Abstract and Summary
163 songs of humpback whales (Megaptera novaeangliae) recorded near Bermuda during April and May of 13 years between 1957 and 1975 have been analysed as continuous sound spectrograms and compared. In each year's sample, all whales were singing basically the same song. However, the song was changing conspicuously and progressively with time so that songs separated by a number of years were very different in content. All the songs showed basic structural similarities so that it is possible to define a song form which characterizes songs from many years. We present some basic characteristics of the song form and consider the nature and extent of changes in content as a function of time. An analysis of the songs sung by groups of whales shows that normal singing continues even when whales are close enough, presumably, to hear each other. Such analysis demonstrates inter– and intra– individual variability, none of which is as great as the variation between songs of consecutive years. We do not understand the significance of changing songs. We know of no other non-human animal for which such dramatic non-reversing changes appear in the display pattern of an entire population as part of their normal behavior.