![]() ![]() ![]() ![]() A general prediction from signal detection theory is that modalities characterized by shorter ranges, faster transmission and/or reduced longevity-as typifies visual and vibratory signals-should comparatively reduce the risk to signallers, while the increased range and longevity of olfactory and auditory signals may lead to a heightened risk of predation, on balance ( table 1). A signal's modality, in particular, may drive differential predation, with the expectation of modality-specific costs guided by knowledge of the structure of signals and signalling environments. While the risk to sexual signallers is well articulated in general terms, questions remain as to whether and how such costs vary predictably across contexts. Birds localize lizard prey using their colourful ornaments, flies attend the advertisement calls of frogs to extract a blood meal, and wasps use the sex pheromones of aphids as kairomones to identify hosts. The interception of signals by unintended receivers is best known as ‘eavesdropping’ and is the route through which predation costs are imposed upon sexual signallers. In ecological terms, predation-induced shifts in signalling behaviour and mate choice are now well documented, while at evolutionary scales the need to balance between signalling to conspecifics and avoiding predators can culminate in genetic polymorphisms and population divergence. This simple observation has since been borne out in a wealth of empirical tests, while more recent efforts have built on these foundations to highlight predation as a selective force in sexual systems more generally. Early work noted that the conspicuousness of sexual signals should also attract unwanted attention from predators, thereby establishing a fundamental trade-off. These benefits are met by countervailing costs, however, with predation and parasitism standing among the most exhaustively studied. ![]() Their ubiquity speaks to their importance in sexual reproduction where they advertise, among other things, the location, identity, availability or quality of prospective mates. Sexual signals rank among the most elaborate and conspicuous innovations showcased by animals. Our results reveal unexpected complexity in a central viability cost to sexual signalling, while also speaking to applied problems in invasion biology and pest management where signal exploitation holds promise for bio-inspired solutions. Variation in outcome measures was universally high, suggesting that contexts in which sexual signalling may incur no cost, or even reduce the incidence of predation, are common. Olfactory and acoustic signals increased the incidence of eavesdropping relative to visual signals, which experienced no greater risk than controls on average. We found a similarly strong overall increase in predation on sexual signallers in the wild, though here it was modality specific. We found that predators and parasitoids express strong and consistent preferences for signals in forced-choice contexts. Here we synthesized 187 effects from 78 experimental studies in a meta-analytic test of two questions namely, whether predators, parasites and parasitoids express preferences for the sexual signals of prey, and whether sexual signals increase realized predation risk in the wild. Tests of the predicted predation costs have produced mixed results, however. Sexual signals are often central to reproduction, and their expression is thought to strike a balance between advertising to mates and avoiding detection by predatory eavesdroppers. ![]()
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