Low Numbers of Sperm Retained in the Spermatheca May Explain the High Values of Sperm Precedence in the Migratory Locust, Locusta migratoria (Latr.)

Four hypotheses about the temporal variation of the number of spermatozoa in the spermatheca of once-mated females were tested in the migratory locust, Locusta migratoria. The best fit provided a regression model that assumed a sudden drop in sperm numbers being indicative of sperm ejection by females. Thereafter, one-fifth of an average ejaculate is stored. Low numbers of sperm stored do not lead to female sperm limitation, as none of the fitness parameters measured, fertilization success, hatching success, and offspring sex ratio, were correlated with the number of sperm present. Fertilization success decreased with successive egg pods. The offspring sex ratio was slightly male biased and tended to increase throughout the laying period. By applying our sperm ejection model to an independent data set of Parker and Smith (1975), we show that the low numbers of sperm retained by the female and subsequent sperm mixing rather than direct replacement may explain the high P₂ values found in this species.