According to Rebecca Burch and Gordon Gallup, researchers at the State University of New York, sex researchers are paying too much attention to sperm and not enough to the makeup of the fluid that carries them.
When Burch and Gallup tested samples of human semen, they found luteinizing hormone and follicle-stimulating hormone, both of which can induce a woman to release eggs earlier than normal. Those hormones were conspicuously scant in semen from primates, like chimpanzees, that advertise their most fertile periods with red, swollen backsides.
Sexual evolution is like an arms race, Burch says. When women evolved concealed ovulation, men most likely developed semen full of hormones to wrest back control of paternity.