Evidence of signature whistles produced by Indian Ocean bottlenose dolphins (Tursiops aduncus) in Mozambique

Individually distinctive signature whistles are used by common bottlenose dolphins (Tursiops truncatus) during social interactions and to facilitate group cohesion. There is evidence from a few populations that Indian Ocean bottlenose dolphins (T. aduncus) also use signature whistles. We investigated this from a coastal resident population of T. aduncus in Mozambique. Video-audio data were collected during recreational swim-with activities over 12 years, where potential signature whistles were identified from 28 individuals. Of the 405 whistles documented, 75% were produced in SIGID bouts. Visual assessment of individual signature whistle contours demonstrated temporal stability for up to 8 years. Overall, most signature whistle types had upsweep frequency modulation and were emitted as multiloop whistles. Comparing all whistle contours to each other indicated low discrimination between individuals, with contours frequently categorised together. However, sex differences in the frequency characteristics of whistles were identified, with females whistling at lower frequencies than males. Our results indicate either a shared whistle repertoire or identity encoding with subtle contour features, requiring acute auditory perception and discrimination to decipher caller identity. More widespread geographic investigation into signature whistle use may demonstrate variation in acoustic communication systems for bottlenose dolphins, which are thus far not well understood.