Putting two calls together

A very brief paper by Arnold and Zuberbüler in Nature presents a case of a complex call in guenons:

Putty-nosed monkeys rely on two basic calling sounds to construct a message of utmost urgency.
Syntax sets human language apart from other natural communication systems, although its evolutionary origins are obscure. Here we show that free-ranging putty-nosed monkeys combine two vocalizations into different call sequences that are linked to specific external events, such as the presence of a predator and the imminent movement of the group. Our findings indicate that non-human primates can combine calls into higher-order sequences that have a particular meaning.

The two calls (called "pyow" and "hack") generally are used to signify two different predators, leopards and crowned eagles, respectively. The different calls help the monkeys use different escape routes -- they "sometimes" move through the canopy to escape from leopards, but avoid such a route in response to eagles; the paper indicates that the group does not move in response to the "hack" warning call alone.

Sometimes, males vocalize a sequence involving several of both calls (a "pyow-hack", or P-H sequence). The research found that these sequences are indicators of group movement, in response to either kind of predator:

Using a global positioning satellite unit (see methods in supplementary information), we found that the groups whose males had produced P-H sequences in response to growls had travelled significantly farther than other groups [significance test]....

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The monkeys' response to P-H sequences was not confined to the predator context, but was generally related to whether the group moved. We recorded a total of 72 natural call sequences (that is, not experimentally induced) over two months from the single male of a group habituated to human observers and monitored the group's travelling patterns. A large proportion of the calls contained P-H sequences (40.3%) and elicited movement of the group over significantly greater distances than after P-H-free call series [significance test]....
Most animals have a restricted repertoire of calls, with innate and structurally fixed vocalizations. Combining existing calls into meaningful sequences increases the variety of messages that can be generated. The simple system used by putty-nosed monkeys encodes the presence of different types of danger and triggers group movement with just two basic call types.

It seems especially likely that an animal might exploit already-existing calls in such a way. These are sounds that are both statistically improbable and already recognized, so if there is a way to alter their interpretation by context, they provide ready channels for communication. In this case, the alternative "meaning" is about movement, it is piggybacked upon the simpler calls themselves.

You might call it monkey multiplexing.

References:

Arnold K, Zuberbüler K. 2006. Language evolution: Semantic combinations in primate calls. Nature 441:303. DOI link