What's in a song? Female bushcrickets discriminate against the song of older males

The striking songs of insects such as bushcrickets have traditionally been interpreted as species recognition signals. If this is so, their information content should be relatively simple. Females should discriminate against atypical songs, but not between common differences between males. Alternatively, female preference may have evolved by sexual selection with variation between males providing reliable cues on which females discriminate among potential mates, or sensory biases may predispose females towards a particular form of song. We examined how the calling song of the bushcricket Ephippiger ephippiger varies in natural populations. Song structure changes over the course of a season, reflecting wear of the stridulatory apparatus. Males vary substantially in the rate at which they show ageing effects. Superimposed on this are consistent differences between males and temperature effects. This sexual signal therefore contains potentially reliable cues which females could use during mate choice. When presented with artificial songs that reproduce some of the song variants associated with older males, females show extremely strong preferences for unmanipulated song, typical of young males. These aspects of female preference will exert sexual selection on males, whether they originate for adaptive or arbitrary reasons.