There has been little investigation of speech analogues in the vocal communication system of non-human primates. Hence there has been little development toward a primate model for the study of speech and language disorders. In our previous studies with Callitrichid monkeys (marmosets and tamarins) we have found several production and perceptual phenomena analogous to phenomena of human speech. We will continue these studies along 5 general lines: (1) Complexity of structure: We have found a rudimentary grammar in the sequencing of syllables of monkeys. We will test this grammar by isolating individual syllables and playing back both isolated syllables and syllables combined into grammatically correct or incorrect phrases to the animals, observing their responses. Several calls of similar structure have different functions. These will also be played back to test the correctness of our hypothesized functions. (2) Locatability: Monkeys vary the structure of their calls to include more cues for spatial localization the farther away they are from conspecifics. We will test the animals' acuity for natural calls of different structure to see if calls given at greater distances are most locatable. (3) Individual recognition: Animals have unique distinctive features within their calls that allow them to be recognized as individuals. We will test for individual recognition on vocal cues alone and will synthesize calls where individual distinctive features can by systematically varied to determine which features are perceptually salient for individual recognition. (4) Populational differences: We will record various subspecies of marmosets in the field to see if different subspecies have distinctly different calls. We will try playback experiments where calls of the same and of different subspecies are played as well as calls of a sympatric or non-sympatric species of tamarin are played. (5) Ontogeny: Marmosets babble like human infants producing imperfect versions of adult calls. We will attempt to see if any vocal imitation is evident and whether vocal development correlates with other social and cognitive aspects of development.
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