Females of the grasshopper Chorthippus parallelus (Zett.) do not remate for fresh sperm

The evolution of female multiple mating is still a largely debated field. A mong the benefits that have been proposed to explain this risky behaviour i s the replenishment of sperm reserves. Apart from an increase in total sper m number, it can be an expression of post-copulatory mate choice or can be directed towards the uptake of fresh sperm. Using fresh sperm for fertiliza tion instead of sperm aged by storage in the female genital tract may avoid a lowered fertilization capacity an increase in deleterious effects or a s kewed offspring sex ratio. We investigated the influence of sperm age on fe male fitness in the grasshopper Chorthippus parallelus; a species where fem ales mate multiply. After copulation, females store sperm over the course o f weeks until fertilization. An average ejaculate of 250000 spermatozoa exp onentially declined with time within the female's spermatheca. The number o f days since copulation better explained the variation in actual sperm numb er than the number of pods or eggs laid. We investigated differences in fem ale fitness parameters in two treatments. In the first, females were mated only once, while in the second, females always had freshly ejaculated sperm available. Although in our experiment, multiply mated females had heavier offspring than singly mated females, egg number per pod, hatching and ferti lization success, their composite effects and offspring sex ratio did not v ary with respect to season or sperm age. We therefore reject the hypothesis that the reason for remating in females of this species is the uptake of f resh sperm.