Data for: Isochrony and rhythmic interaction in ape duetting

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.