An investigation of the long-tailed macaque (Macaca fascicularis) vocal repertoire: call types, signal gradations, and phonetical features

Abstract

The acoustic communication of macaques comprises an array of qualities. The present study examined long-tailed macaques (Macaca fascicularis) call types, signal gradations in their vocal repertoire, and vowel-like sounds in their vocalizations. The vocal behaviour of 8 groups of wild long-tailed macaques in Upper Peirce Reservoir Park and Pulau Ubin, Singapore, was sampled on a group level over a 3-month period. N = 1104 calls recorded were categorized by ear based on their distinctive auditory features and by spectrographic observations of their unique spectral features. A total of 8 broad call types were distinguished: coo, squeal, scream, krra, krahoo, copulation call, wraagh, and bark. The krra call types were further classified into 4 subtypes: krra-squeal, krra-scream, krra-trill, and krra-rapid. Systematic descriptions of their defining auditory characteristics, spectrographic features, and contexts of occurrence were provided. MANOVA tests revealed significant acoustic differences between call types, supporting the accuracy of call categorization. Moreover, PCA demonstrated the clusterability of call types too. PCA also showed intra-gradations and inter-gradations within and between the clusters of call types, indicative of a graded vocal repertoire in long-tailed macaques. Meanwhile, formant analysis used to examine vowel qualities in speech revealed vowel-like features in their coo calls. Consonant-like features in their vocal repertoire were discussed in the present paper too. The potential presence of phonetical units in the long-tailed macaque vocal repertoire suggests possible syntactic structures in their vocal repertoire, in which smaller units of sounds could be structurally combined to produce meaningful expressions.