Early vocal production and functional flexibility in wild infant chimpanzees
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. Here, an important finding has been that, prior to speech, human infant vocal behavior exhibits functional flexibility, the capacity of producing protophones that are not tied to one specific function. Nonhuman primate vocal behavior, by contrast, is comparably inflexible, with different call types tied to specific functions. Our research challenges the generality of this claim, with new findings of flexible vocal behavior in infant chimpanzees. We used artificial intelligence consisting of automated feature extraction and supervised learning algorithms to analyze grunt and whimper vocalizations from free-ranging infants during their first year of life. We found that grunt production was highly flexible occurring in positive, neutral and negative circumstances, as already shown in human infants. We also found acoustic variants of grunts produced in different affective contexts, suggesting gradation within this vocal category. By 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. We concluded that the most common chimpanzee vocalization, the grunt, qualifies as functionally flexible, suggesting that evolution of vocal functional flexibility occurred before the split between the Homo and Pan lineages.