Humans, but not songbirds, produce spontaneously isochronous song through the lifespan

Isochrony - as in the regular beat of a metronome - is cross-culturally ubiquitous in music. Is this ubiquity due to a widespread biological inclination for acoustic communication having isochronous structure? If so, it should be present in lesser-studied vocal music, in the absence of musical training, and in comparable non-human species vocalizations. We quantified isochrony in an untrained expression of musicality: improvised songs from non-musician adults and children. We also tested for isochrony in songs from zebra finches, a bird that learns complex songs. We analyzed improvised songs from children (n = 38, 3-10 years old) and adult non-musicians (n = 15, 24-82 years old), and songs from juvenile and adult zebra finches (n= 77, [~]50 and 120 days post-hatch, respectively). Isochrony was expressed in non-musicians improvised songs, and in childrens improvised songs to a comparable degree. In contrast, both juvenile and adult zebra finches produced songs with less isochrony than chance. And although zebra finches learn sequences and durations of their songs elements, we found no evidence for the learning of isochrony. These data show that spontaneous isochrony in learned vocalizations appears differently across these two species. We propose that species variation in chorusing behaviors may explain why.