Costs and consequences of plastic responses to the sperm competition environment in Drosophila melanogaster

Sperm Competition (SC) has been one dominant focus of evolution and behavioural ecology since its conception. Plastic SC responses evolved in response to rapid changes in SC risk. In the model organism Drosophila melanogaster it is known that males reared with rivals mate longer, alter seminal fluid production and transfer an ejaculate containing more seminal fluid proteins, resulting in higher fertilization success and paternity share. Here I investigate potential longevity costs for males expressing plastic SC responses, as well as any longevity cost to females mating to these males. I also explore whether males vary sperm production in response to SC risk. I demonstrate that virgin males reared in high SC environments live significantly longer than males from low SC environments; mating negates this longevity benefit. I expected males to pay a longevity cost for their plastic SC responses: the counterintuitive finding suggests that males may trade-off this cost by reducing other expensive functions. In contrast, females showed no instantaneous longevity cost or benefit to mating with males from a high SC background. In addition to the plastic behavioural and physiological responses to SC risk already reported, I demonstrate here that male D. melanogaster also respond by increasing both sperm quantity and quality. Despite the longstanding use of D. melanogaster as a model species in SC research, this result has never previously been described. The work reported here reveals two major findings: male D. melanogaster adjust sperm quality in response to risk of SC, and that longevity of these males increases above that of males raised alone. This research raises new questions about plasticity in response to SC in this species, in particular, if the costs of the response are so small, why do males not constantly show the elevated response? The answer is likely to lie in the precise balance of trade-offs between different life history variables.