Summary. Human speech is based on rule-governed assemblage of morphemes into more complex vocal expressions. Free-ranging putty-nosed monkeys (Cercopithecus nictitans) provide an interesting analogy, because males combine two loud alarm calls, 'hacks' and 'pyows', into different call series depending on external events [1]. Series consisting of 'pyows' are a common response to leopards, while 'hacks' or 'hacks' followed by 'pyows' are regularly given to crowned eagles [2,3]. Sometimes, males produce a further sequence, consisting of 1-4 'pyows' followed by 1-4 'hacks'. These 'pyow-hack' (P-H) sequences can occur alone, or they are inserted at or near the beginning of another call series. Regardless of context, P-H sequences reliably predict forthcoming group progression [4]. In playback experiments, we tested the monkeys' reactions to 'pyows', 'hacks' and P-H sequences and found that responses matched the natural conditions. Specifically, females started group progressions after hearing P-H sequences and responded appropriately to the other call series. In a second experiment, we tested artificially composed P-H sequences, and found that they were also effective in eliciting group progressions. In a third experiment, we established that group movement could only be triggered by the calls of the group's own male, not those of a stranger. We conclude that, in this primate, meaning is encoded by call sequences, not individual calls. Many birds and primates are limited by small vocal repertoires [5,6], and this constraint may have favored the evolution of such combinatorial signaling.
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