Why do diurnal moths have ears?

Ears exist in moths primarily for the purpose of detecting hunting bats at night to avoid predation. The ears of four species of day-flying Nearctic moths are as sensitive as those of a common, night-flying genus to the frequencies emitted by sympatric bats and show no evidence of being vestigial. We determined that all of the day-flying moths spend 44–73% of their 24-hour cycles active at night when bats hunt. Two of the moths tested are sound-emitting species and may use their ears during intraspecific communication. We conclude that the functions of bat detection and social communication are the only selective forces acting on moth ears, and that in their absence these sensory structures degenerate. Most moths have simple ears on various parts of their bodies that they use to detect the echolocation calls of aerially hunting, insectivorous bats [1]. Where bats are numerous and diverse (e.g., the tropics), the ears of sympatric moths are more sensitive to a broader range of frequencies than those that live in areas of lower bat diversity (e.g., temperate regions) [2]. Certain habitats exist that are spatially and/or temporally bat free (i.e., places and/or times that bats do not exist), and some of the moths in these habitats exhibit ears with varying levels of auditory degeneration (e.g., oceanic island moths [3], wintermoths [4]). Other moths experience release from bat predation and corresponding auditory degeneration by means of extreme behavioral changes (e.g., flightlessness [5–8]). For temporal bat release a pronounced example of auditory degeneration exists in certain members of the Dioptinae, a group of diurnal Neotropical moths [9]. If diurnal habits result in bat-free environments, we should expect auditory degeneration in day-flying moths, assuming that they do not use their ears for other purposes (e.g., social communication). We examined the auditory, activity, and acoustic characteristics of four species of Nearctic dayflying moths to test this hypothesis.