Temporal stability and change in the social call repertoire of migrating humpback whales.

Quantifying the stability of a species vocal repertoire is fundamental for further investigations into repertoire function and geographic variation. Changes to the repertoire of sounds used in the song displays of male humpback whales have been well studied. In contrast, little is known about the stability of this species' non-song vocal calls. The stability of the social call repertoire of east Australian humpback whales was investigated from 1997, 2003-2004, and 2008. Out of 46 qualitatively defined call types, 19 were classified as "song-unit calls" that tended to change with the song, and 15 were "inconsistent" and only found in one or two years. Twelve call types were "stable" and present in all years and were commonly produced (64.2% of calls). Stable calls tended to vary in some of the measured call parameters but there was no clear trend between years. This result could indicate that minor changes to calls are not permanent, but reflect individual differences in call production or the graded nature of calls within different social environments. This research has clearly identified stable calls in the call repertoire of humpback whales and while their function is not well understood, their stability suggests an important role in social interactions.