Monkeys Might Have Syntax

What can monkey communication tell us about human nature?

Beyond the displays of fairness and empathy, described in the last article, what makes for a successful society?  Communication is needed to make social organization possible, especially more complex societies. Even though it’s not necessarily a good thing, the more complex a civilization becomes, the more complex the communication is likely to be. The more advanced communication becomes, the more advanced is the level of syntax.

It’s interesting to note the probability that humans are not the only primate with syntax. Syntax describes the rules governing the order in which signals are given. A form of phonological syntax refers to ways that signals are built onto one another to create bigger signals (as in “sing” vs. “singers”). Phonological syntax is commonly observed in studies of animal signal systems.

Lexical syntax is a more interesting form of syntax because it’s so characteristic of our own languages and evidence for it in nonhuman animals is rare. Lexical syntax refers to the rules of using signals in conjunction with one another to convey a new message, independent of the meanings of the component signals. For example, “The man is tall.” can be turned into “Is the man tall?”. A more complex example might be “In the corner. The man is tall.” can be turned into “Is the tall man in the corner?”. The structure and position of the words gives them different purposes and conveys a different meaning.

Diana monkeys and Campbell’s monkeys are two species that often live along side one another and are attacked by the same predators. Each species has a specific eagle-alarm call and a leopard call. Acoustically the calls of the two species are very different but both are able to recognize and respond appropriately to the other. Klaus Zuberbühler studied the vocal repertoires of these two species and feels it’s likely that they both exhibit a simple lexical syntax.

Under some circumstances Campbell’s monkey alarm calls are preceded by a “boom-boom” call. Contextually these calls are heard when something startles the monkey indicating a possible predator. Diana monkeys will react to a Campbell’s-alarm as an alarm, but if they hear the “boom-boom” part of the alarm they ignore it. So what is happening here? Well it could be that the “boom-boom” call acts as an inhibitor, preventing the expression of the typical behavioral response to an alarm call stimulus.

When Zuberbühler played a tape of the Campbell’s “boom-boom”-alarm followed by their own alarm call, the Diana monkeys responded very differently. This time they ignored the “boom-boom”, took notice of the alarm call, and responded by fleeing. Based on these results it seems unlikely that the “boom-boom” is simply an inhibitor.

It seems that, to a Diana monkey, the “boom-boom” call is only relevant if it precedes a Campbell’s alarm and that in this context it acts as a modifier. It changes the message conveyed by the signal from say “Leopard!” to something else like, “Possible leopard – but no immediate danger”. At the moment however we can probably only speculate on the complexity of the communicatory abilities of our primate cousins. But as our knowledge base increases I am sure that they will continue to amaze us. Perhaps their most amazing feat of all, will be shedding light on the development of human nature.