Defining Song using Primate Calls

Schruth, D., C.N. Templeton, & D.J. Holman. "A definition of song using human music universals observed in primate calls", BioRxiv pre-print (under review)

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 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 syllabic variation and spectral/temporal redundancy 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 confirm that primate species living in small, monogamous groups have song-like calls, while forest habitat had a less pronounced association.