Measuring the sperm competition successes of field males of the yellow dung fly

Abstract 1. Sperm competition is extremely common in many species but conclusions about the relative successes of males, measured as the proportion of a female's offspring fertilised by the second male to mate with it, are based largely on laboratory studies using two laboratory-bred males or on interrupted last copulations.
2. A study was conducted in which virgin female yellow dung flies Scathophaga stercoraria mated uninterrupted with two field-captured males.
3. The results confirmed two previous conclusions: the proportion of a female's offspring fertilised by the second male was related strongly to the copula duration of the second male, and the proportion of a female's offspring fertilised by the second male was lower in larger females, though this effect was weak.
4. The results indicate that previous experiments on laboratory populations of this species are likely to be relevant to wild situations, but researchers are urged to conduct studies that will improve the confidence with which laboratory measures of the proportion of a female's offspring fertilised by the second male can be assumed to reflect the sperm precedence of the last male to mate in natural situations.