Investigating categorical rhythms in crested gibbons (Nomascus spp.)

Music is considered one of the distinguishing characteristics of the human species and expresses biological traits not shared with other animal species. Since the last decades, however, the interest in musicality traits, those prerequisites that may underlie musical production, revealed that species other than humans might share some similar abilities with our species. For instance, besides humans, singing is present in various primate species, in some cases phylogenetically distant (e.g. 80 mya) in others within the Hominoidea clade (e.g. 20 mya), suggesting the possibility of convergent evolution for this trait. One of the fundamental characteristics of songs is rhythm. The study of rhythm is becoming essential to understand the mechanisms of singing behaviour and the evolution of human communication. Recent findings provided evidence that particular rhythmic structures occur in human music and some singing animal species, such as birds and rock hyraxes, but only two species of non-human primates have been investigated so far. Therefore, this study aims to broaden the list of species studied regarding the presence of rhythmic categories consistently. I focused on three species of gibbons of the genus Nomascus, analyzing their rhythmic structures at the genus and species levels. I collected recordings at the Zoological and Botanical Park of Mulhouse (France), which hosts three species of Nomascus: N. gabriellae (N = 3), N. leucogenys (N = 3) and N. siki (N =4). I visually inspected the recordings' spectrogram and annotated each unit's onset timing in a Praat textgrid (N = 257). Then, with the R software, I tested the presence of three rhythmic categories: 1:1, when two following onsets have the same duration; 2:1, when the first one is twice the second one; 1:2, when the second one is twice the first one. I found that the songs of the three species were strongly characterized by isochrony (1:1 ratio), suggesting high similarity in song patterns between them, in agreement with a previous study on lar gibbons. Interestingly, I found some variation at the individual level, with a male of N. siki, housed away from the social group, performing songs showing two distinct rhythmic ratios (on-integer 1:1 and off-integer 2:1). This finding suggests rhythmic structures may vary within a specie and chorusing may play a critical role in enhancing regularities. While isochrony plays an essential role in group coordination, the physical distance and separation from the social group might influence the songs' rhythmic structure.