Volition and learning in primate vocal behaviour

Investigating nonhuman primate vocal communication is often with the intention of elucidating their similarities with human speech and thus reconstructing the evolutionary history of this important behaviour. However, putative parallels between primate and human vocal behaviours have, in some respects, remained elusive. Here, we review two lines of research in marmoset monkeys on volitional vocal control and vocal learning during development that could bridge our understanding of the relationship between primate vocalizations and human speech. Regarding volitional control, we review how changes in vocal output are not solely due to changes in arousal levels and their effect on the vocal apparatus; extrinsic factors like the vocalizations from other conspecifics also have an important influence. With regard to vocal learning, we describe not only how infant marmoset vocalizations undergo dramatic acoustic changes during development that are not wholly explained by physical growth, but also how, as in humans, contingent vocal responses from parents influence the rate of vocal development. We argue that the similarities in the vocal systems of marmoset monkeys and humans may be due to their shared cooperative breeding strategy and prosociality.