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Eavesdropping predators, parasites and parasitoids exploit signals emitted by their prey and hosts for detection, assessment, localization and attack, and in the process impose strong selective pressures on the communication systems of the organisms they exploit. Signallers have evolved numerous anti‐eavesdropper strategies to mitigate the trade‐off between the costs imposed from signal exploitation and the need for conspecific communication. Eavesdropper strategies fall along a continuum from opportunistic to highly specialized, and the tightness of the eavesdropper–signaller relationship results in differential pressures on communication systems. A wide variety of anti‐eavesdropper strategies mitigate the trade‐off between eavesdropper exploitation and conspecific communication. Antagonistic selection from eavesdroppers can result in diverse outcomes including modulation of signalling displays, signal structure, and evolutionary loss or gain of a signal from a population. These strategies often result in reduced signal conspicuousness and in decreased signal ornamentation. Eavesdropping enemies, however, can also promote signal ornamentation. While less common, this alternative outcome offers a unique opportunity to dissect the factors that may lead to different evolutionary pathways. In addition, contrary to traditional assumptions, no sensory modality is completely ‘safe’ as eavesdroppers are ubiquitous and have a broad array of sensory filters that allow opportunity for signal exploitation. Wemore »discuss how anthropogenic change affects interactions between eavesdropping enemies and their victims as it rapidly modifies signalling environments and community composition. Drawing on diverse research from a range of taxa and sensory modalities, we synthesize current knowledge on anti‐eavesdropper strategies, discuss challenges in this field and highlight fruitful new directions for future research. Ultimately, this review offers a conceptual framework to understand the diverse strategies used by signallers to communicate under the pressure imposed by their eavesdropping enemies.

Haematophagous insects can rely on specialized host‐seeking behaviors to locate hosts. Some frog‐biting flies, for example, eavesdrop on the conspicuous acoustic signals produced by male frogs and toads. Using such auditory cues to locate a host imposes an additional challenge: how to recognize appropriate sounds when different frog species produce calls with varying acoustic properties. The limited knowledge of antennal hearing in dipteran insects hinders our ability to understand how eavesdropping flies detect and recognize frog calls. Behavioral studies suggest that frog‐biting flies use broad acoustic templates to detect and recognize their victims. Here, we use within‐species call variation to examine the acoustic preferences in frog‐biting flies. Specifically, we examine the attraction of frog‐biting mosquitoes (Uranotaeniaspp.) and midges (Corethrella nippon) to the calls of a Japanese treefrog, the Ryukyu Kajika frog (Buergeria japonica), on Iriomote Island, Japan. Male Ryukyu Kajika frogs produce two call types. While both calls have a high frequency peak (3 kHz), the first call type (Type I) also contains a lower frequency peak (1.8 kHz) absent in the second call type (Type II). Using field phonotaxis experiments we found that Type I calls are more attractive to both frog‐biting mosquitoes and midges. Thus, our results suggest thatmore »the frog‐biting Nematoceran flies in this community are biased towards the acoustic properties of Type I calls. We discuss this finding in the context of the evolution of antennal hearing in flies.

In the evolutionary arms race between predators and their prey, prey often evolve to be as cryptic as they can, while predators in turn hone their sensory strategies to detect prey. Examinations of the sensory strategies implemented by predators to detect their prey, as well as the ecological consequences of these interactions, are at the crux of understanding and predicting predator–prey dynamics.

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