Evolution of directional hearing in moths: conversion of bat detection devices to asymmetric pressure gradient receivers

In most acoustic animals directional hearing evolved alongside basic structure and function of the ears. Moths differ somewhat because their ears generally function as simple bat detectors with little or no directional ability. Those that do use sound for mating communication represent a more special case: These species can localize sound sources, but singing and the ability to localize conspecific song evolved after the origin of hearing. Thus, directional hearing may be constrained by fundamental auditory features that were not initially adapted for the task of localization. We studied this problem in the lesser waxmoth, a species in which males broadcast a long-range advertisement song attractive to females. Our analyses revealed a novel localization mechanism wherein the geometry and structure of the tympanal membrane of each ear afford sharp sensitivity to sound arriving from a distinct angle. Females can localize singing males, but they only do so by following an indirect trajectory that may be interrupted by wide deviations. Such inefficiency may be characteristic of specialized perceptual traits that rely on general ones having already undergone extensive prior evolution.