Spatial synchrony is a widely observed phenomenon in ecological fluctuations, but self-organizing phase synchronization of self-sustaining oscillations is a case of special interest. The 17 year periodical cicada Magicicada cassini exhibits true phase synchronization in both its emergence cycles (period 17 years) and in the regularly oscillating sound volume of `choruses' of male cicadas.
We organized local volunteers to record the calling activity of the Brood IV cicada outbreak throughout an area of woodland habitat in Kansas USA, using a simple protocol and consumer recording equipment. Cicada song volume fluctuations were strongly periodic and synchronized. We use 20 spatially distributed recordings of sound volume and a wavelet transform approach to determine how the spatial synchrony falls off with distance. A simultaneous video recording of the forest canopy was used to monitor the light level, which varied with cloud cover.
Amplitude and frequency correlations were determined from the wavelet transforms associated with each recording site. Synchrony falls off with distance and so we determined both the best-fit parameter associated with synchrony decay in space, and a forest-wide synchrony index based on Shannon entropy of cycle phase values, both of which varied in time.
We implemented a relaxation-oscillator ensemble computer model of interacting cicadas, and investigated the effects of sound attenuation and delay on self-synchronization.
Results/Conclusions
We demonstrate how the calling rate of the local cicada chorus is responsive to changes in ambient light level, and how this has a knock-on effect on the degree of synchrony that they can maintain. The amplitude of the cycles in volume, their rate, and the entropy of the distribution of phases obtained from all sites in the forest were found to vary, correlated with changes in the observed illumination.
The model suggests that a tendency to call more often driven by an environmental stimulus (higher light level) results in increased cycle amplitude and greater self-organized spatial synchrony. The ability of the cicadas to spatially synchronize their calling is ultimately limited by the travel time of sound propagation between chorus sites, but the measured cicada activity was intermittent and did not approach this limit.