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Geographic variation in song indicates both isolation of Arabian Sea humpback whales and presence of Southern Hemisphere whales off Oman
Salvatore Cerchio, Andrew Willson, Charles Muirhead, Suaad Al Harthi, Robert Baldwin, Marco Bonato, Tim Collins, Jacopo Di Clemente, Violaine Dulau, Vanessa Estrade, G. Latha, Gianna Minton, Maia Sarrouf Willson
International Whaling Commission
Existing genetic, demographic and behavioral evidence indicates that Arabian Sea humpback whales represent an isolated and unique population. The population exhibits a Northern Hemisphere breeding cycle, is believed to feed year-round, and lacks the typical latitudinal migrations and seasonal separation of breeding and feeding ecology exhibited by other populations of humpback whales globally. A key feature of humpback whale breeding behavior is the male acoustic breeding display, song, studied extensively around the world. Key characteristics of humpback whale song include: all males within a population share the same song patterns (phrases); a population’s song changes progressively over time; and populations that do not overlap or exchange individuals have distinctly different songs, whereas populations in contact share some or all phrases. Here we use long-term acoustic monitoring off the coast of Oman to further assess the isolation of the Arabian Sea population. A total of 76 samples (amounting to 4,434 minutes of recording) of Arabian Sea song collected between 2011 and 2013 were examined, and phrase content was characterized and compared to 23 samples (totaling 202 min of recording) collected during the same years in the Southwest Indian Ocean from Reunion Island and the Comoros Islands. Song from the Arabian Sea and the Southwest Indian Ocean was distinct across the entire study period, with no evidence for shared phrases in any year. In addition, song fragments recorded off western India in 2011 were composed of two phrases present in the Oman song, suggesting continuity across the Arabian Sea. Moreover, the Arabian Sea song exhibited a markedly atypical low level of temporal variation, with song phrases remaining virtually unchanged during the three examined breeding seasons. Notably, Southwest Indian Ocean song was recorded off the coast of Oman in August 2012 (Boreal summer, Austral winter). This song was recorded on multiple days and included multiple simultaneous singers over a 25 day period indicating the presence of more than a single accidental vagrant Southern Hemisphere animal. We suggest that these Southern songs were produced by Southwest Indian Ocean animals moving into the Arabian Sea, and that this may be more common than is currently thought. The low level of temporal variation shown by the Arabian Sea males along with the lack of adoption of the Southwest Indian Ocean song material, further indicate the uniqueness and distinct nature of this population. It seems possible that isolation mechanisms exist that may inhibit the mixing of the Arabian Sea population with Southern Hemisphere animals, and that this may be reflected in the observed atypical song behavior.