Orangutans show active voicing through a membranophone

Abstract

Active voicing – voluntary control over vocal fold oscillation – is essential for speech. Nonhuman great apes can learn new consonant- and vowel-like calls, but active voicing by our closest relatives has historically been the hardest evidence to concede to. To resolve this controversy, a diagnostic test for active voicing is reached here through the use of a membranophone: a musical instrument where a player’s voice flares a membrane’s vibration through oscillating air pressure. We gave the opportunity to use a membranophone to six orangutans (with no effective training), three of whom produced a priori novel (species-atypical) individual-specific vocalizations. After 11 and 34 min, two subjects were successful by producing their novel vocalizations into the instrument, hence, confirming active voicing. Beyond expectation, however, within <1 hour, both subjects found opposite strategies to significantly alter their voice duration and frequency to better activate the membranophone, further demonstrating plastic voice control as a result of experience with the instrument. Results highlight how individual differences in vocal proficiency between great apes may affect performance in experimental tests. Failing to adjust a test’s difficulty level to individuals’ vocal skill may lead to false negatives, which may have largely been the case in past studies now used as “textbook fact” for great ape “missing” vocal capacities. Results qualitatively differ from small changes that can be caused in innate monkey calls by intensive months-long conditional training. Our findings verify that active voicing beyond the typical range of the species’ repertoire, which in our species underpins the acquisition of new voiced speech sounds, is not uniquely human among great apes.

The many functions of vocal learning

“…Furthermore, it may restrict the evolution of open-ended vocal learning to relatively long-lived species. However, this still does not fully explain the lack of vocal learning in certain species: orangutans have the longest period to parental independence of any non-human animal (up to 8 years), but evidence for VPL in orangutans remains very limited [138,139]. Orangutans are mostly solitary, suggesting that large social groups may also be crucial.…”

Multimodality and the origin of a novel communication system in face-to-face interaction

“…[39]). However, Perlman [24] argues that there is at least some evidence of vocal learning in nonhuman primates, and that this might plausibly allow a fully multimodal system right from the start (see also [21,27,40]).…”

Understanding Language Evolution: Beyond Pan‐Centrism

“…While the strand of research on great ape FOXP2 orthologs is still in its infancy, these phenotypical effects seems to be potentially linked and help explain some of the remarkable language-like and sophisticated vocal learning and vocal invention capacities described in Pongo. [26][27][28][29] If measured by the number of FOXP2 amino-acid substitutions since the last great ape common ancestor, only orangutans and humans have accumulated functional substitutions.…”

“…This demonstrates that this substitution has structural consequences in individuals’ phenotype and that their biological makeup would be different without this substitution in terms of the circuitry involved in motor skills and vocal articulation and control. While the strand of research on great ape FOXP2 orthologs is still in its infancy, these phenotypical effects seems to be potentially linked and help explain some of the remarkable language‐like and sophisticated vocal learning and vocal invention capacities described in Pongo . If measured by the number of FOXP2 amino‐acid substitutions since the last great ape common ancestor, only orangutans and humans have accumulated functional substitutions.…”