Sex, death and genetic variation: natural and sexual selection on cricket song

Male field crickets, Gryllus integer, in Texas, USA, produce a trilled calling song that attracts female crickets, resulting in enhanced mating success. Gravid female parasitoid flies, Ormia ochracea, are also attracted to male cricket calling song, resulting in the death of the male within about seven days. Using playbacks of field-cricket calling song in the natural habitat, we show that both female crickets and female parasitoid flies prefer male calling song with average numbers of pulses per trill. Thus female crickets exert stabilizing sexual selection, whereas flies exert disruptive natural selection on male song. Disruptive natural selection will promote genetic variation and population divergence. Stabilizing sexual selection will reduce genetic variation and maintain population cohesiveness. These forces may balance and together maintain the observed high levels of genetic variation (ca. 40%) in male calling song.