The evolution of female multiple mating is one of major questions in evolutionary biology. The fitness of males increases linearly with the number of mates, whereas the fitness of females is limited by their own offspring production. However, recent studies show that sperm production could also be restricted, and males strategically allocate their sperms. Here, we are concerned with male and female reproductive strategies under female biased sex ratio conditions. Under such conditions, our model predicts that males provide a small number of sperms to each female to increase the number of mates, whereas females mate multiple males to obtain a sufficient number of sperms.

Melittobia is a parasitoid wasp which shows an extremely female biased sex ratio (1-5% males), and whose females actually mate multiply. We examined whether females made their decisions of re-mating depending on the number of sperms that they possessed. We did not find any evidences that females directly perceived the number of sperms. However, we showed that males provided a larger number of sperms when they courted for a longer period, and that females decreased re-mating rates when they had been courted for a longer period in their previous matings. These results suggest a possibility that females indirectly know the number of sperms obtained through the courtship period in the previous mating.