Flexibility in wild infant chimpanzee vocal behavior

Abstract

How did human language evolve from earlier forms of communication? One way to address this question is to compare prelinguistic human vocal behavior with nonhuman primate calls. An important finding has been that, prior to speech and from early on, human infant vocal behavior exhibits functional flexibility, or the capacity to produce sounds that are not tied to one specific function. This is reflected in human infants’ use of single categories of protophones (precursors of speech sounds) in various affective circumstances, such that a given call type can occur in and express positive, neutral, or negative affective states, depending on the occasion. Nonhuman primate vocal behavior, in contrast, is seen as comparably inflexible, with different call types tied to specific functions and sometimes to specific affective states (e.g. screams mostly occur in negative circumstances). As a first step toward addressing this claim, we examined the vocal behavior of six wild infant chimpanzees during their first year of life. We found that the most common vocal signal, grunts, occurred in a range of contexts that were deemed positive, neutral, and negative. Using automated feature extraction and supervised learning algorithms, we also found acoustic variants of grunts produced in the affective contexts, suggesting gradation within this vocal category. In contrast, the second most common call type of infant chimpanzees, the whimpers, was produced in only one affective context, in line with standard models of nonhuman primate vocal behavior. Insofar as our affective categorization reflects infants’ true affective state, our results suggest that the most common chimpanzee vocalization, the grunt is not affectively bound. Affective decoupling is a prerequisite for chimpanzee grunts (and other vocal categories) to be deemed ‘functionally flexible’. If later confirmed to be a functionally flexible vocal type, this would indicate that the evolution of this foundational vocal capability occurred before the split between the Homo and Pan lineages.

Socially learned arbitrary call use in a wild primate

“…For example, bonobos ( Pan paniscus ) produce ‘peep’ calls across several contexts, encompassing various emotional valences [15]. The same has been found for the ‘grunt’ calls in juvenile chimpanzees ( Pan troglodytes ) [16] and ‘coo’ calls in different macaque species [17–19].…”

The Complexity and Phylogenetic Continuity of Laughter and Smiles in Hominids

“…Regarding laugh vocalizations, it seems reasonable to conclude that such emancipation took place only after the last common ancestor of extant great apes and humans existed, because ape laughter is bioacoustically distinct (Davila-Ross et al, 2009 ; Taylor et al, 2021 ) and closely linked to play (Matsusaka, 2004 ; Davila-Ross et al, 2011 ), so that there cannot be variants with similar acoustic properties in other behavioral contexts. Furthermore, human infants within their first year of life produce various speech-related vocal types (protophones) free from contexts, but not laughter (Oller et al, 2013 ; for a comparative approach, see Dezecache et al, 2020 ). Consequently, laughter must have been used more flexibly closer toward humans, occurring in a wide range of everyday social interactions with gradually modifying acoustic properties (Owren and Bachorowski, 2003 ; Davila-Ross et al, 2010 ), perhaps accompanying key changes in language evolution (for laughter in conversation, see Vettin and Todt, 2004 ; Flamson and Bryant, 2013 ), when it also became an expression of other motivations and functions.…”

Laughter, play faces and mimicry in animals: evolution and social functions

“…Despite overlapping contexts, play vocalizations and open-mouth faces are at least to some extent likely to differ in function. Play vocalizations seem to be more limited to the context of play than open-mouth faces ([ 84 ]; for functional flexibility, see [ 103 , 104 ]). Although open-mouth faces occur predominantly during play bouts, they have also been observed shortly prior to them in order to invite a conspecific to play [ 105 ].…”