Individual or Group Signatures? Stereotyped Whistle Contour Types by Free-Ranging Risso’s Dolphins

Individual recognition mechanisms have played a key role in odontocete ecology, maintaining social cohesion and therefore ensuring the maintenance of long-term social ties. The current study was the first to identify and describe stereotyped whistle contour types by both a visual similarity value judgment task and acoustic and temporal parameters in free-ranging Risso's dolphins (Grampus griseus) in Mexican Pacific waters of the Gulf of the California. In total, 64.1% (118) of all analyzed whistles (184) were classified as stereotyped whistles (STW); 82.2% of them (97) met the SIGnature IDentification bout criteria and were, subsequently, ascribed into one of the five possible signature whistle (PSW) types. These signature call types accounted for 52.7% (97/184) of the acoustic repertoire. All PSW types were classified as sine (wavering sinusoidal whistles) and showed mean acoustic and temporal variable values similar to other studies of Risso's dolphins that did not differentiate signature whistles (SWs). External naive independent observers who lacked expertise in bioacoustics have confirmed the adopted classification created at high agreement level and near perfect match. Accordingly, the present study provided additional acoustic evidence of individually distinct vocalization in this species, reinforcing the need for further studies to help deepen and broaden the knowledge about the function, production and incidence of individual and “group signatures” and cognitive and recognition processes in the herein investigated species, among others.