Let's face it, males and females of most species are promiscuous. And among so-called monogamous species, only 10 percent actually remain faithful. Mating among Drosophila melangaster involves post-sex battling on a tiny scale. Females not only sleep around and have children by different males, they also usually mate with several males before producing a single batch of offspring. Which means each male's sperm competes with others for access to the female's eggs.
Sperm competition decides who gets to sire offspring.
There's an opportunity for tremendous competition. The trick to discovering the last male's advantage lies in glowing sperm. Before flies were genetically engineered to produce a fluorescent jellyfish protein in their sperm tails, researchers had no way to tell one male's sperm from another's, so determining what happened to each male's sperm was impossible. They surprised themselves and got two answers, reported in this week's issue of Nature . By counting the green sperm from genetically engineered males and regular sperm from another strain of males in a doubly-mated female, Cathy Price, a graduate student at the University of Chicago, and her colleagues found that the last male wins because his sperm, and not his seminal fluid, displaces the first male's sperm.