Self-organizing cicada choruses respond to the local sound and light environment.

Lawrence W Sheppard,Jonathan A Walter,Brandon Mechtley,Daniel C Reuman

doi:10.1002/ece3.6213

Abstract

Periodical cicadas exhibit an extraordinary capacity for self‐organizing spatially synchronous breeding behavior. The regular emergence of periodical cicada broods across the United States is a phenomenon of longstanding public and scientific interest, as the cicadas of each brood emerge in huge numbers and briefly dominate their ecosystem. During the emergence, the 17‐year periodical cicada species Magicicada cassini is found to form synchronized choruses, and we investigated their chorusing behavior from the standpoint of spatial synchrony.Cicada choruses were observed to form in trees, calling regularly every five seconds. In order to determine the limits of this self‐organizing behavior, we set out to quantify the spatial synchronization between cicada call choruses in different trees, and how and why this varies in space and time.We performed 20 simultaneous recordings in Clinton State Park, Kansas, in June 2015 (Brood IV), with a team of citizen‐science volunteers using consumer equipment (smartphones). We use a wavelet approach to show in detail how spatially synchronous, self‐organized chorusing varies across the forest.We show how conditions that increase the strength of audio interactions between cicadas also increase the spatial synchrony of their chorusing. Higher forest canopy light levels increase cicada activity, corresponding to faster and higher‐amplitude chorus cycling and to greater synchrony of cycles across space. We implemented a relaxation‐oscillator‐ensemble model of interacting cicadas, finding that a tendency to call more often, driven by light levels, results in all these effects.Results demonstrate how the capacity to self‐organize in ecology depends sensitively on environmental conditions. Spatially correlated modulation of cycling rate by an external driver can also promote self‐organization of phase synchrony.

Highlights

In ecology, spatial synchrony is often observed in fluctuations of populations produced by the action of environmental changes, the activity of predators, or other synchronous drivers, without the necessity of direct interactions between the populations in space
We show how conditions that increase the strength of audio interactions between cicadas increase the spatial synchrony of their chorusing
We found a significant correlation between light level (Figure 3c) and cicada cycling frequency (Figure 3a; p = .0055) and amplitude (Figure 3b; p = .0457), using a Fourier randomization-based test that properly accounts for temporal autocorrelation in these quantities (Methods: Statistics)

Summary

| INTRODUCTION

Spatial synchrony is often observed in fluctuations of populations produced by the action of environmental changes, the activity of predators, or other synchronous drivers, without the necessity of direct interactions between the populations in space. The most notable feature of this variability in local cicada sound volume is the periodic five-second cycling of the chorus, and the relative timing of the peaks and troughs in these cycles can be compared accurately between sites to definitively demonstrate intersite type 2 spatial synchrony in volume, irrespective of the incomparable absolute volume levels recorded by the devices. Female periodical cicadas are observed to use light cues to select oviposition sites (Yang, 2006) This motivates an investigation of the effects of sunlight on spatial synchrony of calling: the degree of type 1 synchronization of calling activity may determine the degree of type 2 synchronization of call-volume oscillations. We view our efforts to characterize the spatiotemporal acoustic signal associated with M. cassini and to formulate initial models which can reproduce important aspects of this signal as first steps of an approach to understanding the reasons for chorusing in this species that could be practical in light of the particular ecological characteristics of the species

| MATERIALS AND METHODS

Findings

| DISCUSSION