Efficacy constraints on female directional preference stabilize a male call component in a multimodal cricket duet

Mate advertisement signals can vary in their variability, with some signals or signal components being highly stereotyped and others highly variable. One hypothesis for differences in variability suggests that receivers provide stabilizing selection for signal components important for mate recognition, resulting in low variability, while also providing directional selection for signal components indicating mate quality, correlating with greater variability. Factors that affect signal efficacy, however, can also influence signal design – and might interact with receiver preferences to influence variability. We investigated these hypotheses in the duetting cricket species Lebinthus bitaeniatus Stål. Like many other insects, these crickets use call-and-response signalling to find mates. Males produce an acoustic signal, females reply with a vibrational signal, and males use these vibrations to find females. Male calls have two components: a series of isolated syllables (ticks) followed by a rapid series of syllables (a trill). The number of ticks per call is highly variable, whereas the number of pulses in the trill is stereotyped. Male size was correlated positively with the number of trill pulses per call, but not with the number of ticks. Female response amplitude was not related to the number of ticks in the male call (the variable component) and it increased with trill duration (the stereotyped signal component), contrary to the stabilizing directional selection hypothesis. We also found, however, that the latency of the female vibrational reply became erratic as the trill increased beyond the typical duration. Since males in duetting insect species rely on the species-specific latency of the female reply to recognize the signal, this reduces the efficacy of the female reply for communication despite the females’ greater amplitude response. Given the prevalence of call-and-response signalling in insects, this relationship between female response amplitude and signal efficacy could be a common factor influencing signal variability across many species.