Rhythmic structure of the songs of Nomascus siki in the wild

Gibbons are the smallest apes and the only singing primates among apes. They live in tropical and temperate forests of Southeast Asia and produce loud, distinctive, and melodious songs that resemble that of songbirds more than the songs of other singing non-human primates. Gibbons' songs' primary functions are advertising pair bond presence, attracting mates, defending territories, and repelling conspecific intruders. As for songbirds, gibbon songs are species-specific. In most gibbon species, mated pairs combine their songs in a precisely timed and complex pattern to produce coordinated duet songs with sex-specific male and female contributions. Gibbons of the genus Nomascus produce songs with unique characteristics: the highest degree of sex-specificity, no overlap in note and phrase repertoire between sexes, and only non-mated individuals seem to produce solo songs. My study aimed to investigate the rhythmic patterns, particularly the presence of categorical rhythms, in the songs of Nomascus siki. Categorical rhythms are defined as rhythms in which temporal intervals between note onsets (inter-onset interval, IOI) are distributed categorically rather than uniformly and are related by small integer ratios. For this study, songs of wild Nomascus siki were collected in the Nakai-Nam Theun National Protected Area (NNT NPA) forests in Lao PDR. I analyzed the rhythmic structure of the final output of the duets, considering the overall silent and voiced intervals. The results of my study suggest the presence of only one rhythmic category in wild Nomascus siki: isochrony (1:1). This result fits well with previous findings on the individual rhythms of captive males and females of Nomascus siki, which also showed only isochrony. Once again, in my study, we suggest that isochrony is a universal and ancestral rhythmic category among singing primates and other singing taxa. Raimondi and colleagues’ research (2023) on Hylobates lar suggested it derives from morphological and biomechanical constraints and successively may serve several functions, such as facilitating song coordination, synchrony in duetting, song processing, and learning and serving as a quality indicator of the emitter.