Density-dependent natural selection in Drosophila: Trade-offs between larval food acquisition and utilization

Natural selection at high densities has often been postulated to favour the evolution of greater efficiency of food use. Contrary to this expectation, a previous study suggested the existence of a trade-off between larval feeding rate and efficiency at using food to complete larval development in populations of Drosophila melanogaster subjected to crowding for many generations. In this paper, we confirm the generality of such a density-dependent trade-off between food acquisition and utilization by demonstrating its occurrence in a new set of Drosophila populations subjected to extreme larval crowding. We suggest that such trade-offs between food acquisition and food use may represent a general phenomenon in organisms exhibiting scramble competition. We test and reject the possible mechanistic explanation that decreased efficiency of food use in faster-feeding larvae may merely be a consequence of a faster passage of food through the gut, leading to incomplete assimilation of nutrients and energy.