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In mammals, ejaculated sperm are progressively motile yet do not have an immediate capacity for fertilization. In the case of in vivo fertilization they gain this ability after residing in the female genital tract or, in the case of IVF, after incubation for some time in an appropriate medium. The physiological changes that render the sperm competent to fertilize is referred to as capacitation (see Austin 1952; Yanagimachi 1994).