Sexual selection on female copulatory devices in an insect with nuptial gifts

Abstract

Male genitalia are rapidly evolving structures, often driven sexual selection to increase fertilization success. Although sexual selection on females can be strong in systems where males provide offspring care or feed their mates, sometimes resulting in the evolution of female ornamentation, there are no actual estimates of direct sexual selection on female genitalia. In a New Zealand ground weta, Hemiandrus pallitarsis (Orthoptera: Ensifera, Anostostomatidae), females possess a genitalic device (the accessory organ) that is necessary for successful copulation and the acquisition of glandular food-gifts from males. These nutritious gifts are known to result in sexual competition among females in other ensiferan species. In ground weta, the gifts are probably important in avoiding starvation during a months-long period when caring for (their lifetime production of) eggs and offspring. Here, we test the hypothesis that the accessory organ is a sexually selected device in H. pallitarsis by measuring the female Bateman gradient and directional sexual selection on the accessory organ. Using newly developed and characterized microsatellite loci, we analyze offspring and/or stored sperm to estimate female mating frequency for the first time in ground weta. As predicted, we found positive Bateman gradients for females, and some evidence of directional sexual selection on accessory organ length. Although organ length does not correlate well with female fecundity, it may increase mating success by indicating her condition and thus quality of her offspring care.