SPERM COMPETITION OR SPERM SELECTION - NO EVIDENCE FOR FEMALE INFLUENCE OVER PATERNITY IN YELLOW DUNG FLIES SCATOPHAGA-STERCORARIA

Recent studies of non-random paternity have suggested that sperm selec tion by females may influence male fertilization success. Here we argu e that the problems originally encountered in partitioning variation i n non-random mating between male competition and female choice are eve n more pertinent to interpreting patterns of non-random paternity beca use of intense sperm competition between males. We describe an experim ent with the yellow dung fly, Scatophaga stercoraria, designed to part ition variance in the proportion of offspring sired by the second male , P-2, between males and females, and to control for sperm competition . Large males were shown to have a higher P-2 than small males but P-2 was independent of the size of the female's first mate. This result m ight suggest an absolute female preference for large males via sperm s election. However, large males have a higher constant rate of sperm tr ansfer and displacement. After controlling for this effect of sperm co mpetition, large males did not achieve higher paternity than small mal es. We argue that a knowledge of the mechanism of sperm competition is essential so that male effects can be controlled before conclusions a re made regarding the influence of sperm selection by females in gener ating non-random paternity.