Since the groundbreaking work of Payne and McVay (1971), humpback songs have been conceptualized in terms of the upward hierarchical organization of sound units within phrases, and phrases within themes. Songs change within each season and all whales in a geographical region detect and sing the reshaped song. Here we propose two properties that act to make relearning the evolving song possible. First, each level of the hierarchical structure constrains the others. We estimated the degree of constraint using information theory and found that the theme, phrase, and prior sound unit reduced the source entropy of the current sound unit equally. Different sound units are found in each theme and phrase; each theme and phrase ‘uses up’ some of the sound units. Second, the rhythm of the sound units acts to simplify the phrase structure. The timing between sound units often separates adjacent phrases by longer silent durations, and in spite of huge differences in the number of sound units within phrases, the overall duration of the phrases often are equal, allowing the whale to anticipate phrase repetitions. Humpback and human songs share the same hierarchical structure, but there are striking differences in the sound unit sequence.
humpback songs, hierarchical, source entropy, rhythm