Fig. 2. Syllable classification. (A) Sound spectrogram of four syllables produced by six different individuals (Ind; individuals are identified by a number corresponding to the location and a letter that differentiates them from their neighbours inside the location). (B) Examples of syllables (Syl) classified as `identical' or `different'. Sound spectrograms of 11 pairs of `identical' syllables produced by two different individuals. Double lines separate groups of syllables that have a similar shape but that were classified as `different'.