Evolutionary natural selection is usually thought of as something that happens prior to mating. For example, humans are believed to assess a prospective mating partner's physical features to maximize the chances of a healthy pairing that will carry on the genetic lineage. Likewise in the animal kingdom, where the lioness will choose a mate with a healthy mane, or a peacock will choose a partner with the most impressive plumage. But an article appearing in Current Directions in Psychological Science suggests that the human male has evolved more devious mechanisms to successfully pass on his genes. The researchers, Todd Shackelford and Aaron Getz, from the Florida Atlantic University, describe this competition among sperm as "the inevitable consequence of males competing for fertilizations." They add that extra pair copulations (i.e. affairs) appear to be a significant part of our ancestral history and could be responsible for everything from penis shape to copulatory thrusting style.
The male imperative to thwart competition in the mating stakes appears to affect sperm count, say the researchers, who note that when men spend more time away from their partners (time that their partners could have spent with other males), the number of sperm in their ejaculate increases upon their next copulation.
Even more bizarrely, the researchers note that simulated phalluses resembling the human penis removed an ejaculate-like substance from an artificial vagina. They believe that this could indicate that that the human penis developed its shape to act as an anatomical squeegee, clearing the vagina of a competitor's sperm. Additionally, sexual behaviors such as deep copulatory thrusting are thought to remove rival sperm more efficiently.
The researchers also said that women report that men thrust more deeply and quickly into the vagina following allegations of infidelity. The same periods of separation that increase sperm number in male ejaculates may also help to explain the increasingly lustful feelings human males develop after long periods of time apart from their mate. That is, the human male may want to copulate as soon as possible as insurance against possible infidelities.
How many other ways have humans evolved in a world dictated by survival of the fittest? "Sexual conflict between males and females," the researchers say, "produces a co-evolutionary arms race between the sexes, in which an advantage gained by one sex selects for counter-adaptations in the other sex."
Source: Association for Psychological Science
