Researchers have made what they say is the first experimental demonstration that a primate other than humans conveys meaning by combining distinct alarm calls in particular ways. The study appears in the March 11th issue of Current Biology, a publication of Cell Press.
In linguistics, morphemes are usually defined as the smallest meaningful units in the grammar of a language, said Klaus Zuberbühler of the University of St. Andrews—for instance, a word such as cat or a prefix such as un-. Our research has revealed some interesting parallels in the vocal behavior of forest monkeys and this crucial feature of human language.
Male putty-nosed monkeys (Cercopithecus nictitans) produce different alarm-call series in response to a number of disturbances, including leopards and crowned eagles, earlier studies had shown. Call series consisting of pyows are a common response to leopards, while series of hacks and hacks followed by pyows are given to eagles, the researchers said. In addition, males assemble pyows and hacks into unique pyow-hack sequences.
Zuberbühler and his colleague Kate Arnold, also of the University of St. Andrews, now provide evidence that the various hacks and pyows of male putty-nosed monkey contain at least three types of information: 1) the event witnessed, 2) the callers identity, and 3) whether he intends to travel, all of which are recognized by other monkeys.
The new findings challenge the notion, commonly held by theorists, that the transition from non-combinatorial to combinatorial communication was an essential step in the evolution of human language, they said.
Scientists had even thought that such language may have emerged relatively late in human evolution, Zuberbühler said, based on the notion that signals would be combined only once the number of them had grown sufficiently. At some point, according to the theory, it becomes more economical to combine existing elements, rather than add new ones to a large repertoire.
Our research shows that these assumptions may not be correct, Zuberbühler said. Putty-nosed monkeys have very small vocal repertoires, but nevertheless we observe meaningful combinatorial signaling. In fact, he added, most primates are limited in the number of signals they can physically produce because of their lack of tongue control.
The only way to escape this constraint may be to combine the few calls [they have] into more complex sequences, he said. In other words, it may be harder for non-human primates to evolve large repertoires than to evolve the ability to combine signals. Hence, the evolution of combinatorial signaling may not be driven by too many signals but rather by too few.