Culturally transmitted song exchange between humpback whales (Megaptera novaeangliae) in the southeast Atlantic and southwest Indian Ocean basins

Abstract

In migratory marine species, investigating population connectivity and structure can be challenging given barriers to dispersal are less evident and multiple factors may influence individual movement patterns. Male humpback whales sing a song display that can provide insights into contemporary connectivity patterns, as there can be a cultural exchange of a single, population-wide shared song type with neighbouring populations in acoustic contact. Here, we investigated song exchange between populations located on the east and west coasts of Africa using 5 years of concurrent data (2001-2005). Songs were qualitatively and quantitatively transcribed by measuring acoustic features of all song units and then compared using both Dice's similarity index and the Levenshtein distance similarity index (LSI) to quantitatively calculate song similarity. Song similarity varied among individuals and potentially between populations depending on the year (Dice: 36-100%, LSI: 21-100%), suggesting varying levels of population connectivity and/or interchange among years. The high degree of song sharing indicated in this study further supports genetic studies that demonstrate interchange between these two populations and reinforces the emerging picture of broad-scale connectivity in Southern Hemisphere populations. Further research incorporating additional populations and years would be invaluable for better understanding of fine-scale, song interchange patterns between Southern Hemisphere male humpback whales.

Keywords: Africa; cultural transmission; humpback whale; population structure; song.

Figure 1.

Map adapted from Rosenbaum et al. [12] that illustrates sub-stock structure of breeding stocks B and C in relation to the IWC's Southern Ocean feeding ground management areas. Humpback whale songs were recorded at three study sites: Iguela (green star), Mayumba (blue star) and Antongil Bay (red star). Samples from Iguela (green) and Mayumba (blue) were presumably collected from individuals in the breeding sub-stock B1 (BSB1), but with uncertainty regarding sub-stock boundaries we used the population-level demarcation (BSB) when referring to the population. Samples representing breeding stock C (BSC) were sampled from sub-stock C3 (BSC3) in Antongil Bay (red), Madagascar.

Figure 2.

Spectrograms presenting an example of theme composition and progression of song similarity for Gabon and Madagascar songs in 2004 and 2005. In 2004, both populations shared themes 6, 7s (shifting theme), 23, 24 and 25. Themes 17 and 18E (theme evolution) were also present in Gabon 2004. In 2005, the degree of song similarity was even greater with both populations singing the same themes: 7s, 24, 26 and 27. In both years, some variability was observed in units sung between shared themes. Note that unit labels are provided below each individual unit (unit abbreviations listed in electronic supplementary material, table S1) and may differ from the median strings presented in table 2.

Figure 3.

Average-linkage cluster dendrogram of the median or most representative song (theme sequence) per individual for (a) the Levenshtein similarity index and (b) Dice's similarity index. Multiscale bootstrap resampling (AU, left, red) and normal bootstrap probabilities (BP, right, green) are considered significant if p > 95% and if p > 70%, respectively. Branches with high AU values are strongly supported by the data. Individuals within a red box represent a song type and potentially a ‘vocal population’, labelled by year, location and individual number (note: individual numbers are arbitrary; e.g. individual 1 in 2001 Gabon is not the same individual in 2003 Gabon).