Optimal polyandry in fruit flies

The study of polyandry has received increasing scientific attention with an emphasis on the fitness benefits and costs that females derive from mating with multiple males. There are still gaps in our understanding of how polyandry affects female fitness, however, as many previous studies compared the fitness outcomes of a single mating vs. two or three matings and did not separate the consequences of multiple mating from the costs of sexual harassment. We therefore conducted controlled mating trials with female fruit flies (Drosophila melanogaster) that could mate at either low (every eight days), medium (every four days), or high (every other day) rates while controlling for exposure to harassment from males. We found that female lifetime fitness was highest under the high followed by the medium mating-rate conditions. Moreover, we did not detect reductions in lifespan as a consequence of higher rates of polyandry. Our results demonstrate that even at realistically high rates, polyandry can lead to net fitness benefits for females, which can have major implications for sexual selection. Specifically, we discuss the significance of our findings as they relate to competition and the evolution of secondary sex characteristics in females, and sperm competition amongst males.