The NYTimes has a lovely primer on sperm. Money quote:
Sperm do not really hit their stride until they are deposited in the female reproductive tract, at which point chemical signals from the vaginal and cervical mucus seem to spark them to life. Released from the buffering folds of their seminal delivery blanket, they at first swim straight ahead, torpedo-style, "with very little back and forth of the head," Dr. Tash said. They may linger in the cervical mucus for a couple of days, or cross the cervix and enter the uterus.
If an egg has burst from its ovarian follicle and been plucked by a fallopian tube, sperm can sense its signature, a telltale shift in calcium ions. The sperm become "hyperactivated," said Moira O'Bryan, a sperm expert at Monash University in Australia, switching to "a crazed figure-eight motion" ideal for boring through barriers. The ovum eggs them on, signaling some to play the sacrificial kamikaze and explode their enzyme sacks prematurely, loosening the corridor for other, shapelier sperm to pass through intact. A few dozen fine-figured sperm find their way to the final barrier, the egg's plasma membrane, where they waggle with all their crazy-eight might and beg to be chosen -- but only one will be taken, will fuse with the egg and be absorbed into its rich inner sanctum.
Read the whole thing.
I had found an interesting article a while back about how sperm swim together to get where they are going faster. It has videos.
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