Based on new evidence, it shouldn’t be long before Las Vegas oddsmakers start accepting wagers on the intense battles that have now been observed within the sex organs of females.
According to the March 19, 2010 issue of Science (available online only to subscribers; it is page 1443 in the print edition), sexual selection continues on in the most intimate of arenas in at least some species in which the females sometimes mate “more than once in quick succession, filling their reproductive tract with rival sperm that must compete for access to the unfertilized eggs.” The Science article, by Elizabeth Pennisi, is entitled “Male Rivalry Extends to Sperm and Female Reproductive Tract.” According to Pennisi, two recent studies have shown that the seminal fluid of some ants and bees contains “toxins that impede rival sperm.” She also notes that some female fluids seem to counter these toxins. The studies cited in science indicates that “the competition between males continues in a very fierce way inside the female.”
In the first of two studies, the scientists genetically coaxed one species of fruit flies to produce a green fluorescence, and the other to produce a red fluorescence. This allowed the scientists to study the interactions between the rival teams of sperm; they were able to tell who is who. The scientists then discovered that the sperm of the first male was displaced by the sperm of the second male (after a while, however, the sperm settled down and all then seem to have an equal chance of fertilizing an egg).
A second study similarly studied females who mated multiple times during a single courtship period.
For the multiple mating species studied, two leaf cutter ants and a honeybee, seminal fluid from a given male enhanced survival time of its own sperm in a lab dish but damaged unrelated sperm and even sperm from a brother.… Sperm facing competition have evolved some as yet-to-be-defined seminal fluid components that somehow recognize and thwart rivals.
Therefore, if sperm could talk, you might imagine a sperm telling a rival sperm, “Let’s step inside and duke it out.” The literature on sexual selection was already extensive. This new information documenting sperm wars bolsters this magnificent explanatory story even further.
In addition to sperm wars, there is evidence that the human penis is designed to remove sperm of competitors before depositing its own sperm:
So how did natural selection equip men to solve the adaptive problem of other men impregnating their sexual partners? The answer, according to [evolutionary psychologist Gordon] Gallup, is their penises were sculpted in such a way that the organ would effectively displace the semen of competitors from their partner’s vagina, a well-synchronized effect facilitated by the “upsuck” of thrusting during intercourse. Specifically, the coronal ridge offers a special removal service by expunging foreign sperm. According to this analysis, the effect of thrusting would be to draw other men’s sperm away from the cervix and back around the glans, thus “scooping out” the semen deposited by a sexual rival.
http://www.scientificamerican.com/article.cfm?id=…