Speech-Like Rhythm in a Voiced and Voiceless Orangutan Call

Abstract

The evolutionary origins of speech remain obscure. Recently, it was proposed that speech derived from monkey facial signals which exhibit a speech-like rhythm of ∼5 open-close lip cycles per second. In monkeys, these signals may also be vocalized, offering a plausible evolutionary stepping stone towards speech. Three essential predictions remain, however, to be tested to assess this hypothesis' validity; (i) Great apes, our closest relatives, should likewise produce 5Hz-rhythm signals, (ii) speech-like rhythm should involve calls articulatorily similar to consonants and vowels given that speech rhythm is the direct product of stringing together these two basic elements, and (iii) speech-like rhythm should be experience-based. Via cinematic analyses we demonstrate that an ex-entertainment orangutan produces two calls at a speech-like rhythm, coined “clicks” and “faux-speech.” Like voiceless consonants, clicks required no vocal fold action, but did involve independent manoeuvring over lips and tongue. In parallel to vowels, faux-speech showed harmonic and formant modulations, implying vocal fold and supralaryngeal action. This rhythm was several times faster than orangutan chewing rates, as observed in monkeys and humans. Critically, this rhythm was seven-fold faster, and contextually distinct, than any other known rhythmic calls described to date in the largest database of the orangutan repertoire ever assembled. The first two predictions advanced by this study are validated and, based on parsimony and exclusion of potential alternative explanations, initial support is given to the third prediction. Irrespectively of the putative origins of these calls and underlying mechanisms, our findings demonstrate irrevocably that great apes are not respiratorily, articulatorilly, or neurologically constrained for the production of consonant- and vowel-like calls at speech rhythm. Orangutan clicks and faux-speech confirm the importance of rhythmic speech antecedents within the primate lineage, and highlight potential articulatory homologies between great ape calls and human consonants and vowels.

Rhythm in speech and animal vocalizations: a cross‐species perspective

“…Focusing on individual vocal rhythms, early descriptive work remarked temporal regularities in gelada monkeys’ vocalizations, a claim which is intriguing but purely descriptive, unfortunately not supported by quantitative data or statistical inference. More recent work in macaques and orangutans noted a 5‐Hz isochronous pattern during lip‐smacking, facial movement, or vocalization . Primate perspectives on speech rhythm can be found elsewhere (e.g., see Ref.…”

“…More recent work in macaques and orangutans noted a 5-Hz isochronous pattern during lip-smacking, facial movement, or vocalization. [35][36][37] Primate perspectives on speech rhythm can be found elsewhere (e.g., see Ref. 21), but it is clear that we do need to understand more about vocal rhythms in our closest living relatives, the primates.…”

From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans

“…In one study, macaque monkeys were even observed to spontaneously modify the vocal properties of their contact call for requesting different objects from the experimenter ( Hihara et al , 2003). Anecdotal reports of more generalized volitional vocal control, albeit rudimentary, in apes ( Hayes & Hayes, 1952; Hopkins et al , 2007; Kalan et al , 2015; Koda et al , 2007; Koda et al , 2012; Lameira et al , 2015; Laporte & Zuberbühler, 2010; Perlman & Clark, 2015; Taglialatela et al , 2003; Wich et al , 2008) suggest that the direct connections between the IFG and the brainstem motor nuclei were strengthened prior to our divergence from our apian relatives.…”

The Evolution of Rhythm Cognition: Timing in Music and Speech

“…Other primates' vocal tracts are capable of producing a human-like range of vowels Boë et al, 2017) and consonants (Lameira et al, 2015(Lameira et al, , 2016(Lameira et al, , 2017. The overall conclusion is that the complexity of human speech, including its rhythmical nuances, must have neural, rather than morphological, bases (Ravignani et al, 2014b;Fitch et al, 2016;Belyk and Brown, 2017).…”