The influence of male and female eyespan on fertility in the stalk-eyed fly Cyrtodiopsis dalmanni

When males produce ejaculates that ensure only partial fertility of females' eggs, we expect adaptations to increase fertility in both males and females. Males are predicted to allocate their ejaculates strategically between females with respect to female reproductive quality, while females are predicted to prefer to mate with highly fertile males. Males and females cannot directly assess a potential partner's reproductive quality, but quality may be reliably signalled by visible phenotypic traits. We investigated the potential influence of eyespan, a sexually selected trait, on fertility after a single mating in the sexually dimorphic stalk-eyed fly Cyrtodiopsis dalmanni. We found that female eyespan was an accurate predictor of fecundity and large-eyespan females laid more than twice as many eggs as small-eyespan females. Large-eyespan females also laid more fertile eggs after a single mating than small-eyespan females did. This difference can in part be explained by males strategically allocating more ejaculate to large-eyespan females, and we found evidence for this as males produced larger spermatophores when mated to large-eyespan females. In contrast, we failed to find evidence that male eyespan had any effect on female fertility after a single copulation. However, female mate preferences for large-eyespan males may still be an adaptation to cope with male ejaculate limitation in C. dalmanni as males with a large eyespan have larger accessory glands and mate more frequently.