THE PROCESS OF HUMAN CONCEPTION is almost absurdly inefficient. During copulation, a man expels tens of millions of sperm, with considerable force, into his partner's vaginal canal. Despite the head start, most of the tiny, tadpole-shape, self-propelled cells never come within shouting distance of the woman's egg, floating deep inside the Fallopian tube. And if one does finally complete the journey, it may or may not have the energy left for fertilization.
With these abysmal odds, a man clearly needs every last sperm cell he's got. Any fewer than 20 million or so per ml of semen--40 million to 120 million...
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