Progression in humpback whale song structure and complexity on a subarctic feeding ground in Northern Norway

Abstract

Male humpback whales (Megaptera novaeangliae) sing structurally complex songs traditionally associated with low latitude breeding grounds. This vocal behaviour is increasingly reported outside these areas. All singers in a given population sing the same version of a song that is constantly evolving with modifications on different levels within the song structure. This study provides the first detailed analysis of humpback whale songs recorded on a subarctic feeding ground in Northern Norway. Passive acoustic data from the Lofoten-Vesterålen Ocean Observatory were collected using bottom-moored underwater hydrophone and included the months January – June 2018 and December 2018 – January 2019. Two measures of the song structure were examined: (1) sequence similarities using the Levenshtein distance and (2) song complexity. More than 440 hours of recordings spread over 199 days were inspected for humpback whale song occurrence using a semi-automated approach. Overall, 750 hours of humpback whale singing activity was detected spanning 79 days between December and April. The first singing activity was detected beginning of January 2018, followed by a peak in February and was heard until mid-April. No song was found during the summer months and was detected again in December 2018, continuing over January 2019. Song structure analyses resulted in a total of 18 distinct themes; 11 themes in 2018 and 7 themes in 2019. The themes clustered into two song types, one for each year, suggesting an event of rapid song progression. As all sampled animals were singing the same version of the song within each year, this might indicate that the singers are either from the same breeding population or that song conformation was performed before the study period. Song complexity increased as songs evolved over the months in 2018 and decreased drastically between the years 2018 and 2019. The results confirm that humpback whale song can be heard over multiple months and years in Northern Norway. Finally, this study identified song progression on a shared subarctic feeding ground, indicating potential song exchange between the North Atlantic humpback whale populations already before reaching their breeding grounds. Tracing changes in whale song will help to undercover the drivers underlying this vocal display and contribute to the understanding of animal culture and its evolution.