Temporal Selection for Communicatory Optimization: The Dawn-Dusk Chorus as an Adaptation in Tropical Cicadas

Henwood and Fabrick (1979) discuss the optimal adaptive properties of the dawn chorus phenomenon, well documented for various vertebrate species in a broad range of environments, including the tropical rain forest. They argue quantitatively that the relative magnitude of broadcast coverage and other sound transmission properties of chorusing vertebrate animals is greatest during the dawn period. Sound attenuation and broadcast area are maximized during the dawn period according to their model, which takes into account complex atmospheric conditions characteristic of many terrestrial habitats throughout he day. Particularly for tropical rain forests, they argue effectively for the occurrence of increased background noise at midday, and emphasize the marked absence of such interference for callers at dawn. In their figure 2, they illustrate the cyclic trajectories of relative air humidity and temperature throughout he day, and also show relative changes in broadcast area (their fig. 4) throughout he day. These graphs and the general arguments developed contain information regarding dusk properties not discussed directly by Henwood and Fabrick. Strikingly, these data indicate that the dusk period is also a time of relatively low background noise (although not quite as pronounced as for the dawn period), and given the adaptive value of convergence for dawn chorusing in vertebrates, it may be anticipated that some organisms in terrestrial environments would exhibit dusk chorusing as well. The purpose of this note is to call attention to the very pronounced daily dawn-dusk chorusing in one group of insects in the American tropics, the cicadas (Homoptera: Cicadidae), and to extend the results of Henwood and Fabrick (1979) to these organisms. The males in many species of cicadas produce distinctive free songs (Pringle 1954). The free song of a cicada is the song produced by the undisturbed insects in the habitat. Presumably selection for such behavior is high since the songs, generally species-specific (Moore 1966), are a major component of reproductive behavior in these insects, and thus intimately related to fitness. Different genera of cicadas employ different sound-producing mechanisms (D. Young 1972a, 1972b) that primarily involve the buckling of ribbed cuticular membranes called tymbals by tymbal muscles (Pringle 1954), resulting in the production of pulses of sound. Pulse structure and other properties (e.g., pitch, intensity) of free song patterns in cicadas vary considerably among different genera and species. The following general behavioral properties related to cicada free songs are evident for many neotropical species, based on my observations on cicadas in Central America over the past 10 yr. (1) In three distinct tropical environments, the lowland-to-premontane tropical wet forest, montane rain forest, and lowland tropical dry forest, the majority of cicada species (Young 1976) exhibit intense bursts of dawn-dusk chorusing from trunks of large trees and from the forest