Toward a Functional Interpretation of Synchronous Flashing by Fireflies

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Abstract
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. Each display tree probably approximates a steady-state population in which a constant number of males, stabilized by a balance of inflying and eventual death, interacts with a constant flux of females, stabilized by a balance of in- and out-migration. Different tree populations may be partially isolated and somewhat inbred. 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 P. cribellata and Luciola pupilla courtship and mating seem to involve 2 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 attracton are enhanced by the flash synchronization. Simultaneously males established individually in the tree and and flashing rhythmically in small, defended territories compete for in-tree females. Probably, in extension of Lloyd''s analysis, mutual recognition is 1 or more sexual differences in light emission and the female does most of the short-range moving connected with pair formation. The female probably 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 selection must depend on a genetic predilection of females for males that flash synchronously. Poststimulus refractoriness is suggested as the mechanism for the predilection. The synchronized tree congregation performs a long-range attraction of both males and females to the tree and simultaneously operates as a huge permanent lek. In various in-tree interactions, flash synchronization may improve mating opportunity for all participants and exclude possible conspecific cheaters. Arguments for questioning the orthodox view that large-scale synchrony must be only the incidental or statistical consequence of a small-scale adaptive synchrony are presented.