Abstract
From the first months of life, human infants produce “protophones,” speech-like, non-cry sounds, presumed absent, or only minimally present in other apes. But there have been no direct quantitative comparisons to support this presumption. In addition, by 2 months, human infants show sustained face-to-face interaction using protophones, a pattern thought also absent or very limited in other apes, but again, without quantitative comparison. Such comparison should provide evidence relevant to determining foundations of language, since substantially flexible vocalization, the inclination to explore vocalization, and the ability to interact socially by means of vocalization are foundations for language. Here we quantitatively compare data on vocalization rates in three captive bonobo (Pan paniscus) mother–infant pairs with various sources of data from our laboratories on human infant vocalization. Both humans and bonobos produced distress sounds (cries/screams) and laughter. The bonobo infants also produced sounds that were neither screams nor laughs and that showed acoustic similarities to the human protophones. These protophone-like sounds confirm that bonobo infants share with humans the capacity to produce vocalizations that appear foundational for language. Still, there were dramatic differences between the species in both quantity and function of the protophone and protophone-like sounds. The bonobo protophone-like sounds were far less frequent than the human protophones, and the human protophones were far less likely to be interpreted as complaints and more likely as vocal play. Moreover, we found extensive vocal interaction between human infants and mothers, but no vocal interaction in the bonobo mother–infant pairs—while bonobo mothers were physically responsive to their infants, we observed no case of a bonobo mother vocalization directed to her infant. Our cross-species comparison focuses on low- and moderate-arousal circumstances because we reason the roots of language entail vocalization not triggered by excitement, for example, during fighting or intense play. Language appears to be founded in flexible vocalization, used to regulate comfortable social interaction, to share variable affective states at various levels of arousal, and to explore vocalization itself.
Highlights
Roots of Language Envisioned Through Cross-Species ComparisonsThe claim that language is the quintessential feature of humanity has distant roots (Condillac, 1756)
Our findings suggest that infants of one of our nearest genetic relatives, bonobos, do appear to produce protophone-like sounds, suggesting a commonality in language-like foundations with humans
Our report suggests human infant vocal communication contrasts sharply in frequency of occurrence with that of the bonobos we studied, and the difference calls for evolutionary explanation
Summary
Roots of Language Envisioned Through Cross-Species ComparisonsThe claim that language is the quintessential feature of humanity has distant roots (Condillac, 1756). Such research has revealed much about animal intelligence, including cognitive domains in which animals exceed humans (de Waal, 2016). It has made clear that even if animals do not display language the way humans do, many of the features of language have foundations in other animals (Snowdon, 2004). Of special interest are the dramatic achievements of a variety of taxa in learning fundamental language features including, importantly, symbolism in the context of human training (e.g., Menzel, 1999; Pepperberg, 2004), especially when it begins early in the life of the animal and is consistent over extended time periods (Griebel et al, 2016). Research has provided mounting physiological and genetic evidence about vocalization across species, revealing shared systems as well as species-typical and age-dependent properties of vocal control (DeVoogd et al, 1993; Jürgens, 1995; Wild, 1997; Newman, 2007; Ravbar et al, 2012; Ackermann et al, 2014; Takahashi et al, 2015; Hage et al, 2016; Mello and Clayton, 2016; Roy et al, 2016; Loh et al, 2017)
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