How Common Is Homosexuality in Animals?

Fruit Flies

Scientists have long been fascinated by the mating behaviors of the common fruit fly. The male members of the Drosophila melanogaster species engage in an elaborate courtship ritual, beginning with a courtship song played by extending and vibrating their wings.

The mating practice typically lasts about 15 minutes, but it’s the fluidity of the performance of sex roles that has researchers buzzing. Starting in the 1960s, geneticists found that they could modify the sexual behavior of fruit flies by manipulating specific genes. The genetically modified flies displayed drastically different sexual patterns, such as females engaging in active courtship, males becoming sexually passive, and male fruit flies attempting to mate with other males. 

Sheep

Researchers have found that as many as 8% of rams (male sheep) demonstrate sexual attraction to other rams. A larger percentage demonstrates attraction to both males and females. While researchers continue to examine why these differences in sexual behavior occur, they have made one significant discovery related to animal brains.

The difference occurs in a region of the brain called the anterior hypothalamus, where researchers identified the existence of what they called an “ovine Sexually Dimorphic Nucleus”, or oSDN.  A 2004 study found that the oSDN of male-oriented rams is, on average, smaller than those of female-oriented rams. The oSDN of the heterosexual rams also produced more aromatase, an enzyme that converts the hormone testosterone into an estrogen called estradiol. These findings present a potential pathway towards understanding the biological basis of sexual behavior in sheep.

Laysan Albatross

Scientists often point to the frequency of same-sex child-rearing among birds as a potential explanation for same-sex pairings across multiple species. There are, in fact, more than 130 bird species that engage in same-sex behavior, which researchers have concluded may have adaptive benefits. 

A total of 31% of laysan albatross belong to same-sex pairings (primarily female-female). Researchers suggest that female-female pairings increase fitness in colonies with fewer males than females, as the female birds can ensure their eggs are fertilized by the fittest males even if that male already has a partner and thus will not participate in raising the chick. 

Atlantic Molly Fish

Certain fish species have demonstrated same-sex attraction and mating patterns, including the Atlantic molly fish. A researcher at the University of Frankfurt found that female Atlantic mollies are more likely to mate with males who engage in the greatest number of sexual interactions, regardless of the genders of the male mollyfish's partners. Thus, the study concluded, male mollyfish could increase their reproductive fitness by interacting sexually with fellow males. 

Bonobos

Among bonobos, a great ape native to the Congo region in Africa, female-female sexual interactions account for around 60 percent of all sexual activity. Primatologists have long surmised that the exchange of sexual favors among same-sex and opposite-sex pairings serves functions such as settling conflicts, strengthening social bonds, and climbing the social hierarchy. 

A study conducted at Emory University concluded that some female bonobos engage in sexual activity as a strategy for improving their social status. The researchers found that, during sexual activity, lower-ranking females appeared to make louder 'copulation calls' whenever a dominant alpha female was nearby. They also made similarly loud vocalizations during sex if the partner was an alpha female, which served to signal their stature to the group. The study concluded that, among bonobos, sexual behavior serves social purposes beyond the act of reproduction.

Sources

• Bagemihl, Bruce. Biological Exuberance: Animal Homosexuality and Natural Diversity. St. Martins Press, 2000.
• Bierbach, D., et al. “Homosexual Behaviour Increases Male Attractiveness to Females.” Biology Letters, vol. 9, no. 1, Dec. 2012, pp. 20121038–20121038., doi:10.1098/rsbl.2012.1038.
• Clay, Zanna, and Klaus Zuberbühler. “Communication during Sex among Female Bonobos: Effects of Dominance, Solicitation and Audience.” Scientific Reports, vol. 2, no. 1, Jan. 2012, doi:10.1038/srep00291.
• Harmon, Katherine. “No Sex Needed: All-Female Lizard Species Cross Their Chromosomes to Make Babies.” Scientific American, 21 Feb. 2010, www.scientificamerican.com/article/asexual-lizards/.
• Roselli, C. E., and F. Stormshak. “Prenatal Programming of Sexual Partner Preference: The Ram Model.” Journal of Neuroendocrinology, vol. 21, no. 4, 2009, pp. 359–364., doi:10.1111/j.1365-2826.2009.01828.x.
• Roselli, Charles E., et al. “Sexual Partner Preference, Hypothalamic Morphology and Aromatase in Rams.” Physiology & Behavior, vol. 83, no. 2, 2004, pp. 233–245., doi:10.1016/j.physbeh.2004.08.017.
• Young, L. C, et al. “Successful Same-Sex Pairing in Laysan Albatross.” Biology Letters, vol. 4, no. 4, 2008, pp. 323–325., doi:10.1098/rsbl.2008.0191.