Toward a Functional Interpretation of Synchronous Flashing by Fireflies

In certain firefly species of tropical Southeast Asia the males habitually congregate in trees in huge numbers and flash in rhythmic synchrony all night, each night. Though females do not synchronize, they are attracted to the trees and mate there. Many fireflies remain in the trees by day. We believe that each display tree approximates a steady-state population in which a constant number of males, stabilized by a balance of inflying and eventual death, is interacting with a constant flux of females, stabilized by a balance of in- and out-migration. There are some very tenuous indications that different tree populations may be partially isolated and somewhat inbred. The mass-display fireflies are unique in possessing a neural mechanism that automatically synchronizes the rhythmic flashing of conspecific males. Mass congregation and mass synchrony are therefore inseparable. All evidence agrees in indicating that the mass synchronized assemblies are exclusively reproductive. In the Thai Pteroptyx malaccae and the Melanesian Pteroptyx cribellata and Luciola pupilla we believe that courtship and mating involve two successive photic interactions. Distant males and females are being attracted indiscriminately into the tree by the massed rhythmic luminescence of the in-tree males. The range and species-specificity of this attraction are enhanced by the flash synchronization. Simultaneously males established individually in the tree and flashing rhythmically in small, defended territories compete for in-tree females. In extension of Lloyd's analysis, we postulate mutual recognition via one or more sexual differences in light emission and surmise that the female probably does most of the short-range moving connected with pair formation. We postulate that the female selects her mate on the basis of the intensity of his signal relative to those of other males visible to her simultaneously. As in Lloyd's model the selection must depend on a genetic predilection of females for males that are flashing synchronously. Poststimulus refractoriness is suggested as the mechanism for the predilection. The synchronized tree congregation thus performs a long-range attraction of both males and females to the tree and simultaneously operates as a huge permanent lek. In the various in-tree interactions, flash synchronization is thought to improve mating opportunity for all participants and to exclude possible conspecific cheaters. The orthodox view that large-scale synchrony must be only the incidental or statistical consequence of a small-scale adaptive synchrony is questioned on the following bases: 1. From natural selection theory, flash synchronization per se, being a group behavior, cannot serve as the competitive agent for promoting the reproductive fitness of an individual participating male, no matter how small the synchronizing group. 2. Flash synchronization is, for physiological reasons, obligatory between two or more close-together males. Mass synchrony depends on mass congregation. 3. Because most fireflies court by continuous line-of-sight photic contact, environmental obstacles such as abound in the jungly vegetation of Southeast Asia could be a major obstacle to dialogue-type communication. Conversely, the formation by males of a large, concentrated communal light beacon, visible through vegetation for long distances in all directions, might attract more females per unit time than a given male would meet in solo search through a population dispersed in dense vegetation. 4. Such a congregation, it is argued, would benefit all participating males, penalize none, and be safeguarded against nonflashing or nonsynchronizing cheaters by the female's requirement for flash coincidence during comparison of male flash intensities. Mass congregation could thus, we suggest, be a group adaptation, made possible by unique physiological and ecological circumstances. The observed evolutionary perpetuation of the behavior is made possible by the fact that flashing-the act that, when synchronized, permits nonspecific sexual gain at two levels (increased mate accessibility for both sexes and eligibility for individual competition between males)-can also be modulated to serve as the competitive instrument between conspecific males vying for selection by females. Any synchronizing male in the congregation who mates will therefore transmit the genetic basis for the flash synchronization as well as that controlling competitive use of flash intensity, and the female will gain by having selected a competitive mate.