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The function and evolution of child-directed communication

['Johanna Schick', 'Department Of Comparative Language Science', 'University Of Zurich', 'Zurich', 'Center For The Interdisciplinary Study Of Language Evolution', 'Isle', 'Caroline Fryns', 'Department Of Comparative Cognition', 'University Of Neuchatel', 'Neuchatel']

Date: 2022-05

Abstract Humans communicate with small children in unusual and highly conspicuous ways (child-directed communication (CDC)), which enhance social bonding and facilitate language acquisition. CDC-like inputs are also reported for some vocally learning animals, suggesting similar functions in facilitating communicative competence. However, adult great apes, our closest living relatives, rarely signal to their infants, implicating communication surrounding the infant as the main input for infant great apes and early humans. Given cross-cultural variation in the amount and structure of CDC, we suggest that child-surrounding communication (CSC) provides essential compensatory input when CDC is less prevalent—a paramount topic for future studies.

Citation: Schick J, Fryns C, Wegdell F, Laporte M, Zuberbühler K, van Schaik CP, et al. (2022) The function and evolution of child-directed communication. PLoS Biol 20(5): e3001630. https://doi.org/10.1371/journal.pbio.3001630 Published: May 6, 2022 Copyright: © 2022 Schick et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Writing this article was supported by the National Centre of Competence in Research (NCCR) Evolving Language, Swiss National Science Foundation Agreement 51NF40 180888 for JS, CF, FW, KZ, CPvS, SWT and SS. SWT was additionally funded by Swiss National Science Foundation grant PP00P3_198912. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. Abbreviations: CDC, child-directed communication; CSC, child-surrounding communication; IDC, immature-directed communication

The features and functions of immature-directed vocalizations in animals To identify both the evolutionary roots and adaptive functions of CDC in humans, we must examine similar phenomena in animals. We already noted that preliminary work on great apes suggests our common ancestor featured few, if any, of the elements of CDC as listed in Table 1, at least in the vocal domain. However, it must be stressed that this absence may simply reflect a lack of focused research effort rather than actual absence. But if it is confirmed, this would suggest that surrounding vocalizations provide the primary input for the learned part of the vocal development in great apes and that CDC originated de novo in the human lineage (Fig 1), presumably linked to the emergence of natural pedagogy, which may have preceded, and in fact facilitated, language evolution [53]. We now turn to possible convergent cases. First, we already discussed calls by great ape mothers, but they also occur in other primates [123,124], as well as in many nonprimate species, where mothers call to their infants to retrieve them. Examples include domestic cats (Felis silvestris catus [125]), and ungulates such as domestic sheep (Ovis aries [126]), cattle (Bos taurus [127]), goitred gazelles (Gazella subgutturosa [128]), or saiga antelopes (Saiga tatarica tatarica [129]). Second, immature-directed calls may serve to aid recognition of the mother’s voice, as in domestic cats [125], Mexican free-tailed bats (Tadarida brasiliensis mexicana [130]), fur seals (Arctocephalus tropicalis [131]), or domestic sheep [126]. These examples show that even if IDC exists in an animal species, it is unlikely that these cases are functionally equivalent to human CDC. However, in a third category of species, we find immature-directed calls related to their capacity for vocal accommodation (small alterations of vocalizations as a result of experience [132]) and vocal learning (Box 1). Orcas (Orcinus orca) produce family-typical calls at higher rates after the birth of a calf [133]. Likewise, common marmosets (Callithrix jacchus), which show evidence of accommodation learning, and thus some level of vocal plasticity [134], modify call rates and repeat various different call types before and after birth of infants [135]. In agile gibbons (Hylobates agilis), duetting by mothers with inexperienced young has also been argued to represent IDC, serving to aid the acquisition of the species-specific song [136]. In these cases, the calls may serve to acquire the group’s vocal signature. Finally, some cases show suggestive parallels to human CDC. In cooperatively breeding marmosets, adults give contingent vocal feedback specifically to infants, which is suggested to impact vocal ontogeny since infants exposed to more of such calls by adults produce and properly use adult-like calls earlier [28,137], possibly owing to increased practice or because vocal feedback reduces stress [13]. This contingent vocal feedback may help infants acquire the underlying rules of dyadic vocal communication (i.e., turn-taking [138], but see [139]). Outside primates, in zebra finches, male tutors use a more stereotypic song when they are near immature birds [140]. In greater sac-winged bats (Saccopteryx bilineata), mothers adjust the pitch and timbre when they use immature-directed vocalizations [141]. Despite these parallels, no study has asked exactly which features of the vocalizations (Table 1) are essential and which functions they serve. It is therefore too early to conclude the common incidence of CDC-like functions of immature-directed vocalizations in either primate or nonprimate species [28,40,140–142]. Systematic comparisons are needed to assess the extent of convergence and the determinants, but it remains plausible that IDC serves to facilitate the learning of vocal signatures (in accommodators) or call repertoires (in vocal learners sensu stricto), similar to the language acquisition function of human CDC.

The function of CDC relative to CSC in humans Although considerable attention has been paid to CDC and its structuring and function, comparatively less is known about the relative role of surrounding communication that children are exposed to (CSC). Indeed, in some linguistic communities surrounding communication is the primary source of input since adults rarely directly address infants (e.g., Kaluli and Samoan [106]; Yucatec Mayan [16] and Tsimane [17]), at least in their first year of life. Despite these differences in input type, children still become competent native speakers [106,109,143,144]. This inevitably begs the question how important CDC actually is for speech development and suggests that CSC, although currently still underresearched, may have an equally important, perhaps compensatory role in facilitating language acquisition. In small-scale societies, which arguably represent the more typical human condition, children are continuously surrounded by individuals of all ages [145], suggesting that the amount and variation of CSC will be higher than in WEIRD societies. To date, the few studies that to our knowledge have quantitatively assessed this [17,109,146] have not revealed an effect of CSC on vocabulary development [16,101]. However, more work is needed to understand whether CSC supports the learning of other properties of language such as grammatical features. To obtain a full understanding of how communicative competence develops in both humans and animals, it is critical to account for both sources of input—CDC and CSC—and the interplay between them. Are both CSC and CDC essential for proper language learning, or are they to some extent compensatory? If so, do the large amounts of CDC in WEIRD societies serve to compensate for the much lower quantity of CSC? In animals, immature-surrounding vocalizations might well be the predominant form of input, yet very little research has attempted to quantify their occurrence and assess their influence on the development of communicative competence. Filling this gap should be a high priority for research. The question arises whether the relative amounts of CDC and CSC seen in humans are comparable to those found in great apes. The one study on chimpanzee infants suggests that immature-surrounding communicative events total approximately 15 gestures, 50 vocalizations, and 3 gesture–call combinations per hour [147]. This is considerably more than what is known so far about the above mentioned low rate of immature-directed vocalizations. In all likelihood, therefore, immature-surrounding vocalizations were the most important source for the learnt part of the vocal system (usage and comprehension learning) in early hominins.

Conclusions and future directions In human language learning, the amount and quality of CDC is one of the key facilitators of learning. But how the various features that make up CDC change with age, especially relative to the 9-month revolution, is not clear and should be the target of future studies because they may vary in function from creating attachment, to establishing joint attention, to supporting specific details of language acquisition. Despite its universality, research across and within cultures has shown enormous variation in a child’s exposure to directed communication. Studies of a few non-WEIRD societies show much lower rates of CDC than found in the typical studies of WEIRD societies. This suggests that the amount of CDC children are exposed to in WEIRD societies might be atypical for the rest of the world and most of human history. Given the fact that all children learn the language of their culture, independent of culture-specific variation in input, the role of CSC for language learning might have been underestimated. The increased amount of CDC in WEIRD societies seems to result mainly in a refinement of skills, involving the size of the vocabulary and the construction inventory involved. This raises the question of how CDC produces this refinement. Its impact may relate to the interactional situations in which it occurs. In these contexts, joint attention is the key component that actually facilitates learning [52,148,149]. Such joint attentional frames allow the reduction of interpretation space of form-meaning associations. Given the extreme cross-linguistic variability of CDC, we must ask the questions of whether and how much CDC is really essential to language learning, whether CSC would do an equivalent job but just more slowly, or whether CDC is essential at particular stages only. Daylong recordings in naturalistic conditions are likely to provide answers to these questions. To shed light on how CDC evolved, we examined research on our closest relatives, the great apes. So far, very little directed input to infants has been documented. Concerning the features of human CDC (Table 1), few have been found in ape communication, except for repetition of gestures. Repetition is arguably the best predictor of language acquisition in human infants and children [88,89,150]. These findings suggest that short-term repetitive use of communicative acts is potentially an ancestral feature of CDC. We therefore propose that more research is needed on structural repetition to complement the usual emphasis on acoustic features of CDC. With regard to other animal species, there is more evidence for immature-directed vocalizations in species that engage in vocal learning. This supports the idea that CDC in hominins arose to support the acquisition of highly culturally variable acoustic and structural features of language. However, much more systematic comparisons are needed, which should indicate which of the features characterizing human CDC are also found in these convergent cases. Obviously, more targeted work on great apes is a high priority, if only to see whether repetition is the only CDC-like feature present and why gestures appear to be the exception. In sum, the current state of research suggests that most features of human CDC have evolved anew in our hominin ancestors. It serves to engage children in social interaction with caretakers and thus to facilitate language acquisition and, in later phases, more explicitly in the acquisition of semantics and grammar. In other words, there is no doubt that CDC is an implicit teaching device. Doubt remains, however, whether it is the only facilitator.

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