Language Evolution and Computation Bibliography

Syntax is widely considered the feature that most decisively sets human language apart from other natural communication systems. Animal vocalisations are generally considered to be holistic with few examples of utterances meaning something other than the sum of their parts. Previously, we have shown that male putty-nosed monkeys produce call series consisting of two call types in response to different events. They can also be combined into short sequences that convey a different message from those conveyed by either call type alone. Here, we investigate whether ‘pyow–hack’ sequences are compositional in that the individual calls contribute to their overall meaning. However, the monkeys behaved as if they perceived the sequence as an idiomatic expression rather than decoding the sequence. Nonetheless, while this communication system lacks the generative power of syntax it enables callers to increase the number of messages that can be conveyed by a small and innate call repertoire.

Primates communicate not only because they are biologically hardwired to do so, but also because they pursue specific goals during social interactions. This is well documented in the context of ape gestural signals, which have revealed a considerable degree of flexibility in interesting ways. In terms of vocal behavior, however, both monkeys and apes appear to be much less flexible, which raises important questions about how and why vocal flexibility evolved in the human lineage. The emerging picture is that, across the primate order, flexibility is widespread in call comprehension but largely restricted to humans in call production. Non-human primates, including the great apes, are curiously constrained by weak motor control over their vocal apparatus, resulting in limited vocal repertoires. A key transition in the evolutionary origins of language may have been when early humans began to interact with each other collaboratively. Another prediction is that, the species in which infants are exposed to competition over non-maternal caregivers should be more likely to exhibit elaborate vocal behavior than species in which their mothers raise infants only. Research on the communicative skills of other cooperative breeders, particularly communal breeders, may provide interesting empirical data to test this hypothesis.

Primate vocal behavior is often considered irrelevant in modeling human language evolution, mainly because of the caller's limited vocal control and apparent lack of intentional signaling. Here, we present the results of a long-term study on Campbell's monkeys, which has revealed an unrivaled degree of vocal complexity. Adult males produced six different loud call types, which they combined into various sequences in highly context-specific ways. We found stereotyped sequences that were strongly associated with cohesion and travel, falling trees, neighboring groups, nonpredatory animals, unspecific predatory threat, and specific predator classes. Within the responses to predators, we found that crowned eagles triggered four and leopards three different sequences, depending on how the caller learned about their presence. Callers followed a number of principles when concatenating sequences, such as nonrandom transition probabilities of call types, addition of specific calls into an existing sequence to form a different one, or recombination of two sequences to form a third one. We conclude that these primates have overcome some of the constraints of limited vocal control by combinatorial organization. As the different sequences were so tightly linked to specific external events, the Campbell's monkey call system may be the most complex example of 'proto-syntax' in animal communication known to date.

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.