PREDICTING VARIATION IN SPERM PRECEDENCE

Sperm competition theory predicts that males are adapted for success i n sperm competition by the production of large numbers of sperm. This is supported by both inter- and intraspecific studies showing that mal es mating under high sperm competition risk increase investment in spe rm production. Such an increase in sperm production is an advantage if sperm mix randomly or if sperm displacement occurs. When two males ma te with the same female, the measurement of the proportion of eggs fer tilized by the second male to mate (termed P-2) has been used to help elucidate sperm competition mechanisms. P-2 is usually quoted as a mea n value, with little attention being paid to its variance, although P- 2 estimates are notoriously variable. By predicting an expected varian ce for P-2, additional information on sperm competition mechanisms may be obtained. Here we present a technique for analysing the variance i n P-2 when a given mechanism of P-2 is assumed. We apply this techniqu e to P-2 data collected from Plodia interpunctella (Lepidoptera, Pyral idae), assuming a 'fair raffle' mechanism of sperm competition. We com pare observed distributions of P-2 with theoretical distributions gene rated assuming random mixing of two ejaculates drawn randomly from a p opulation of known mean and variance in sperm numbers. Ejaculates of k nown size were obtained by counting the number of sperm ejaculated by males mating for the first (large ejaculate) or second (small ejaculat e) time. Females either received two small, or one small and one large ejaculate, and the distribution of P-2 (estimated using the sterile m ale technique) was compared with our theoretical predictions. The obse rved variance in P-2 was greater than our model prediction, thus we co nclude that sperm from P. interpunctella do not mix randomly before fe rtilization.