Characterizing the rhythm and tempo of sound production by singing whales

Singing humpback whales often produce sound patterns that are predictably timed. Phrase duration, in particular, has been noted as one of the most stable properties of humpback whale songs. Most descriptive analyses of songs focus, however, either on consistency in sequential structure or on gradual changes in song composition, rather than on the timing of sounds and silences within songs. As a result, little is known about how the timing of sound production varies within or across song sessions. Here, we present an approach to analysing temporal structure within song sessions that can be applied to any recording of a nearby, isolated singer, without requiring subjective classification of songs, themes, phrases or units. Our analyses confirm past subjective impressions of high temporal regularity within song sessions, but also show that the timing of sound production can vary greatly. Singing humpback whales alternate between producing sequences with highly stable temporal structure, sequences with progressive shifts in timing and sequences with rapidly varying temporal features. The amount of time a singer spends producing sequences of each type can vary considerably across song sessions. These findings raise important questions about the extent to which singing humpback whales actively control the rhythm and tempo of sound production.