Oscillatory whistles—the ups and downs of identifying species in passive acoustic recordings

Sympatric short- and long-beaked common dolphins in the Southern California Bight (Delphinus delphis and D. delphis bairdii) are challenging to identify acoustically because their whistles overlap in many time-frequency characteristics. We therefore asked whether frequency modulation patterns can help with species identification. Whistle contours from single-species encounters (short-beaked = 902 whistles, 14 schools, long-beaked = 872 whistles, 10 schools) were extracted and categorized based on frequency content and shape using dynamic time warping and artificial neural networks. This analysis resulted in 447 whistle types with 38% being produced by both species. Of the remaining species-specific whistle types, 22% (n = 60) were recorded from more than one school. Thirty-two of these were specific to short-beaked common dolphins and 28 were specific to long-beaked common dolphins. Almost half of the short-beaked common dolphin species-specific whistle types (47%) were oscillatory (contour shape containing at least two cycles with the maximum and minimum of each cycle separated by at least 1 kHz), while only 3% of long-beaked common dolphin species-specific types were oscillatory. Thus, oscillatory whistles appear to be diagnostic of short-beaked common dolphins in this area. More broadly, our findings suggest that repertoire-wide comparisons of acoustic features may overlook possible species recognition via specific signals.