Acoustic interference in bush crickets; a factor in the evolution of singing insects?

Abstract

This paper examines forms of acoustic interference in some European bush crickets, in particular the phenomenon of song modification, that occurs when individuals of different species sing together. This can result in inhibition of song, changes in the species-specific temporal pattern of the song, or, if the singers simply chorus in unison, songs may be masked. One of the principal causes of song modification is similarity of the song frequency spectra; insects with dissimilar frequency spectra do not interfere. The second requirement for song modification to occur is that one or both singers normally respond with temporal adjustments to chirp rate or duration to the songs of neighbouring conspecifics. These species then ‘attempt’ to respond to the inappropriate heterospecific signal of the correct frequency. The excitatory and inhibitory properties of these signals are also considered to contribute to the formation of often completely novel song patterns. Field surveys of natural communities indicate that acoustic interference is probably uncommon in the wild though some instances of song modification have been recorded. The adaptations that might have evolved to reduce acoustic interference between species have been considered. Differences in frequency spectra of the broadcast songs, and habitat and microhabitat preferences in those species with similar song spectra are two of the main adaptations that have been suggested.