Singing to the sound of their own tune: Uncovering an increasingly complex vocal repertoire for the east Indian Ocean pygmy blue whale

In an underwater world, acoustic signalling is an important aspect of the social communication of marine mammal species with the complexity of a species’ vocal repertoire often considered to reflect the social complexity of the population. The acoustic behaviour of blue whales is relatively well studied, though much of what is known is limited to the characteristically loud, low frequency songs that are believed to be produced as a reproductive display by male animals. Blue whales are known to produce song units outside of stereotypical song sequences, along with short duration down swept signals known as ‘D calls’ leading researchers to believe their acoustic communication, and by proxy their social cognition is relatively less complex when compared to other baleen whales such as humpback and bowhead whales. Drawing from a multidecadal data set of acoustic recorders deployed throughout the migratory range of blue whales, this paper characterises four previously undescribed signals for the East Indian Ocean pygmy blue whales and presents the first known evidence of a large baleen whale producing these social sounds in stereotyped patterned sequences that bear similarity to song. This indicate a higher level of complexity in the social communication of blue whales than previously understood and provides further support that blue whales have a higher level of social cognition than has been considered previously.