Exploitation of sexual signals by predators and parasitoids

Signals used to attract mates are often conspicuous to predators and parasites, and their evolution via sexual selection is expected to be opposed by viability selection. Many secondary sexual traits may represent a compromise between attractiveness and avoidance of detection. Although such signal exploitation appears to be widespread, most examples come from species that use acoustic or olfactory mating signals, and relatively few cases of visual signal exploitation can be substantiated. Because males are usually the signaling sex, they are more at risk from predators or parasitoids that locate prey or hosts by sexual signals; this differential selection on the two sexes can affect the intensity of sexual selection on male ornamental traits. The notable exception to male signaling and female attraction occurs in pheromone-producing insects, particularly lepidopterans, which show an opposite pattern of female odor production. Exploitation of such sex pheromones is relatively rare. We discuss reasons for the reversal in sex roles in these species and its implications for signal exploitation. Changes in signals that appear to be adaptations to avoid predation include the use of different signal modalities, changes in signaling behavior, loss of signals, and alteration of signal characteristics such as pitch. Selection pressure from signal exploiters could lead to the production of a novel signal and thus facilitate speciation. Relatively little work has been done on adaptations on the part of the exploiting species, but such adaptations could indirectly influence the mating system of the predator or parasitoid. Signal exploitation is also expected to be a fruitful source of examples of coevolution. Finally, plants emit attractants analogous to secondary sex characters in animals, and may also be vulnerable to signal exploitation.