THE FUNCTION OF FINE-SCALE SIGNAL TIMING STRATEGIES: SYNCHRONIZED CALLING IN STREAM BREEDING TREE FROGS

In dense mating aggregations, such as insect and anuran choruses, signals produced at the same time can overlap and interfere with one another, reducing the ability of receivers to discriminate between individual signals. Thus, evolution by sexual selection is expected to result in mating signal timing strategies that avoid overlap. Patterns of signal alternation between competing males are commonly observed in leks and choruses across taxa. In some species, however, signalers instead deliberately overlap, or ‘synchronize’, their mating signals with neighboring conspecifics. Given the assumed high cost of reduced mate attraction when signals overlap, mating signal synchronization has remained an evolutionary puzzle. Synchronization may be beneficial, however, if overlapping signals reduce the attraction of nontarget receivers (predator avoidance hypothesis). Synchronized signals could also constructively interfere, increasing female attraction to the mating aggregation (the beacon effect hypothesis). I investigate these functions of synchronized signaling in two species of tree frogs that synchronize their mating calls: the pug-nosed tree frog (Smilisca sila) and the Ryukyu Kajika frog (Buergeria japonica). To examine the trade-offs imposed by call synchronization in each species, I conduct a series of field and laboratory playback experiments on target (female frogs) and nontarget (eavesdropping predators) receivers of frog calls. Results from these experiments support both hypotheses, suggesting that synchronized frog calls can reduce the attraction of predators and attract mates to the chorus. In addition, I found reduced preferences for fine-scale call timings in female S. sila and B. japonica, deviating from the expected preferences observed in many other anuran and non-anuran species. Thus, while males may enjoy multiple benefits from synchronized mating signals, relaxed sexual selection for non-synchronous signals may be key to the evolution and maintenance of mating signal synchrony.