Evidence of Vocal Gradation in the Vocal Repertoire of Wild Moor Macaques (Macaca maura)

Abstract

Nonhuman primates constitute an ideal model to study the evolutionary origins of human language, because of their close phylogenetic distance to humans and their reliance on complex communication systems that include different signal types. In this study, we investigated the vocal repertoire of Macaca maura (moor macaques), a highly tolerant primate species endemic to Sulawesi, Indonesia. We conducted detailed acoustic analyses on 1116 high-quality vocalizations recorded from a well-habituated wild group of 42 individuals. Using discriminant function and random forest analyses, we found that moor macaques show a graded vocal repertoire, with considerable acoustic overlap between several call types and a high number of acoustic parameters needed for accurate classification. These findings were supported by the failure of unsupervised clustering to detect robust categorical structures beyond a two-cluster solution. Our findings provide novel insights into the vocal behavior of a yet understudied species, and provide preliminary evidence that moor macaques show graded communication systems.