Birds do it, bees do it, even educated fleas do it: postcopulatory sexual selection. It took nearly a century after Darwin first described how competition between males forms the process of sexual selection before researchers realized that selection does not end there. Instead, males have to fight off their competitors even after copulation has started, through postcopulatory selection, either directly (through sperm competition) or indirectly (through sperm selection by the female).
Postcopulatory selection has certainly led to some interesting adaptations. Recently discovered examples include exploding male genitalia – which prevent other males from mating – and seminal compounds that enter the female’s nervous system and modify her behavior, such as the Drosophila seminal fluid proteases (Sfps), which make her resist the amorous approaches of other males. However, no one had tried to measure the relative importance of postcopulatory selection in determining reproductive success until a study recently published in PNAS by Alison Pischedda and William Rice. They addressed this question by identifying the precopulatory and postcopulatory components of sexual selection in a lab-grown population of the fruit fly (Drosophila melanogaster).
The authors first quantified the extent of sexual selection within this population by measuring and comparing the reproductive success of male individuals. They found that there was great variability in male reproductive success: a large number of the males produced no or few offspring, while others produced large numbers of offspring. This suggested that there is indeed strong sexual selection.
The total number of offspring a male produces is determined mainly by two factors: the number of females that it mates with (mating success), which is the product of precopulatory selection, and the proportion of offspring it sires with these females (fertilization success), which is the product of postcopulatory selection. To measure the relative importance of postcopulatory selection in the process of sexual selection, the duo divided the variability in reproductive success into its precopulatory and postcopulatory components.
They found that the variability in these two parameters was roughly the same, suggesting postcopulatory selection is as important as precopulatory selection in the process of sexual selection. This is the first quantitative evidence that the number of offspring a male produces is determined in equal measure by how well it can find a willing mate and what happens during and after mating.
Knowing that Drosophila males rely on a complicated sequence of courtship behaviors to ensure mating success, the duo then decided to identify the factors that determine fertilization success. As the last male that copulates with a female is known to sire most of her offspring, Pischedda and Rice focused on finding how much of the postcopulatory success can be attributed to this bias by looking at the correlation between the mating order of males and their fertilization success.
They found that the vast majority of the fertilization success can be attributed to mating order. After finding a willing mate, the timing of mating is therefore the most important factor that determines the number of offspring a male produces.
The authors have established that postcopulatory selection is a crucial factor in determining the overall reproductive success of male fruit flies. While the biological mechanisms that are behind this selection are still unknown, the timing of mating is essential. The same principle could apply to other non-monogamous species, such as birds, bee, and, yes, even educated fleas.