Sperm competition experiments between lines of crickets producing different sperm lengths.

Sperm numbers can be important determinants of fertilization success in sperm competition. However, the importance of variation in sperm size is less well understood. Sperm size varies significantly both between and within species and comparative studies have suggested that some of this variance can be explained by sperm competition. In this study we examine whether variation in sperm length has consequences for fertilization precedence using controlled sperm competition experiments in the field cricket Gryllus bimaculatus. This species is an ideal model for such investigations because the mechanism of sperm competition generates complete mixing of different males' spermatozoa in the female (thereby allowing individual sperm to express their own competitive abilities). We successfully bred lines of crickets, the males of which produced short, medium and long sperm types with narrow and non-overlapping distributions. Males of different lines were then sequentially mated with control females in order to create two-male sperm competitions. The paternity outcomes of these competitions were measured after matings using an irradiated male technique (with a full reciprocal design that controls for natural fertility and any irradiation effects on gamete competitiveness) over a 12 day oviposition period. However, having successfully bred diverging sperm length lines and competing males that differed in sperm length, we found no evidence that a male's sperm size explained any of the variation in their relative fertilization success. Males from lines producing longer sperm showed no fertilization advantage over males producing shorter sperm across 97 double matings. There was also no advantage for males producing a sperm length close to the population mean over those competitors whose sperm length had been selectively diverged across 63 matings.