Abstract
Analyzing the rhythm of animals' acoustic signals is of interest to a growing number of researchers: evolutionary biologists want to disentangle how these structures evolved and what patterns can be found, and ecologists and conservation biologists aim to discriminate cryptic species on the basis of parameters of acoustic signals such as temporal structures. Temporal structures are also relevant for research on vocal production learning, a part of which is for the animal to learn a temporal structure. These structures, in other words, these rhythms, are the topic of this paper. How can they be investigated in a meaningful, comparable and universal way? Several approaches exist. Here we used five methods to compare their suitability and interpretability for different questions and datasets and test how they support the reproducibility of results and bypass biases. Three very different datasets with regards to recording situation, length and context were analyzed: two social vocalizations of Neotropical bats (multisyllabic, medium long isolation calls of Saccopteryx bilineata, and monosyllabic, very short isolation calls of Carollia perspicillata) and click trains of sperm whales, Physeter macrocephalus. Techniques to be compared included Fourier analysis with a newly developed goodness-of-fit value, a generate-and-test approach where data was overlaid with varying artificial beats, and the analysis of inter-onset-intervals and calculations of a normalized Pairwise Variability Index (nPVI). We discuss the advantages and disadvantages of the methods and we also show suggestions on how to best visualize rhythm analysis results. Furthermore, we developed a decision tree that will enable researchers to select a suitable and comparable method on the basis of their data.
Highlights
Rhythms can be found anywhere in the world: our hearts have rhythms, circadian rhythms are all around, music across all cultures shares certain components such as rhythm, public transportation follow a certain schedule which is nothing but rhythm
In the analysis of animal communication more and more interest is shown in rhythm of animal communication and what information this might convey
We chose three different datasets for the analysis with very different properties: 1) monosyllabic, short isolation calls of the neotropical bat Carollia perspicillata, 2) multisyllabic, medium long isolation calls of the neotropical bat Saccopteryx bilineata–both social vocalizations–and 3) monosyllabic, very long echolocation click trains of the sperm whale Physeter macrocephalus used for orientation and foraging (Fig 2)
Summary
Rhythms can be found anywhere in the world: our hearts have rhythms, circadian rhythms are all around, music across all cultures shares certain components such as rhythm, public transportation (should) follow a certain schedule which is nothing but rhythm. We learn more and more about how important a certain temporal structure is in human language, in their production as well and probably even more so in their perception; stuttering, for example, is most likely connected to a misfunction of rhythm perception [1]. This raises the question of whether rhythms, or temporal structures to use a more precise terminology, play an important role in animal communication and sound production. We still use the term ‘rhythm’ as a concept that will be understood by a broad audience, as most people have an intuitive understanding of ‘rhythm’, independent of whether this study analyses ‘rhythm’ in the musicological sense of the term
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