Table 2. Acoustic terms and definitions of gibbon song (following Konrad and Geissmann [33], Ruppell [34] and Feng et al. [35]).

Term	Definition
Note	A single continuous sound of distinct frequency or frequency modulation that may be produced during either inhalation or exhalation
Roll	A characteristic of notes produced by the male wherein each roll includes a steep increase in frequency followed by a steep decrease
Phrase	A single vocal activity consisting of a larger or looser collection of notes or elements or both that may be produced together or separately
Male sequence	A complete sequence produced by the male, is made up of boom, aa note, pre-modulated note and modulated figure. The female dose not sing during this period
Song bout	All song notes of a gibbon group with periods of silence of <10 min