Facts about signature whistles of bottlenose dolphins, Tursiops truncatus
Data collected from wild and captive bottlenose dolphins, Tursiops truncatus, over the past five decades indicate that they produce individually distinctive signature whistles that function in individual recognition and in maintaining group cohesion. However, a recent study by McCowan & Reiss (2001, Animal Behaviour, 62, 1151–1162) failed to detect signature whistles in nine of 12 captive bottlenose dolphins. These authors suggested that previous studies of signature whistles had used biased data sets, and claimed that the visual classification methods used by most researchers were inferior to their computerized technique. To evaluate their claims, we randomly selected 20 whistles from each of 20 bottlenose dolphins from recordings made during brief capture–release events in Sarasota Bay, FL, U.S.A., and asked 10 judges to visually group spectrograms based on similarity of their contours. Judges consistently grouped whistles according to the identity of the vocalizer; the mean number of whistles of a single dolphin in the judges' groups was 18.9 ± 1.6 out of 20 possible; resampling simulations indicate that it is highly unlikely (P < 0.0001) for this many whistles to be grouped together randomly. In contrast, the McCowan method was highly inaccurate at classifying whistles according to the identity of the vocalizer; whistles produced by multiple dolphins were grouped together. This study provides strong external validation for the reliability of visual classification of whistles, and confirms the presence of individually distinctive signature whistles in the vocal repertoire of bottlenose dolphins.