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

Conspicuous mating signals attract mates but also expose signalers to predators and parasites. Signal evolution, therefore, is driven by conflicting selective pressures from multiple receivers, both target and nontarget. Synchronization of mating signals, for example, is an evolutionary puzzle given the assumed high cost of reduced female attraction when signals overlap. Synchronization may be beneficial, however, if overlapping signals reduce attraction of nontarget receivers. We investigate how signal synchronization is shaped by the tradeoff 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 túngara frogs (Engystomops pustulosus), in which males alternate their calls. To examine the tradeoff imposed by signal synchronization, we conducted field and laboratory playback experiments on eavesdropping enemies (bats and midges) and target receivers (female frogs). Our results suggest that, while synchronization can be a general strategy for signalers to reduce their exposure to eavesdroppers, 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 the origin of mating signals and displays.