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

Abstract

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 overlap. Synchronization may be beneficial, however, if overlapping signals reduce attraction of non-target receivers. 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 túngara frogs (Engystomops pustulosus), in which males alternate their calls. 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). 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. This study highlights the role of relaxed selection in our understanding of the origin of mating signals and displays.

Keywords: acoustic communication; communication network; eavesdroppers; relaxed selection; synchrony.

Figure 1.

Oscillogram (top) and spectrogram (bottom) of the mating calls of the pug-nosed tree frog (a) and túngara frog (b). The oscillogram shows two calls synchronized with a latency of 79 ms, the average natural synchrony of pug-nosed tree frogs. The spectrogram shows only the first call. Although both pug-nosed tree frogs and túngara frogs can produce a more complex, multi-note call, only playbacks of simple calls (as shown) were used in this study.

Figure 2.

Eavesdropper preferences for synchronized and unsynchronized calls, measured for both pug-nosed tree frog (left) and túngara frog (right) calls during each species' respective breeding season. Number of bat attacks per night (a,b) and number of midges captured per night (c,d) for calls were compared for different call timings: near-perfect synchrony (5 ms of latency), average pug-nosed tree frog call synchrony (79 ms of latency) and alternating calls (alt). Values are the least squared means and bars show standard error. Note that for midge attraction, the y-axis range for pug-nosed tree frogs (c) is different than túngara frogs (d).

Figure 3.

Female preference for synchronized and unsynchronized calls, measured for both pug-nosed tree frog (a) and túngara frog (b) calls during each species' respective breeding season. Female choice was compared between calls broadcast in synchrony with 79 ms of latency between calls (synch), or broadcast in alternation (alt). Each choice represents a single female.