Micro- and macro-geographic variations of the whistles of striped Dolphins (Stenella coeruleoalba)

Stenella coeruleoalba, a cosmopolitan species distributed in tropical and temperate pelagic waters, is an extremely vocal dolphin with wide acoustic repertoire. This study analyses the geographic variability of the acoustic parameters of whistles between the Mediterranean Sea and the Atlantic Ocean, taking into account the intra basin variation. Acoustic data from Eastern and Western basins of the Mediterranean Sea and from two archipelagos of the Macaronesia (the Azores and the Canary islands) in the Atlantic Ocean were collected from 1994 to 2011, using multiple platforms. 51 independent sightings were made, approximately 22 hours of recordings were inspected and 1152 high quality sounds were extracted and analysed. 12 acoustic parameters of whistles (1 for temporal duration, 5 for frequency and 6 for modulation of the sound) were measured and compared. At the macro geographic level, among the Atlantic Ocean and the Mediterranean Sea, significant variations were expressed for duration, maximum frequency, frequency range and all the modulation parameters. Discriminant function analysis correctly classifies the sounds in the 75% of the cases. At the micro geographic scale, the same pattern of variation is present among the Eastern and the Western basins of the Mediterranean Sea, with a significant variation also for minimum frequency. The sounds of Eastern and Western areas can be discriminated in the 68% of the cases. Within the Atlantic Ocean, between Azores and Canary archipelagos, only modulation parameters are significantly different. The results of this study suggest that genetic differences, identified in other studies both between Atlantic Ocean and Mediterranean Sea and within the Mediterranean, can affect the variation of frequency parameters and promote the variability among these areas, while habitat adaptation, individuality, sociality and vocal learning may be the factors promoting differences for modulation parameters between Atlantic groups.