The identification of units used in primate vocal behaviour and the isolation of components essential for intraspecific communication (i.e. having a function) from possibly inessential ones (i.e. having no function) is an absolute prerequisite to a successful analysis of primate vocalizations. For most calls special methods are required because neither the essentials of structure nor their relation to function are known, nor the context involved. For calls without any distinct frequency modulation we showed that the animals utter calls containing categorical amplitude changes. The changes within one call are either very small or very large. The positions or the very large amplitude changes within a call depend on the social situation the animals were subjected to and affect the immediately succeeding vocalization of the conspecific. This proves that in vocal communication discrete components are produced (structure) as well as used (function).