Variation in the structure of a referential signal (or demolishing the Cartesian paradigm: the bowdlerized version)

Some animals produce highly-specific calls in response to predators. Playbacks evoke adaptive responses. systems with these properties are functionally referential. Production and perception have been studied extensively at the level of call type, but very little is known about fine-grained variation in signal structure. We report experiments exploring the relationship between the behaviour of a simulated terrestrial predator and the structure of alarm calls in male fowl. Ground alarm calls are not a homogenous signal class. Rather, there are several distinct call sub-types, which can be objectively classified based upon spectral and temporal characteristics. Birds move abruptly between these sub-types during bouts of calling. They also introduce graded changes in call elements. When we mapped these structural changes onto stimulus properties, we discovered orderly variation in usage. Playback experiments to hens demonstrate that this is sufficient to cause changes in the duration of anti-predator responses. Our results reveal a hierarchically organized system in which gross changes denote predator category, which subtle ones reveal immediacy of threat or affective state.