As one might expect, sexuality is a complex topic that often brings up many of those perplexing “why” questions. Not surprisingly, these age-old inquiries have been tackled by a variety of thinkers, each of whose ideas have been grounded in various schools of thought. It’s likely, though, that no single school of thought can explain human sexuality in totality, but rather contributes a small piece of the puzzle. What I’d like to do is to introduce our readers to the biological perspective of human sexuality. Even within this single framework, a variety of theories have been proposed. The perplexing problem when studying the biology of human thoughts, emotions, and behaviors is that we enter into the realm of the brain. While some phenotypes, such as a person’s blood type, have a clearly worked out and predictable biology, we still have much to discover when it comes to the human brain and behavior. Despite the long road ahead, we have indeed learned much about the biology of human sexuality. In this series of blog posts, I’ll summarize some of our major and recent discoveries in the field. The blog series I’m entitling “Gay, Straight and the Biology of Why” is inspired by Dr. Simon Levay’s “Gay, Straight and the Reason Why – The Science of Sexual Orientation”. Should you want to explore more on these topics this text is a great place to start. If you’d like to delve into the primary literature yourself, I’ll be including a link to recent articles at the end of every post.
The chapters that we’ll explore will include the following:
- Ch1: Evolutionary Psychology and Survival of the Fabulous
- Ch2: Do I look Gay in these Genes?
- Ch3: Oh Brother! Male Siblings and Birth Order
- Ch4: Hormones and Behaviour
- Ch5: Neuroanatomy
- Ch6: Penis Size & Other Anatomy
Evolutionary Psychology and Survival of the Fabulous
Evolutionary theory—or natural selection—is the idea that whatever traits lead to procreation and offspring are those that have the advantage to persist and live on (also known as fitness), while those traits that do not die out. By this logic, one could think that the 1.9% of men and 1.3% of women in the U.S that identify as “homosexual, gay, or lesbian” have no evolutionary fitness (1). Yet, evidence for homosexual behavior in humans has persisted across eras and cultures and is found widely across the animal kingdom as well. This begs the question: why has homosexual behavior survived in so many species for so long if it does not encourage reproduction? And is it possible that there is actually an evolutionary advantage to being gay?
To address these questions, I interviewed Amanda Brown, a behavioral geneticist with research experience in evolutionary genetics and behavioral ecology.
Do other animals in our kingdom display homosexual behaviors?
As mentioned previously, humans are not the only species in which homosexual behavior is observed: over 450 animal species have been observed to partake in homosexual behaviors such as mounting behavior, oral sex, and penetrative sex, as well as less overtly sexual behavior such as same-sex pair bonds (2). Some of these cases demonstrate clear evolutionary strategies, such as in species like flour beetles, where sperm ejaculated onto another male has a likely chance of being transferred to that male’s next female mating partner, or in the case of Laysan albatrosses where female-female pair bonds arise to raise chicks in response to a shortage of males (3,4). While these examples from nature are interesting, we hardly see men engaging in sex with each other or single mothers opting for lesbianism for these reasons in the human world.
On the other hand, some animals seem to engage in homosexual behavior for benefits that could be far more akin to our own. Other mammals, like macaques, bonobos, and dolphins have been observed to have sex for pleasure, and these couplings- both heterosexual and homosexual in nature- seem to play an important role in creating social bonds and providing comfort (5,6,2). However, in these cases, none of the animals have been observed to be exclusively homosexual, and still participate freely in heterosexual procreation. So while using sex for emotional support and bond building between members of a group could easily explain the evolutionary advantage of bisexuality, it paints an incomplete picture of how homosexuality could be naturally selected for.
In fact, other than humans, the only species currently shown to engage in exclusively homosexual behavior is domestic sheep. About 8% of male sheep in flocks have been observed to unanimously choose sex with other rams, even in the presence of available and fertile ewes (7).
So, why would such a high rate of homosexual sheep persist across flocks if they aren’t mating?
Contrary to popular belief, traits that are evolutionarily adaptive are not just limited to those that result in the formation of offspring. Any gene that results in a trait that helps perpetuate itself successfully into the next generation can be considered evolutionarily adaptive. While, yes, this often means passing on your genes directly to fertile offspring, it can also encompass any genes that result in traits that increase the chance that family members with your genes survive and mate. This type of natural selection is called kin selection: evolutionary strategies where the reproductive success of an individual’s relatives is increased, even if it’s at a cost to the individual’s reproductive success (8). This could be the case for the gay sheep, as neuroscientist Simon LeVay has suggested that the sisters of these sheep are more fertile and produce more offspring, counter-acting the reproductive output of their brothers and allowing them to persist (7).
And it’s not just the sheep: evidence suggests that similar forms of kin selection could be responsible for the continued existence of gay men. In 2004, one ground-breaking Italian study found that the maternal female relatives (sisters, mothers, aunts etc) of homosexual men had higher fecundity than women who only reported heterosexual male relatives (9). These women also have more advantages than just fertility, researcher Andrea Camperio-Ciani notes that they present with fewer complications during pregnancy and tend to have more extroverted and happier dispositions that make them more ideal mothers and partners. Interestingly, this effect was not observed for paternal female relatives of homosexual men, leading researchers to speculate that a gene on the X-chromosome could be (at least partly) responsible for both the increased fertility of female recipients and sexual orientation of male recipients. After all, a mother’s ability to pass on a gene that increases her daughter’s chances of attracting male partners and having healthier children could represent a net gain for her fitness, even if it decreases their son’s chances of producing offspring (9).
Is there such thing as the gay gene?
A DNA linkage study conducted in 1999 has even suggested an X-chromosome linked candidate gene for male homosexuality-—Xq28-—although their results have been difficult to replicate (10). Instead of there being one candidate gene, however, the researchers from the Italian study have suggested a model where more than one x-linked female-fertility gene may need to be inherited to result in male homosexuality. This explanation not only accounts for the difficulties in finding one causal “gay gene”, but also why homosexuality is so relatively uncommon even if it is linked to a common source of fitness (9).
But wait, is it possible that homosexuality may actually be adaptive?
The existence of gay men may create a “null” effect in the continuation of their family tree compared to the increased genetic output of their female relatives, allowing the homosexual phenotype to persist. But, some researchers have taken the line of evolutionary reasoning farther than that, even suggesting that the homosexual phenotype itself provides an evolutionary advantage to these families. In many bird species there exists a form of kin selection wherein non-mating “helper” birds increase the chance of survival of their relatives in large broods by providing care (such as feeding, grooming, and protection) associated with parents (11). Without the input of these helpers, a smaller percentage of the brood would survive that would not be offset by the helper mating and trying to raise their own offspring alone. This variety of kin selection increases “inclusive fitness” of the helper. That is, the number of surviving offspring produced not just by the individual themselves, but also through their efforts of rearing and supporting relatives. One could easily see how, in families where women have increased fecundity, a male relative who is less naturally inclined to reproduce would increase inclusive fitness (particularly if that motivates them to be active in the lives of their many nieces and nephews).
So what you’re telling us is that gay men make evolutionary “super uncles”?
The phenomena of the gay “super uncles” has been studied extensively in the Pacific Island of Samoa: where a third gender (called fa’afafine) consisting of biological males who are exclusively homosexual and display feminine gender expression, is culturally sanctioned (12). Fa’afafine are notable for altruistic investment in the rearing of their nieces and nephews, who they regularly babysit, tutor, and help support financially. These observations in Samoan culture have led researchers to investigate whether this investment was observed in other countries such as Canada, the US, and Japan. After all: how many families have a beloved gay uncle? However, empirical research has largely been unable to find evidence that gay uncles in these countries showed increased investment in their nieces and nephews compared to straight uncles, and some studies even found less investment due increased estrangement of gay men from their biological families (8). Perhaps, this result is due to the difference in the cultural acceptance and value placed upon fa’afafine compared to gay men in other cultures. If gay men can persist as a neutral to slightly “fit” force when balanced out by their fertile female relatives, could greater societal acceptance and appreciation for gay uncles actually make male homosexuality an actively evolutionarily adaptive trait? It’s entirely possible.
Are there any final things you’d like to leave us with?
Considering natural selection, when you look at the robust occurrence of homosexual behaviour in the animal kingdom, or the potential benefits of homosexual men (or at least their shared genes) to family pedigrees, it seems the gay liberation activists of the 1960s were really on to something when they declared that “gay is good”.
Sources:
- Copen, C. E., Chandra, A., & Febo-Vazquez, I. (2016). Sexual Behavior, Sexual Attraction, and Sexual Orientation Among Adults Aged 18–44 in the United States: Data From the 2011–2013 National Survey of Family Growth. National Health Statistics Reports, 88, 1-13.
- Bagemihl, B. (2000). Biological exuberance: Animal homosexuality and natural diversity. New York: St. Martins Press.
- Levan, K. E., Fedina, T. Y., & Lewis, S. M. (2009). Testing multiple hypotheses for the maintenance of male homosexual copulatory behaviour in flour beetles. Journal of Evolutionary Biology, 22(1), 60-70.
- Young, L. C., Zaun, B. J., & Vanderwerf, E. A. (2008). Successful same-sex pairing in Laysan albatross. Biology Letters, 4(4), 323-325.
- Clay, Z., & Zuberbühler, K. (2012). Communication during sex among female bonobos: Effects of dominance, solicitation and audience. Scientific Reports, 2(1).
- Vasey, P. L., Foroud, A., Duckworth, N., & Kovacovsky, S. D. (2006). Male–Female and Female–Female Mounting in Japanese Macaques: A Comparative Study of Posture and Movement. Archives of Sexual Behavior, 35(2), 116-128.
- Buckley, P. (2006). The Volume of a Sexually Dimorphic Nucleus in the Ovine Medial Preoptic Area/Anterior Hypothalamus Varies With Sexual Partner Preference. Yearbook of Psychiatry and Applied Mental Health, 2006, 304.
- Bobrow, D., & Bailey, J. (2001). Is male homosexuality maintained via kin selection? Evolution and Human Behavior, 22(5), 361-368.
- Camperio-Ciani, A., Corna, F., & Capiluppi, C. (2004). Evidence for maternally inherited factors favouring male homosexuality and promoting female fecundity. Proceedings of the Royal Society B: Biological Sciences, 271(1554), 2217-2221.
- Rice, G. (1999). Male Homosexuality: Absence of Linkage to Microsatellite Markers at Xq28. Science, 284(5414), 665-667.
- Nam, K., Simeoni, M., Sharp, S. P., & Hatchwell, B. J. (2010). Kinship affects investment by helpers in a cooperatively breeding bird. Proceedings of the Royal Society B: Biological Sciences, 277(1698), 3299-3306.
- Vasey, P. L., & Vanderlaan, D. P. (2008). Avuncular Tendencies and the Evolution of Male Androphilia in Samoan Fa’afafine. Archives of Sexual Behavior, 39(4), 821-830.