Host Call Frequency Drives Attractiveness to an Eavesdropping Micropredator

Acoustic sexual signals are vulnerable to interception by eavesdropping predators, micropredators and parasitoids. Signal traits can impact attractiveness to eavesdroppers and potential mates alike. Thus, signals are influenced by interacting selection pressures and drive predator/prey interactions and their consequences (e.g., disease dynamics where eavesdroppers are vectors). We assessed the spectral preferences of eavesdropping micropredators in a near cosmopolitan system-frog-biting midges (Corethrella spp.) and frogs-for the first time in Oceania. Frequency treatments that spanned the spectral profile of the study area's frog fauna (sinusoidal pure tones at 0.1, 0.3, 0.5, 0.8, 1.4, 2.3 and 3.2 kHz) were offered in situ in no-choice experiments. Frequencies of 0.5-1.4 kHz attracted significantly more midges than silent controls with a peak at 0.8 kHz (74 mean flies vs. 0.36 to control). The lower spectral limit of attractiveness to Corethrella closely matched the lowest-calling local frog species, and was higher than in other regions where even lower-calling frogs occur, suggesting potential evolutionary adjustment of Corethrella's preferred spectral profiles to what is available. The smallest-bodied frogs called above the frequency range that attracted Corethrella, and we posit that this may avoid negative selection pressures that are conceivably stronger for smaller hosts, rather than being purely a result of small bodies' biomechanical constraints. Our results are consistent with broader observations that eavesdroppers and intended receivers commonly share preferences, and competing selection pressures are likely incurred by intended and unintended signal receivers, with implications for signal evolution, disease dynamics and potentially biodiversity conservation.