How did rhythm originate in humans, and other species? One cross-cultural universal, frequently found in human music, is isochrony: when note onsets repeat regularly like the ticking of a clock. Another universal consists of synchrony, e.g., when individuals coordinate their notes so that they are sung at the same time. An approach to biomusicology focuses on similarities and differences across species, trying to build phylogenies of musical traits. Here we test for the presence of, and a link between, isochrony and synchrony in a non-human animal. We focus on the songs of one of the few singing primates, the lar gibbon (Hylobates lar), extracting temporal features from their solo songs and duets. We show that another ape exhibits one rhythmic feature at the core of human musicality: isochrony. We show that an enhanced call rate overall boosts isochrony, suggesting that respiratory physiological constraints play a role in determining the song's rhythmic structure. However, call rate alone cannot explain the flexible isochrony we witness. Isochrony is plastic and modulated depending on the context of emission: gibbons are more isochronous when duetting than singing solo. We present evidence for rhythmic interaction: we find statistical causality between one individual’s note onsets and the cosinger’s onsets, and a higher than chance degree of synchrony in the duets. Finally, we find a sex-specific trade-off between individual isochrony and synchrony. Gibbons' plasticity for isochrony and rhythmic overlap may suggest a potential shared selective pressure for interactive vocal displays in singing primates. This pressure may have convergently shaped human and gibbon musicality while acting on a common neural primate substrate. Beyond humans, singing primates are promising models to understand how music and, specifically, a sense of rhythm originated in the primate phylogeny.

Animals and recordings

Six habituated gibbon familiar groups were followed, with a total of 12 individuals and 215 songs, and specifically 157 female contributions to duet, 157 male contributions to duet and 58 male solos. Four groups were inhabiting the forests of Huai Kha Khaeng Wildlife Sanctuary (Thailand), and two other ex-situ groups were living at the Cappeller faunistic park (Corvigliano, Italy) and the Falconara zoo park (Ancona, Italy). 

We recorded the animals between 6:00AM and 12:00AM using a Sennheiser ME67 microphone connected to a solid-state digital audio recorder Tascam DR-100MKII (44.1KHz sampling rate). All vocalizations were recorded at 5–50 meters distance from the animals, aiming the microphone toward the individual vocalizing to maximize recording quality.

Acoustic analyses

We edited the songs using Praat 6.0.14 and saved them as WAV audio files. Field notes and video recordings allowed us to recognize and separate individual contributions to each song; each annotated contribution was subsequently saved as a single Praat TextGrid, an object featuring onset and offset of each note. A computing cluster (OCCAM) processed all 196768 vocal units via a custom Praat script and exported all onsets of song units from separate TextGrids into one .csv datasheet. We calculated the temporal interval between an onset and the next one, which defines an inter-onset interval (t_k). We focused on all t_k ≤5 sec, as this value is typically hypothesized as the upper limit for meter perception and performance in humans, and there is no quantitative evidence about an upper threshold on other apes. We calculated the tempo frequency as the inverse of peak values of t_k per song type (Hz). The ratio (r_k) was then calculated between a t_k and the next one, t_k+1, as t_k/(t_k+t_k+1).