Abstract:

Vocal homologues between humans and other primates shape our understanding on the evolution of spoken language. Voiced calls (or “vocalizations”) characterize primate vocal repertoires overall and are present in human speech, predominantly in the form of vowels. Voiceless primate calls exhibit, however, distinct acoustic and articulatory traits with affinities much closer with human consonants. Whilst voiced calls date back to the last common ancestor of all primates, voiceless calls are presumed to derive from the last common great ape ancestor. Despite such recent emergence in primates’ evolutionary timeline and putative precursor role of human consonants, great ape voiceless calls have remained largely understudied. Here, we address the fundamental question of whether, while showing “exotic” acoustics, consonant-like calls could have provided equal adaptive value as vowel-like calls, and thus, whether similar selective pressures governed their evolution. We analyzed the most comprehensive dataset of great ape voiceless calls ever assembled, comprising 4487 orangutan kiss-squeaks collected across 4 wild populations and >5,500 observation hours. Analyses of two acoustic parameters defining calls’ overall spectral profile revealed that great ape voiceless calls exhibit clear geographic, sex, size, individual and context signatures – the five major levels of acoustic variation hitherto described in nonhuman primate voiced calls. Our findings demonstrate that consonant-like acoustics in hominids allow the transmission of information-dense content. The evolution of proto-consonants and proto-vowels in the human lineage were most likely characterized by similar communicative functions and selective regimes.