When the sperm of different male insects meet inside a female, they use everything from wrestling to chemical warfare to try and fertilize as large a share of her eggs as possible, according to two studies published this week. The studies also show that females don't just let the battle take its course, but manipulate it to their own ends. A US team has genetically engineered fruitflies to produce sperm that fluoresce in different colours. The researchers use the technique to watch the sperm of different males as they jostled for position inside a female, giving a first look at sperm competition in action.
And researchers in Denmark and Australia have shown that the seminal fluid of some ants and bees aids a male's own sperm and attacks his rivals. But queen ants, which need huge sperm reserves for the long years of egg-laying ahead, suppress this competition. Both studies are published in Science. For the fruitfly Drosophila melanogaster the most recent male to mate with a female fertilizes most of her eggs — 80% — and his predecessors lose out. But the mechanism by which the last male got this advantage wasn't known. “The female reproductive tract has been a black box,” says Scott Pitnick of Syracuse University in New York.