Simultaneous sound production in the bowhead whale Balaena mysticetus—Sexual selection and song complexity

Different components of bowhead whale Balaena mysticetus song were localized using hydrophone arrays. In 2008 recordings were made using two hydrophones spaced 20–35 m apart. In 2009 a linear GPS synchronized array of four hydrophones with an aperture of ~1400 m was used. The localization results confirm the co-location of the sound sources. The analyses show amplitude modulation of one signal caused by the onset of the second signal, which provides additional evidence of simultaneous sound production. Sound, when used as an indicator of fitness forces the vocalizing animal to improve the quality of its signal to compete with other vocalizing conspecifics. Several methods can be used to improve signal quality and these include 1) large repertoire size, 2) annually/seasonally changing repertoire, 3) broad frequency band, and 4) simultaneous sound production. Bowhead whales show all these features in their acoustic behaviour and we suggest that these complex songs have evolved as the result of sexual select...