For decades, scientists have known that female Drosophila reject sex after they come into contact with seminal fluid. This may give a reproductive edge to the first male with whom she has sex. A study led by Geoffrey Findlay at the University of Washington has finally shed some light on the phenomenon. Apparently, there are proteins in seminal fluid, many of which transfer to the female’s system after sex. Some of these proteins are apparent warriors — they fight the sperm of rival males. Other proteins are like hypnotists — by affecting the female’s hormones, they dampen her interest in other males. Somewhat disturbingly, the more semen a female is exposed to, the more influence the male has over her reproductive tract and her mating behavior. Semen, it seems, has mind-control properties.