Keeping up with Bats: Dynamic Auditory Tuning in a Moth

Many night-flying insects evolved ultrasound sensi- tive ears in response to acoustic predation by echolo- cating bats [1–10]. Noctuid moths are most sensitive to frequencies at 20–40 kHz [6], the lower range of bat ultrasound [5, 11–13]. This may disadvantage the moth because noctuid-hunting bats in particular echo- locate at higher frequencies shortly before prey cap- ture [7, 11–13] and thus improve their echolocation and reduce their acoustic conspicuousness [6–10, 12–16]. Yet, moth hearing is not simple; the ear’s non- linear dynamic response shifts its mechanical sensi- tivity up to high frequencies. Dependent on incident sound intensity, the moth’s ear mechanically tunes up and anticipates the high frequencies used by hunt- ing bats. Surprisingly, this tuning is hysteretic, keep- ing the ear tuned up for the bat’s possible return. A mathematical model is constructed for predicting a lin- ear relationship between the ear’s mechanical stiff- ness and sound intensity. This nonlinear mechanical response is a parametric amplitude dependence [17, 18] that may constitute a feature common to other sen- sory systems. Adding another twist to the coevolu- tionary arms race between moths and bats, these re- sults reveal unexpected sophistication in one of the simplest ears known and a novel perspective for inter- preting bat echolocation calls.