No evidence for killer sperm or other selective interactions between humanspermatozoa in ejaculates of different males in vitro

This study examines one of the possible mechanisms of sperm competition, i. e. the kamikaze sperm hypothesis. This hypothesis states that sperm from di fferent males interact to incapacitate each other in a variety of ways. We used ejaculates from human donors to compare mixes of semen in vitro from t he same or different males. We measured the following parameters: (i) the d egree of sperm aggregation, velocity and proportion of morphologically norm al sperm after 1 and 3 h incubation in undiluted semen samples, (ii) the pr oportion of viable sperm plus the same parameters as in (i) in 'swim-up' sp erm suspensions after 1 and 3 h incubation, (iii) the degree of self and no n-self sperm aggregation using fluorescent dyes to distinguish the sperm of different males, and (iv) the extent of sperm capacitation and acrosome-re acted sperm in mixtures of sperm from the same and different males. We obse rved very few significant changes in sperm aggregation or performance in mi xtures of sperm from different males compared with mixtures from the same m ale and none that were consistent with previously reported findings. The in capacitation of rival sperm therefore seems an unlikely mechanism of sperm competition in humans.