PREQUEL: Supervised phonetic approaches to analyses of great ape quasi-vowels

There is renewed interest in potential vowel production by nonhuman primates, but no agreed?upon methodologies for its estimation from real?life vocalizations. Here, we present a set of supervised approaches for estimating primate vowel-like articulation, with reference to orangutan long call pulses (N=36). We summarize our approach as a cohesive framework, the Primate Quasi-Vowel (PREQUEL) protocol. We (1) estimated f 0 from correlograms, (2) and vocal tract resonances (formants) from spectrograms, (3) the results of which were then compared against synthesized vowels for those frequency values; and (4) presented to uninformed listeners (N=16), who largely agreed on the categorization of vowel-like qualities for vocalizations (Cronbach’s alpha=.701). We also provide descriptions of methods that are seemingly inadequate for formant estimation in great ape calls. We argue that a combination of phonetic methods is required to develop a science of nonhuman primate articulation.