Identification and classification of repeated whistle types from free-ranging rough-toothed dolphins (Steno bredanensis)

Acoustic communication is vital for marine mammals. Many delphinids rely on frequency-modulated tonal signals – known as whistles – used during social communication. Among these, individually distinctive “signature whistles” have been extensively studied in bottlenose dolphins (Tursiops truncatus) over the past 60 years. These stereotyped whistles convey individual identity and motivational information. Recent studies have identified similar stereotyped whistles in other delphinids, including a rehabilitated rough-toothed dolphin (Steno bredanensis). However, the function, occurrence, time–frequency characteristics, and degree of stereotypy of whistles in free-ranging individuals of this species remain unexplored. This study analysed acoustic recordings from three encounters with rough-toothed dolphins in summer 2023 off Madeira Island (Eastern North Atlantic). Of 4928 whistles analysed, 1015 were identified as repeated whistles and classified into 25 categories based on frequency modulation contours using visual inspection. Visual classification was verified using an inter-observer reliability test and compared to results from an unsupervised neural network (ARTwarp), employing both sequential and global analyses. The results revealed repeated contour types, with the degree of stereotypy varying across categories and generally lower than that reported for other well-studied delphinid species. Like other delphinids, rough-toothed dolphins exhibit a fission–fusion social structure, where repeated signals may help maintain group cohesion, potentially functioning as individual- or group-level calls. Future research incorporating detailed contextual and behavioral information will be crucial to investigate the function, occurrence and social relevance of repeated calls in rough-toothed dolphin whistle repertoires.