Abstract
The origins of human speech are obscure; it is still unclear what aspects are unique to our species or shared with our evolutionary cousins, in part due to a lack of a common framework for comparison. We asked what chimpanzee and human vocal production acoustics have in common. We examined visible supra-laryngeal articulators of four major chimpanzee vocalizations (hoos, grunts, barks, screams) and their associated acoustic structures, using techniques from human phonetic and animal communication analysis. Data were collected from wild adult chimpanzees, Taï National Park, Ivory Coast. Both discriminant and principal component classification procedures revealed classification of call types. Discriminating acoustic features include voice quality and formant structure, mirroring phonetic features in human speech. Chimpanzee lip and jaw articulation variables also offered similar discrimination of call types. Formant maps distinguished call types with different vowel-like sounds. Comparing our results with published primate data, humans show less F1–F2 correlation and further expansion of the vowel space, particularly for [i] sounds. Unlike recent studies suggesting monkeys achieve human vowel space, we conclude from our results that supra-laryngeal articulatory capacities show moderate evolutionary change, with vowel space expansion continuing through hominoid evolution. Studies on more primate species will be required to substantiate this.This article is part of the theme issue ‘Voice modulation: from origin and mechanism to social impact (Part II)’.
Highlights
The origins of human speech are obscure, and the order of emergence of components required for speech to evolve is much debated [1,2,3,4,5]
Superimposing the non-human primate formant space with that of American English speakers [40], we find that the formant usage of chimpanzees, and of primate species investigated in previously published studies, overlaps with that of humans
Chimpanzee vocalizations can be broadly classified into four major call types: hoos, grunts, barks and screams, using either discriminant or principal component classification approaches
Summary
The origins of human speech are obscure, and the order of emergence of components required for speech to evolve is much debated [1,2,3,4,5]. These studies demonstrate formant structure (gorilla: double grunts [2]), or formant shifts, indicating that articulatory changes modulate both voiced and voiceless call production (chimpanzee: rough grunts given to foods of different preferences [18]; orangutans: voiceless clicks and voiced fauxspeech [19]; grumph versus ‘wookie’ calls [20]) These studies suggest that articulatory movements, such as lip and jaw movements, likely contribute to call modulation in great apes, the extent to which such movements assist in the classification of different call types across a species’ repertoire has rarely been assessed. We assess the contribution of acoustic features often explanatory in speech sound and animal call classification to classify the main four call types in the chimpanzee vocal repertoire (hoos, grunts, screams and barks), each of which is used widely across the chimpanzee vocal repertoire
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