Homosexuality occurs in nearly all animal species, including birds, reptiles and apes, but nobody knows how same-sex coupling, which from an evolutionary point of view should have disappeared long ago, is still widely prevalent. Time Magazine explores the possible reasons in its June 19 edition, citing articles by biologists at the University of California at Riverside led by professors Nathan Bailey and Marlene Zuk.

First is the "boys-in-the-locker-room" theory. It draws an analogy from boys developing a sense of solidarity while making jokes about homosexuality and slapping each other with towels in locker rooms after sports. Male bottlenose dolphins engage in same-sex behaviors to reinforce social relationships. Bonobo chimpanzees, when food is in front of them, have sex with each other regardless of gender, and share the food. Their behavior is interpreted as a means of alleviating tensions and facilitating reconciliation within the group.

Then there is the "emasculation theory." It argues that certain male animals voluntarily adopt homosexual activities to prevent other males from approaching females. Male dung flies enter fierce competition to mate with a limited number of female dung flies, and in the course of the competition, they also engage in homosexual acts. In the end, those who initiated the same-sex sexual behaviors get the females.

The "oops" theory, meanwhile, says some animals have same-gender sex because of misidentification. Male toads are unable to tell the difference between male and female toads because they lack a gene to distinguish sexes.

The fourth theory -- the "let's-see-how-this-thing-work" theory -- argues that young animals, mainly males, "sometimes engage in same-sex sexual behavior as practice, which may improve their reproductive success when they are ready for a heterosexual relationship later." Young fruit flies are one example.

"What all these theories have in common is that same-sex sexual activity is either an accident or a quirky genetic method of helping males impregnate females," Time writes. However, the magazine concluded that the evolutionary theories still do not provide clear answers to same-sex romance among humans.