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

Male field crickets, Gryllus integer, in Texas, USA, produce a trilled call ing song that attracts female crickets, resulting in enhanced mating succes s. Gravid female parasitoid flies, Ormia ochracea, are also attracted to ma le cricket calling song, resulting in the death of the male within about se ven days. Using playbacks of field-cricket calling song in the natural habi tat, we show that both female crickets and female parasitoid flies prefer m ale calling song with average numbers of pulses per trill. Thus female cric kets exert stabilizing sexual selection, whereas flies exert disruptive nat ural selection on male song. Disruptive natural selection will promote gene tic 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 vari ation (ca. 40%) in male calling song.