Most communication occurs in a network, where signals are detected and used by multiple receivers, both target and non-target. This study highlights the role of relaxed selection in our understanding of the origin of mating signals and displays. Our results suggest that, while synchronization can be a general strategy for signallers to reduce their exposure to eavesdroppers, relaxed selection by females for unsynchronized calls is key to the evolution and maintenance of signal synchrony. To examine the trade-off imposed by signal synchronization, we conducted field and laboratory playback experiments on eavesdropping enemies (bats and midges) and target receivers (female frogs). We investigate how signal synchronization is shaped by the trade-off between natural and sexual selection in two anuran species: pug-nosed tree frogs ( Smilisca sila), in which males produce mating calls in near-perfect synchrony, and tĂșngara frogs ( Engystomops pustulosus), in which males alternate their calls. Synchronization may be beneficial, however, if overlapping signals reduce attraction of non-target receivers. Synchronization of mating signals, for example, is an evolutionary puzzle, given the assumed high cost of reduced female attraction when signals overlap. Signal evolution, therefore, is driven by conflicting selective pressures from multiple receivers, both target and non-target. Conspicuous mating signals attract mates but also expose signallers to predators and parasites.
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