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

Abstract

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.

Non-song social call bouts of migrating humpback whales

“…The spectrograms of social calls were produced using Adobe Audition software (v1.5, Adobe Systems) with a 4096 point fast Fourier Transform, 75% overlap, 50 s window, yielding 15.625 Hz frequency resolution. Once a social call was located, the start and end times of the call in the recording were noted manually and the call was categorized qualitatively, where possible, using the catalogue of calls outlined by Dunlop et al (2007) and Rekdahl et al (2013). Dunlop et al (2007) and Rekdahl et al (2013) measured parameters of all social calls and performed both discriminant function analysis and principal components analysis (Dunlop et al, 2007) or classification tree analysis (Rekdahl et al, 2013) to quantify call categories.…”

“…Once a social call was located, the start and end times of the call in the recording were noted manually and the call was categorized qualitatively, where possible, using the catalogue of calls outlined by Dunlop et al (2007) and Rekdahl et al (2013). Dunlop et al (2007) and Rekdahl et al (2013) measured parameters of all social calls and performed both discriminant function analysis and principal components analysis (Dunlop et al, 2007) or classification tree analysis (Rekdahl et al, 2013) to quantify call categories. In the process of assigning calls to call types for this paper, it was noted that a number of calls commonly found in the recordings did not fit any of the previously defined call types.…”

“…Six new call types were identified and named that did not subjectively match any of the previously defined call type categories from this population (Dunlop et al, 2007;Rekdahl et al, 2013). One of these call types, the bop call, was the most common call type found to be produced within bouts [ Fig.…”

“…Humpback whales' use of social calls in bouts is of particular interest given males' ability to produce complex song sequences, as well as the potential for social and contextual information to be conveyed by social calls. The well documented social call repertoire of migrating east Australian humpback whales (Dunlop et al, 2007;Dunlop et al, 2008;Rekdahl et al, 2013) provides an ideal opportunity to further investigate whether humpback whales, like some social terrestrial species, also produce their calls within ordered call bouts.…”

Source levels of foraging humpback whale calls

“…All humpback whale calls were annotated and assigned to vocal class based on a known catalog for the region . Some humpback whale calls are highly variable, and appear to persist along a continuum, which may confound classification at fine scales Rekdahl et al, 2013;Stimpert et al, 2011;Murray et al, 1998). Calls in Southeast Alaska, however, follow a hierarchical classification structure which allows for call placement into broader-scale groupings that encompass the variability associated with a repertoire of static-dynamic call types .…”