This study examines one of the possible mechanisms of sperm competition, i. This hypothesis states that sperm from different 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 the same or different males. We measured the following parameters: i the degree of sperm aggregation, velocity and proportion of morphologically normal sperm after 1 and 3 h incubation in undiluted semen samples, ii the proportion of viable sperm plus the same parameters as in i in 'swim-up' sperm suspensions after 1 and 3 h incubation, iii the degree of self and non-self sperm aggregation using fluorescent dyes to distinguish the sperm of different males, and iv the extent of sperm capacitation and acrosome-reacted sperm in mixtures of sperm from the same and different males. We observed very few significant changes in sperm aggregation or performance in mixtures of sperm from different males compared with mixtures from the same male and none that were consistent with previously reported findings.
Cooperative sperm, killer sperm and the competition for reproductive success
Sperm competition - Wikipedia
Sure, nobody really thinks of worm sperm as being cute and cuddly -- assuming one thinks about them at all -- but who would have thought they were stone cold killers? In a paper published Tuesday in PLOS Biology, a team of researchers who apparently enjoy watching nematodes get jiggy were shocked to learn that male sperm will conduct search-and-destroy missions on females and hermaphrodites of other worm species. Animals of the same genus but different species may mate and produce offspring, but that animal is usually sterile. An example is the mule -- which occurs when you breed a horse and a donkey. Researchers mixed and matched three Caenorhabditis species, cross-breeding male, female and hermaphrodite worms -- worms capable of producing both sperm and eggs in order to reproduce.
Do Sperm Battle Other Sperm?
Sperm Wars is a primarily nonfiction book by evolutionary biologist Robin Baker. Through a series of short fictional stories and discussion following them, Baker proposes evolutionary functions for sexual habits, mostly on the principle of competition between sperm of different men for a prized egg. The book is controversial,  both because of its explanations of homosexuality , sexual assault , and prostitution, and because some critics have claimed that several of the hypotheses in the book are not supported by scientific research. The shape of the penis and the thrusting during intercourse serve to remove other men's semen. Baker also proposes that men adjust the amount of sperm they ejaculate based on the time their mate has spent away from them.
In the context of sexual reproduction, natural selection is generally thought of as a pre-copulation mechanism. We are drawn to features of the human body that tell us our partner is healthy and will provide us a fighting opportunity to carry on our genetic lineage. But a new article appearing in the February issue of Current Directions in Psychological Science suggests that the human male has evolved mechanisms to pass on his genes during post-copulation as well, a phenomenon dubbed "sperm competition. In their article, Todd Shackelford and Aaron Getz at Florida Atlantic University describe sperm competition as "the inevitable consequence of males competing for fertilizations. For a monogamous species, sperm competition may seem unlikely.