Synchronized mating signals in a communication network: the challenge of avoiding predators while attracting mates

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