EFFECTS OF SPERM COMPETITION ON RESOURCE ALLOCATION IN MALE DROSOPHILA MELANOGASTER

Evolution and sexual selection have favoured the development of morphological, physiological and behavioural adaptations in males to handle sperm competition risk. Some males however, are also able to respond to this risk within a much shorter time period. Their plastic phenotype allows them to adapt according to their ever-changing environment. There are, however, costs and limitations to plasticity and there is a trade-off between reproduction and survival. In addition, the investment into plastic sperm-competitive traits creates supplementary energy costs. The aim of this project was to study the trade-off between survival and male Drosophila melanogaster responses to sperm competition, and to look at the effect these responses have on both fitness and resource allocation. In a first experiment, the interactions between various nutrient treatments, sperm competition, and survival of virgin males were studied. It was found that males were only able to invest and benefit from sperm-competitive traits with a sufficient energy supply. Male survival was optimum at intermediate nutrient level and, when conditioned to sperm competition, they lived longer and were less active than males not conditioned to sperm competition. In a second set of experiments, using three nutrient levels and sperm competition risk, male survival and reproductive success were measured. It was found that males subjected to low nutrient treatment increased their mating duration and invested more in courtship, which suggests that terminal investment was taking place. This study has added evidence to the terminal investment concept, and has contributed to the research on the energy trade-off between reproduction, socio-sexuality and the individuals’ experience of their environment.