Interruptibility of long call production in tamarins: implications for vocal control.

Vocal production can be highly deterministic, such that once the central nervous system generates a signal to call, the vocalization is emitted immune to external events. Conversely, vocal production can be modulated by auditory feedback such that interference or disruption can cause an individual to stop calling or, if it continues to call, for the acoustic morphology of the signal to change. To explore which of these models best accounts for the control of vocal production in non-human primates, we adapted an interruption technique originally developed for songbirds for use with a New World monkey species, the cotton-top tamarin (Saguinus oedipus). Results from a pilot experiment indicated that an auditory stimulus (white noise) was more effective than a visual stimulus (strobe light) at interrupting the tamarin's species-typical 'combination long call (CLC)'. Data from a second experiment showed that although the duration of the auditory stimulus did not affect the proportion of interruptions that occurred, a 1000 ms white noise stimulus perturbed the temporal structure of the CLC to a greater extent than did a 250 ms white noise stimulus. Furthermore, when call production was interrupted, tamarins stopped vocalizing after the completion of a syllable, suggesting that the syllable represents a unit of organization within the call. Overall, these results provide evidence that tamarins can modify their vocal output based on external events, but the degree of vocal control is significantly less than in oscine songbirds.