Those aspects of an organism's biology that influence the number of progeny produced make up the fertility component of its fitness. The fertility of both male and female Drosophila pseudoobscura is influenced by multiple matings. In the former, rates of multiple matings and the genotypes of females' other mates interact to affect male fitness. Female fecundity and productivity increases with multiple matings, while longevity declines. Mating rates could be the result of these conflicting selection pressures. Density is a non-genetic factor influencing multiple mating. The mechanisms by which multiple matings increase female fertility is not the same in different species of Drosophila. In D. melanogaster, subsequent matings appear to replenish diminished sperm stores, while in D. pseudoobscura, females may absorb sperm for use as a nutrient. These differences emphasize that there need not be single "model systems" even within a genus.