Allopatric humpback whales of differing generations share call types between foraging and wintering grounds

Abstract

Humpback whales (Megaptera novaeangliae) are a cosmopolitan baleen whale species with geographically isolated lineages. Despite last sharing an ancestor ~ 2-3 million years ago, Atlantic and Pacific foraging populations share five call types. Whether these call types are also shared between allopatric breeding and foraging populations is unclear, but would provide further evidence that some call types are ubiquitous and fixed. We investigated whether these five call types were present on a contemporary foraging ground (Newfoundland, 2015-2016) and a historic breeding ground (Hawaii, 1981-1982). Calls were classified using aural/visual (AV) characteristics; 16 relevant acoustic variables were measured and a Principal Component Analysis (PCA) was used to examine within-call and between-population variation. To assess whether between-population variation influenced classification, all 16 variables were included in classification and regression tree (CART) and random forest analyses (RF). All five call types were identified in both populations. Between-population variation in combined acoustic variables (PC1, PC2, PC3) was lower within call types than among call types, and high agreement between AV and quantitative classification (CART: 83% agreement; RF: 77% agreement) suggested that acoustic characteristics were more similar within than among call types. Findings indicate that these five call types are shared across allopatric populations, generations, and behavioural contexts.

Figure 1

World map showing the locations of the two study areas (Newfoundland and Hawaii) in boxes, along with other regions where one or more of the five call types have been described, representing two migration routes and two foraging grounds. From left to right on the map (excluding the study sites): Southeast Alaska, USA (foraging) where all five call types were identified and found to be stable over 36 years (1976–2012)^,,; Massachusetts Bay, USA (foraging; 2008); Angola, Africa (migration; 2012); Harvey Bay and Byron Bay (1997, 2003, 2004, 2008), Australia (migration) where whups (called ‘wops’) were stable over 11 years; and other locations on the East Coast of Australia (migration; 2002–2004; 2002–2004, 2008). This map was created using QGIS 3.8.3-Zanzibar (https://qgis.org/en/site/forusers/download.html) using the ESRI Ocean basemap (https://services.arcgisonline.com/ArcGIS/rest/services/Ocean/World_Ocean_Base/MapServer/tile/%7Bz%7D/%7By%7D/%7Bx%7D&zmax=20&zmin=0) accessed on October 21, 2020.

Figure 2

Spectrograms examples (Hann window, 8192 (NL) and 32,768 (HI) Discrete Fourier Transform, 2.93 Hz resolution, and 50% overlap) of each of the five call types from each population, NL Newfoundland during 2015, 2016, HI Hawaii during 1981, 1982.

Figure 3

PC1, PC2, PC3 by call type and population showing between-population variation in acoustic parameters within call types as well as variation among call types. Boxplots show the mean (middle horizontal bar), 25th percentile (lower bar), 75th percentile (upper bar), and outliers. Positive PC1 values refer to calls of higher frequency and shorter duration, while negative PC1 values refer to calls of lower frequency and longer duration. Positive PC2 values refer to calls with a larger range in frequency and higher entropy, while more negative PC2 values refer to calls with a smaller frequency range and lower entropy. Positive PC3 values refer to calls with more variation in frequency and amplitude, while more negative PC3 values refer to calls with less amplitude and frequency variation.