The always excellent New Scientist has an article synthesizing much of the research into same-sex sexual behavior in animals and the possible evolutionary explanations. They spoke with University of California evolutionary biologists Marlene Zuk and Nathan Bailey, who recently published a paper examining same-sex behavior in various species. Zuk and Bailey note that same-sex sexual behavior in other animals can't necessarily be equated with sexual orientation in humans, researchers have come up with similar questions as to why certain animals have evolved to include members who expend energy on same-sex liaisons.

Evolutionary biologists have come up with various hypotheses for why same-sex behavior has evolved in various animals. In some cases, same-sex behavior has emerged as a result of specific adaptations, such as to foster social bonding, or because certain genes for same-sex attraction hold another survival benefit when only one copy is present. In some cases, though, the behavior is incidental, such as in certain fish that cannot easily tell male and female members apart.

Below, New Scientist charts several of the possible evolutionary explanations for same-sex sexual behavior in various species:

Bailey believes that exploring the evolution of sexual behavior will give us a better understanding of evolution, including the development of our own species:

"Given its persistence in species in many different animal groups, including humans, viewing it as an evolutionary force in its own right promises to provide a much richer understanding of the evolution of reproductive behaviour," Bailey says. He suggests we could make some fascinating comparisons. Might male-male copulation in species as diverse as flour beetles and dolphins have similar, even predictable, evolutionary consequences? More daringly, could understanding the evolutionary consequences of same-sex interactions in animals help us understand our own evolution?

Homosexual selection: The power of same-sex liaisons [New Scientist]

According to LiveScience:

[Lead researcher Andrea] Camperio-Ciani and his team hypothesize that the genes they modeled may cause people of both sexes to be extremely attracted to men, which would lead men with the genes to pursue relationships with other men, while causing women with the genes to have more sexual partners, and become pregnant slightly more often than an average woman.

Scientists involved in the study were also quick to point out that there is no single genetic cause for homosexuality, and that environmental factors are almost certainly involved as well. They also are still completely clueless about female homosexuality. Researchers have yet to figure out what causes lesbianism, and so far nobody has figured out a form of gene therapy I can use to make straight girls want me.

Sexually Antagonistic Selection in Human Male Sexuality [PLoS One via LiveScience]