DOI: 10.5061/dryad.wpzgmsbrm
title: Data for: Isochrony and rhythmic interaction in ape duetting

description: 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.
description: <strong>Animals and
 recordings</strong> Six habituated gibbon familiar groups were
 followed, with a total of 12 individuals and 215 songs, and specifically
 157 <em>female contributions to duet</em>, 157 <em>male
 contributions to duet</em> and 58 <em>male solos</em>.
 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. <strong>Acoustic
 analyses</strong> 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 <em>TextGrid</em>, 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<sub>k</sub>). We focused on all t<sub>k</sub>
 ≤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<sub>k</sub> per song type (Hz). The ratio
 (r<sub>k</sub>) was then calculated between a
 t<sub>k</sub> and the next one, t<sub>k</sub>+1, as t<sub>k</sub>/(t<sub>k</sub>+t<sub>k</sub>+1).