Developmental changes of cognitive vocal control in monkeys.

The evolutionary origins of human language are obscured by the scarcity of essential linguistic characteristics in non-human primate communication systems. Volitional control of vocal utterances is one such indispensable feature of language. We investigated the ability of two monkeys to volitionally utter species-specific calls over many years. Both monkeys reliably vocalized on command during juvenile periods, but discontinued this controlled vocal behavior in adulthood. This emerging disability was confined to volitional vocal production, as the monkeys continued to vocalize spontaneously. In addition, they continued to use hand movements as instructed responses during adulthood. This greater vocal flexibility of monkeys early in ontogeny supports the neoteny hypothesis in human evolution. This suggests that linguistic capabilities were enabled via an expansion of the juvenile period during the development of humans.