sexual selection male fliesNORMAN, Okla.—Survival of the fittest really boils down to reproduction of the fittest. If an animal can survive long enough to pass on its genes to a new generation, it has won out in the evolutionary competition.

So, understandably, many animals put a lot of effort into attracting the attention of the opposite sex, explained Alison Pischedda, of University of California Santa Barbara, showing pictures of sparring male elephants, a colorful peacock and Mike "The Situation" Sorrentino from MTV's Jersey Shore.

But winning the female is only part of the game in many species even more promiscuous than those featured on cable TV. "We often assume mating success was the primary marker of male fitness," Pischedda noted. But in many animals, a female might mate with several males before producing a brood of offspring with mixed paternity. This so-called post-copulatory sexual selection is "kind of a big deal" in the biological sciences, Pischedda said here June 21 at the Evolution 2011 annual conference.

The post-deed weeding can happen either via sperm competition or rejection by the female's body of some mates' goods—a process known as cryptic female choice.

Although both strategies have been described previously, Pischedda noted, there was still one "pretty fundamental question" that had yet to be answered: How much do these processes influence the number of offspring a male actually ends up siring?

So Pischedda set out to figure out just how much post-copulatory sexual selection matters in terms of the number of offspring a male ends up fathering. "I thought it would be a piece of cake—turns out not so much," she said of the project. Rather than try to assess animal populations in the wild, Pischedda and her colleagues used fruit flies (Drosophila melanogaster) in a controlled laboratory experiment, studying the genetic patterns of some 1,800 breeding flies in their "very promiscuous mating system," she explained.

The team found that the number of females that a male mated with was not the only determinant of how many offspring they sired. In fact, post-copulatory factors explained about 27 percent of the males' success rate.

The data also revealed that the number of offspring that a male ended up fathering from a female's batch of eggs had more to do with how recently he had mated with her than with his overall genetic quality. The last male to mate with a female fruit fly before she lays her eggs fathered an average of about 79 percent of the brood even though "those males are not necessarily the best in the population," Pischedda said.

So after factoring that steep statistic in to the analysis, mating order explained about a quarter of a male's reproductive success and number of offspring, whereas any cryptic or other internal sperm selection was responsible for about 2 percent of sexual success. Although that factor is small, Pischedda pointed out that it has "a statistically significant impact" in figuring out fitness, and "should really be included" in future work, she noted.

But just playing the numbers, males that get to mate with the most females might have the best odds of being the last one in, and thereby fathering the majority of the offspring—at least in the fruit fly world. Mating dynamics on Jersey Shore might need further study.

Image courtesy of Wikimedia Commons/TheAlphaWolf