Chapter One: Artificial Anuran Choruses
Frogs and toads (order Anura) can be found just about anyplace where water col- lects regularly. During their large nighttime assemblies, males call out rhythmically, producing loud, washing choruses of advertisement to the unseen and often un- heard females. Choruses commonly include a number of different species, each of whose sound signals fills spectral and temporal niches in the available acoustic bandwidth (Krause 1987).
The structure of anuran choruses can be observed on several time scales. The signal interactions of neighboring callers produce pulses of synchrony and alterna- tion as the callers mutually influence the timing of signal production. Groups of callers engage in competitive bouts lasting several minutes, stimulating other groups, which can spread activation over a large area. These bouts concentrate in one or more periods during a twenty-four-hour cycle, influenced by light, weather, and the presence or absence of predators. Over evolutionary time, female prefer- ences move species through sonic fashions, and the choruses change as genetics drift. It is a natural theme and variation that has been performed continuously for millions of years.
Studies of the signal interactions of anurans, as well as those of insects, have led researchers to develop a general model of an individual’s calling patterns based on a phase-resettable oscillator. In addition to explaining the empirical evidence of call-timing adjustments, this oscillator model also allows for general predictions of patterns of synchrony and alternation in simulated choruses. The parameters in this model can be tuned to accurately reproduce the choruses of many different species.
Although chorus models can be useful to composers working with agent-based artificial-life (a-life) models in music by providing a biologically plausible mecha- nism for rhythmic coordination, these models can alternatively be used to produce a different kind of a-life–based music altogether. Rather than a wholesale appropria- tion of the algorithms for creative inspiration, an artificial chorus can capture and reproduce a musical event from the natural world as a sort of generative nature recording—a literal imitation of life.