By genetically engineering fruit flies, researchers from Syracuse University in the US have been able to track the fate of live sperm cells between insemination and fertilisation. Two male fruit flies were engineered to express either green or red fluorescent sperm which could then be traced through the female reproductive tract and watched in real time using a microscope.

'Our jaws hit the floor the first time we looked through a microscope and saw these glowing sperm', said lead author Scot Pitnick in a statement. He added: 'It turns out that they are constantly on the move within the female's specialised sperm-storage organs and exhibit surprisingly complex behaviour.'

Postcopulatory sexual selection, or the process of deciding which sperm wins, has been difficult to understand until now. Like many species, fruit flies are not promiscuous. Sexual selection therefore becomes increasingly important in evolutionary change if several males are competing for paternity.

The research, published in Science, confirms previous reports that sperm from the most recent male tends to displace those from earlier males inside the female storage organs. The study also concludes that the timing of sperm release from female storage organs, the distribution of competing sperm, the 'fair raffle' sperm use (meaning no sperm favouritism from the female or the environment), and the high level of sperm mobility all influence postcopulatory sexual selection.

According to Pitnick, the previous inability to discriminate between competing sperm of different males coupled with the challenges of directly observing the live sperm inside the female reproductive tract have limited our understanding of postcopulatory sexual selection. Pitnick told Scientific American magazine that 'I suspect we have just scratched the surface of using this material.' The technique could be significant in understanding the complexity of sexual selection and may have important implications in the field of reproductive and evolutionary biology.