Evolution of human language: duetting as part of prosociality and cognition

Abstract

The evolution of human language is a topic that has received undiminished attention. Numerous hypotheses for the origin of human language have been proposed, including gestural communication found specifically among apes. This study advances the hypothesis that human evolution, including human language development, is three-pronged: prosocial, cognitive, and collaborative. Duetting and turn-taking in primates are used as pivotal examples of how bonding leads to joint action and collaboration. It points out that such vocal behavior itself may be a crucial precursor of language evolution in the sense that it is explicitly focused on a conspecific. Some current hypotheses have acknowledged duetting as an important perceptual and behavioral example of synchronicity. Some forms of synchronized behavior, as found in duetting, synchronized dance, or even shared song, were perhaps crucial evolutionary steps preceding the evolution of human language. Duetting signifies more than that, however, because it is an observable and significant cognitive investment that signals attention toward a partner. This study also advances the hypothesis that affect and cognition would have needed to precede any form of duetting or signs of affiliation such as grooming. Hence, this study, asking what duetting in primates signifies in evolutionary terms, takes a multidisciplinary and multimodal approach to suggest important affective and cognitive steps in the evolution of human language and speech, the chief of which is prosociality. Prosociality, as an attitude and awareness of another, be this as a friend or partner for whom one can do favors or whom one can help, is a model for collaboration and cooperation, and also increased cognition.

“The song remains the same”: not really! Vocal flexibility in the song of the indris

“…Although the Levenshtein (used in Zanoli et al 2020 ) and the Jaro distances consider different parameters when calculating the similarity between two strings (Margoliash et al 1991 ; Jaro 1989 ), both algorithms gave robust results when applied to indri songs and showed that dominant females are more flexible than dominant males when concatenating elements into strings, bearing in mind that non-dominant individuals show the highest flexibility degree. These results corroborate previous work that suggested that dominant females contribute to the indris’ songs (De Gregorio et al 2019a , b ) and that they are more flexible in notes’ spectro-temporal parameters than dominant males (Torti et al 2017 ). Although the reproductive couple has a higher rank than that of other group members (usually offspring; Pollock 1975 ), reproductive females seem to be dominant over reproductive males in terms of access to resources (Pollock 1979 ; Kappeler and Pozzi 2019 ), and this could reflect in their songs’ structure.…”

“…Terleph and colleagues proposed that, for turn-taking to occur effectively, males would be the ones adjusting their song to their mate’s one because of females’ noticeable variation in spectral and temporal characteristics of their phrases (Terleph et al 2018 ). Our results did not corroborate the pattern suggested by Terleph and colleagues ( 2018 ) but are in line with previous findings on Northern white-cheeked ( Nomascus leucogenys , Deputte 1982 ), agile ( Hylobates agilis, Koda et al 2013 ), white-handed gibbon ( Hylobates lar, Raimondi et al 2023 ) and indri (Torti et al 2017 ; De Gregorio et al 2019a , b ). In indris, the dominant pair generally orchestrates the overall architecture of the duet: the duet between the reproductive pair is the most common type of singing organization (De Gregorio et al 2022a , b ), only occasionally succeeded by non-dominants, consisting of the offspring, and shows longer durations and higher degrees of overlap (Gamba et al 2016 ).…”

“…The role of dominant females during singing is critical to understanding these results. Indeed, dominant females show more creative potential than dominant males not only in the element concatenation but also by adjusting the rhythmic and temporal structures of their contributions according to the number of singers in the chorus (Gamba et al 2016 ; De Gregorio et al 2019a , b ; De Gregorio et al 2022b ). As suggested for white-handed gibbon songs ( Hylobates lar ), the temporal coordination of a duet relies on the individuals’ capability to adjust their contribution to that of the mate temporally (Terleph et al 2018 ).…”

“…These results remarkably extend previous findings that centered on the sexually dimorphic features of the indris’ duetting. Indeed, female contribution in indris is critical to determine the total song duration (Giacoma et al 2010 ), influences the duration of males’ contribution (De Gregorio et al 2019a , b ), and shows higher flexibility in concatenation (Zanoli et al 2020 ). Further studies may elucidate whether and how the relationship we found in terms of contribution diversity between males and females could be linked to a (or to a lack of a) non-random phrase type combination, where two duetting partners could use a statistical association between the phrase types they use (Logue and Krupp 2016 ).…”

“…First, we created a spectrogram for each record and saved the singer’s identity information into a Praat TextGrid. We then extracted each individual contribution and labeled the vocal units as long notes (LN), single units (SU; De Gregorio et al 2019a , b ), and descending phrases (DPs; sequences of adjacent notes with a descending frequency pattern composed of 2–6 units. Valente et al 2021 ; Torti et al 2013 ).…”