Biphonic calls as signature whistles in a free-ranging bottlenose dolphin

Biphonic signals are nonlinear phenomena (NLPs) occurring in the repertoire of several animal groups and widespread across mammalian species. Although the mechanism of production is unclear, an adaptive meaning was suggested by their communication function, such as group or individual recognition. We document the rare recording event of a biphonic whistle emitted within a free-ranging population of bottlenose dolphins in the waters of south-western Sicily, Italy. The whistle was recorded in 2 different years, always when a photo-identified individual was present. A quantitative description of the signal is provided. The signal presents some unique characteristics in its frequency modulation pattern which is stable for a long period. Furthermore, the synchronized beginning of the two fundamental frequencies, the signal repetition within few seconds and its emission in freely interacting contexts suggest that biphonation is neither temporary nor involuntary. Also, we propose that biphonation can be produced via multiple mechanisms in bottlenose dolphins and that NLPs could represent natural recognizing marks that play a role in communication between bottlenose dolphins.