Spermatophore size and multiple mating: Effects on reproductive success and post-mating behaviour in the Indian meal moth

Abstract:
Ensuring fatherhood is an important issue in polygamous species. In many lepidopteran species, the male inserts a large package (spermatophore) consisting of sperm, accessory gland fluids and nutrients into the female's bursa copulatrix during copulation. In most species it has been shown that a large mate donation causes females either to stop calling, or to prolong the time until re-mating, hereby ensuring fatherhood. We investigated the changes in size of the donation in multiple mating and how the size affected the female post mating behaviour in the Indian meal moth, Plodia interpunctella. In concordance with other previous studies, we found that a male inserted a significantly larger donation during the first mating compared to following mating occasions. The larger donation was not coupled with larval output since it did not decline during mating occasions. Some males were able to mate at least eight times and male fitness, counted as larval output, showed no association with sequence of mating number. Even though females received larger donations during the first mating, there was no effect on female post mating behaviour in terms of pheromone production and calling behaviour. The weight of the mated female's bursa copulatrix decreased linearly over the first eight days after mating, suggesting that females absorb material other than sperm. We discuss the female advantage of receiving a large donation and why males invest more energy into the first mating while female post-mating behaviour is not affected.