To elude bats, a moth keeps its hearing in tune

Moth ears are among the simplest in the insect world—they have only two or four vibration-sensitive neurons attached to a small eardrum. Previously, it was thought that these ears were only partially sensitive to the sound frequencies commonly used by bats, and it would seem likely that by using high ultrasound, bats would make their hunting calls inaudible to moths.

However, as now reported by James Windmill and colleagues at the University of Bristol (UK), moths are not as deaf as they had previously been supposed to be. The findings appear in the December 19th issue of the journal Current Biology, published by Cell Press.

The new work shows that the ear of the yellow underwing moth changes its frequency (pitch) sensitivity in response to sound. As a bat gets closer to a moth, both the loudness and frequency of sound increase. Surprisingly, the sensitivity of the moth’s ear to the bat’s calls also increases, as the moth ear dynamically becomes more sensitive to the frequencies that many bats use when attacking moths. Furthermore, the ear remains tuned up in this way for several minutes after the sound stops, ready in case there is another attack.

To date, this phenomenon has apparently not been reported for insects, or in fact for any other hearing system in the animal kingdom. This finding changes our understanding of the co-evolution of bats and moths—for example, as the moth cleverly tunes its ear to enhance its detection of bats, do some bats in turn modify their calls to avoid detection by moths” The authors point out that, in view of the vast diversity of bat calls, this is to be expected.