Call it sperm competition.

According to a study appearing in Current Directions in Psychological Science, sexual characteristics exhibited by human males indicate that men have evolved to deliver their sperm more effectively to females who have multiple partners to choose from.

"Although many people are familiar with the idea of animals competing for mates before sex occurs, through mating displays such as bright feathers or butting antlers, we are finding more evidence that there is also competition after mating occurs," says author Todd K. Shackelford. "An alternative way of thinking about it is that there is not only competition between males for mates, but competition between males for fertilization."

The research presented in the review covers physical adaptations, including penis shape and style of intercourse, as well as behavior in response to perceived infidelity that all serve to increase the success of fertilization. "The studies have shown that when partners are separated for periods of time, males are more likely to arouse easily, produce more sperm, and even rape their partners," says Shackelford. According to Shackelford and co-author Aaron T. Goetz, this does not mean that women are promiscuous by nature, but it is evidence that humans are not naturally a monogamous species.

Shackelford is quick to point out, however, that females are not passive partners in the sexual relationship. "Although this review focused on male adaptations, sexual conflict between males and females 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: Blackwell Publishing.