Apart from "gay genes" in humans, the main evidence that homosexuality is naturally selected is the fact that gay animals are so common.
Pairs of male penguins court each other, build a nest, and huddle over a round stone in lieu of an egg (1). One captive pair of chinstrap penguins, Roy and Silo of New York's Central Park Zoo went farther. When curious zookeepers gave them a real egg stolen from another nest, they successfully hatched, and raised, the chick. Pairs of female penguins get in on the act as well. Perhaps a tenth of penguin pairings in the wild are same-sex, a remarkable phenomenon when one considers that penguins often mate for life.
The prevalence of homosexual pairings among penguins is something of a puzzle. Even if homosexuality were a sex-linked trait (see previous post) this incidence would be unexpectedly high One of the more common sex-linked traits, red-green color blindness occurs in about 8 percent of men which is far higher than most other deleterious genes.
Homosexuality is influenced by genes but it can also occur when normal gender development goes awry. Farmers are familiar with the phenomenon of freemartin ewes that mount other female sheep after the manner of rams. These butch sheep are twins whose brains were affected by sex hormones produced by their male companion in the womb. Homosexuality can emerge due to perturbations in the hormonal environment of the womb for other species also, humans included.
Regardless of the particular biological mechanism through which homosexuality arises, it is a pervasive phenomenon in the animal kingdom, a fact that discredits the common argument that it is "unnatural." According to Joan Roughgarden, a trans-gendered biologist who studied such phenomena, there is documented evidence of homosexual behavior for some 450 different species and undocumented reports for about another thousand species (1). Such evidence was the basis for a recent photographic exhibition in Oslo.
Groups of male bighorn sheep engage in genital licking and anal intercourse to the point of ejaculation. Interestingly, individuals uninterested in such same-sex contact are kicked out of the group. Field workers describe the all-male orgies of giraffes, dolphins, gray whales, killer whales, and West Indian manatees among others.
For some species same-sex genital contact is more common among females. Japanese macaques live in female groups having a single resident male. They are described as enthusiastic lesbians who mount each other frequently although this behavior is somewhat ambiguous among primates for whom sexual mounting serves as a dominance-submission display.
Among primates, perhaps the most pervasive, and most enthusiastic expression of homosexuality is found amongst bonobos (a species fairly closely related to common chimpanzees). Bonobos live in highly cohesive communities. Social bonds are maintained by a very high level of sexual interaction that include all possible pairings and many different positions. Males and females are bonded together and copulate frequently, sometimes face-to-face. But females frequently rub their genitals together. Males also engage in frequent sexual interactions. Bonobos are perhaps the most bisexual species of vertebrate on the planet. The bonobos do it all. For instance, males engage in highly unusual "penis fencing" that involves rubbing their erect penises together.
Frequent sexual interaction among bonobos defuse aggression and strengthen social bonds. Perhaps for that reason, bonobos are less aggressive towards each other than common chimpanzees are. Bonobo antics provide a fairly compelling argument against anyone who holds that homosexual behavior is "against nature."
Of course, bonobos are not exclusively homosexual, like some penguins, or humans. In the last post, I argued that exclusive homosexuality in humans can be partly explained in terms of genes on the X-chromosome that increase reproductive success for females who carry them. The penguins remain something of a puzzle.
Bisexuality is much more easily explained because bisexuals can form close bonds with either gender, that could provide practical advantages, as hypothesized for bonobos. Anthropologists describe bisexual conduct as common in many societies around the world and conclude that close same-sex relationships provide advantages in education, trade, and the making of political alliances (2).
1. Roughgarden, J. (2005). Evolution's rainbow: Diversity, gender, and sexuality in nature and people. Berkeley: University of California Press.
2. Kirkpatrick, R. C. (2000). Evolution of human homosexual behavior. Current Anthropology, 41, 385-413.