
(real-time) sounds continuous to human ears".
- Phrases are organized groups of units. Typically, a phrase will
consist of two distinct units (e.g., a tone '-' or a moan '|') each
consecutively repeated a variable number times.
- Sequences of one of more similar phrases form a theme. Themes may be
composed of one of several kinds of phrase, e.g.,
- static phrases, which contain virtually the same material (e.g., - - | - - | - - |).
- shifting phrases, whose content gradually drifts from one unit type
to another (e.g., - - - / / / | | |), where '/' represents a unit of
intermediate form.
- Sequences of themes (typically 4-10),arranged in a fairly
inflexible order, make up a song.
- Singing whales repeat the song that is currently typical of their
population in long (up to many hours) 'song sessions'.
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.