Moths’ ears evolved first for bat defense and later for communication

Abstract

Many moths possess simple ears primarily to alert them to the echolocation calls of hunting bats. Since large targets reflect stronger echoes than small ones, bats should detect moths with large wing and body surface areas at greater distances than small moths. The increased conspicuousness of large moths would reduce their safety margin over bats, unless this is offset by a greater sensitivity of their ears. Results seem to support this: The sensitivity of ears of noctuid moths is significantly dependent on size, with larger moths possessing lower thresholds. If moth ears exist for bat detection, moths that live in bat-free niches should exibit reduced acoustic sensitivity. Studies of moths temporally isolated from bats (flying either in the winter or during the day) have confirmed this expectation. Another effect of the release of predation pressure from bats may be the use of the preexisting ears for sexual acoustic communication. All sonorous noctuoid moths described so far produce sounds in the same frequency range as that emitted by bats, presumably because bat-sensitive ears are the ancestral (plesiomorphic) condition and acoustic communication is a facility aquired later. [Work supported by the Danish National Research Foundation.]