Both song and language require species-specific stimulation at a sensitive period in development, as well as subsequent practice (subsong and plastic song in birds and babbling in infant humans) that leads to the development of characteristic vocalizations for each species. This book illustrates how social interactions during development can shape vocal learning and extend the sensitive period beyond infancy, and how social companions can induce flexibility even into adulthood. This book shows how social companions in a wide range of species including birds and humans as well as cetaceans and nonhuman primates play important roles in the shaping of vocal production as well as the comprehension and appropriate use of vocal communication.