General isochronous rhythm in echolocation calls and social vocalizations of the batSaccopteryx bilineata

Abstract

Rhythm is an essential component of human speech and music but very little is known about its evolutionary origin and its distribution in animal vocalizations. We found a regular rhythm in three multisyllabic vocalization types (echolocation call sequences, male territorial songs and pup isolation calls) of the neotropical bat Saccopteryx bilineata. The intervals between element onsets were used to fit the rhythm for each individual. For echolocation call sequences, we expected rhythm frequencies around 6–24 Hz, corresponding to the wingbeat in S. bilineata which is strongly coupled to echolocation calls during flight. Surprisingly, we found rhythm frequencies between 6 and 24 Hz not only for echolocation sequences but also for social vocalizations, e.g. male territorial songs and pup isolation calls, which were emitted while bats were stationary. Fourier analysis of element onsets confirmed an isochronous rhythm across individuals and vocalization types. We speculate that attentional tuning to the rhythms of echolocation calls on the receivers' side might make the production of equally steady rhythmic social vocalizations beneficial.

Rhythm in speech and animal vocalizations: a cross‐species perspective

“…Surprisingly, the neotropical bat Saccopteryx bilineata exhibits isochronous rhythms not only in its echolocation calls, but also in male vocal displays (i.e., “songs”) and pups’ call sequences . In addition, a (post‐hoc) superimposed metronomic grid exhibited a tempo, which matched the wing‐beat of the animals . The finding of temporal similarities between vocal (calls) and nonvocal (wing‐beat) rhythms in bats is one more comparative piece of evidence of the cross‐modality of rhythm, which in humans entails audition, vision, movement, etc.…”

“…The isochrony detection technique used in zebra finches has also been applied to a bat species capable of vocal production learning. Surprisingly, the neotropical bat Saccopteryx bilineata exhibits isochronous rhythms not only in its echolocation calls, but also in male vocal displays (i.e., “songs”) and pups’ call sequences . In addition, a (post‐hoc) superimposed metronomic grid exhibited a tempo, which matched the wing‐beat of the animals .…”

“…and ). Babbling as a kind of vocal play and imitation of adult calls, barks, trills, and songs is also observed in infant and juvenile pygmy marmosets, sac‐winged bat pups, giant otters, and zebra finches . According to the Frame and Content theory, babbling in human infants is a rhythmic motor training, which lays the grounds for basic syllable structure.…”

Comparison of methods for rhythm analysis of complex animals’ acoustic signals

“…Moreover, knowledge about temporal structures is necessary to find coupled biological processes, such as the correlation between beat frequencies in bat's acoustic signals (also called vocalizations) with their wingbeat frequencies (i.e. wingbeats per second), independent from whether a bat might actually be flying in a vocalizing context or hanging in a roost [4]. Rhythmicality might also influence mate choice and individual recognition [5,6].…”

“…Where we speak of rhythms in animals' acoustic signals a musicologist might only talk about different beats and tempi. What we mean in this paper with rhythm and the connotation of rhythm used in other studies on the subject [4,5,10] describes a temporal structure that might have varying complexity but is mostly based on an isochronous beat (i.e. sounds produced by a metronome).…”