Silence as an Overlooked Catalyzer for Primate Vocal Evolution.

Language is inherently relational, prompting comparative studies to prioritize social, gregarious, and voluble primates as evolutionary models. By prevailing theory, solitary and dispersed primates ought to be silent or communicatively impoverished, but this is often presumed, not empirically verified, with these species' capacities seldom being brought to bear on evolutionary theories based on social drivers. This skewed sampling could mask other valuable but hitherto disregarded sources of information and inference obfuscated by the spotlights of the social stage. Orangutans-the only hominids living without permanent social groups-are also the only nonhuman primate known to share with humans two hallmark features of language: displaced vocal reference (where mothers delay alarm calls to teach offspring about past predator encounters) and vocal hierarchies (with vocal motifs nested in self-similar motifs). The description of such traits in the most private and reserved great ape highlights a hidden precondition for vocal evolution: extended and elaborated vocal patterns and functions, which epitomize vocal sophistication, are time-sensitive and require attentive listening without interruption for correct processing. The functional benefits of elaborate and complex sound combinations partly depend on receivers being capable of vocal suppression and impulse control in response to others. Conversely, species with frequent, rapid and reactive social chatter incur "town square taxes" that interrupt and constrain, rather than promote, vocal elaboration and complex patterning. Silence remains a critical but undertheorized resource for fostering communicative and cognitive sophistication, helping reframe the (a)social prerequisites for primate vocal evolution and the emergence of language amongst ancestral hominids.