Do you think humans are made for monogamy?

Well, they may try to stay monogamous, but their genetic background says otherwise.

A recent study in sexual adaptation reveals that male sexual behavior is built for "sperm competition", something not found in the strictly monogamous species.

The review points out the fact that physical and behavioral sexual traits displayed by men are signs that males have evolved to more effectively fertilize females with multiple partners. "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," said 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 sperm competition).

Physical adaptations are the shape of the penis and the style of intercourse, and behavioral ones those displayed when perceiving infidelity, all aiming to ensure the possibility that one particular male will be the one to fertilize the egg.

One behavioral adaptation is the males' high sex drive when reuniting after long periods of time with their long-time partner. "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," said Shackelford.

These evidences show that our species is not a naturally monogamous one and that females are not passive sexual partners. "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."