Defining Song using Primate Calls

Schruth, D., C.N. Templeton, & D.J. Holman. "Primate Vocalizations Inform a Quantitative Index of Song", CSSS Working Paper

Musical behavior is likely as old as our species with song originating as early as 60 million years ago in the primate order. Early singing likely evolved into the music of modern humans via multiple selective events, but efforts to disentangle these causal influences have been stifled by challenges to precisely define this behavior in a broadly applicable way. Detailed here is a method to quantify the elaborateness of acoustic displays using published spectrograms (n=832 calls) culled from the literature on primate vocalizations. Each spectrogram was scored by five trained analysts via visual assessments along six musically relevant acoustic parameters: tone, interval, transposition, repetition, rhythm, and syllabic variation. Principal Components Analysis (PCA) was used to reduce this multivariate assessment into a simplified measure of musical elaborateness. The resulting “acoustic reappearance diversity” index simultaneously captures both spectral and temporal redundancy as well as syllabic variation in a single continuous variable. The potential utility of this index is demonstrated by applying it to several social and habitat based theories of acoustic display origins. Our results corroborate that primate species living in small, monogamous groups do indeed have song-like calls, while forest habitat had a less pronounced association.

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