A syntactic rule in forest monkey communication
Syntactic rules allow a speaker to combine signals with existing meanings to create an infinite number of new meanings. Even though combinatory rules have also been found in some animal communication systems, they have never been clearly linked to concurrent changes in meaning. The present field experiment indicates that wild Diana monkeys, Cercopithecus diana, may comprehend the semantic changes caused by a combinatory rule present in the natural communication of another primate, the Campbell's monkey, C. campbelli. Campbell's males give acoustically distinct alarm calls to leopards, Panthera pardus, and crowned-hawk eagles, Stephanoaetus coronatus, and Diana monkeys respond to these calls with their own corresponding alarm calls. However, in less dangerous situations, Campbell's males emit a pair of low, resounding ‘boom’ calls before their alarm calls. Playbacks of boom-introduced Campbell's alarm calls no longer elicited alarm calls in Diana monkeys, indicating that the booms have affected the semantic specificity of the subsequent alarm calls. When the booms preceded the alarm calls of Diana monkeys, however, they were no longer effective as semantic modifiers, indicating that they are meaningful only in conjunction with Campbell's alarm calls. I discuss the implications of these findings for the evolution of syntactic abilities.