Two decades of song evolution and revolution on a humpback whale migratory corridor

Oceania in the South Pacific contains a metapopulation of humpback whales (Megaptera novaeangliae) with several connected breeding grounds. Male humpback whales sing complex socially learned songs that are socially transmitted between breeding grounds within Oceania. The Cook Islands—a migratory corridor within Oceania—connects different breeding grounds within the region, and the song recorded each season may reflect the migratory streams of whales using the corridor, as well as wider metapopulation connectivity. Song is present here, yet it appears highly unsettled in theme structure and sequencing, compared to song recorded in other Oceania breeding grounds. Here, we quantified fine-scale song evolution (Dice’s Similarity Index) and complexity at the Cook Islands over two decades (1998–2018). Using matched themes, we identified 16 different song types over this period, including at least nine revolutionary song types. All but one song type matched to previously identified song types from the West, showing song types continued to spread east across the central South Pacific region. Finally, in three seasons, multiple song types were recorded within a season, indicating either multiple migratory streams or a shift away from the standard one population-wide song per season and possibly reflecting the increased population growth.