Naturally selected and sexually selected wing structures synergistically enhance the attractiveness of katydid acoustic signals

Naturally selected adaptations are often thought to impair the elaboration of conspicuous sexual signals. In katydids, males produce acoustic signals by rubbing together structures on their forewings; specialized forewing cells then radiate this sound. These dedicated sound-producing structures are found on a physically distinct basal area of the forewing, but in many species, the remainder of the wing has evolved enormous, exaggerated blade-like or leaf-like structures under natural selection. We tested whether removal of such adjacent, naturally selected modifications enhances the attractiveness of male sexual signals by releasing bioacoustic constraints caused by their physical connection on the wing surface. Acoustic and resonance measurements of the leaf-mimic Viadana brunneri before and after removing the non-sound-producing, leaf-mimicking, portion of the forewing demonstrate that these regions decrease the frequency of male song and increase its amplitude (i.e. loudness). Using a playback experiment with calls from the intact versus wing-clipped males, we found that females prefer the lower frequency calls of males with the non-stridulatory portion of their forewings intact. Thus, the function and attractiveness of intraspecific signals can be enhanced by naturally selected adaptations, contradicting the common assumption that adaptations for mate attraction and predator avoidance represent functional compromises between sexual and natural selection.