Currents

Same-sex behavior has been documented in over 1,500 species to date. Faced with this reality, academics and the general public alike have long struggled to explain a so-called “Darwinian Paradox”: how same-sex behavior can be so prevalent when it presumably must come at a huge evolutionary cost — no offspring. New research out of Imperial College London offers intriguing answers and challenges many widespread preconceptions about sexual orientation. The study, published in Nature Ecology and Evolution, combines observational and genetic data to form what Imperial College describes as “the first long-term study of same-sex behavior in males within one species” — in this case, rhesus macaque monkeys. In doing so, the study has poured cold water on a number of long-held misconceptions that have prevented us from better understanding bisexuality and sexual orientation.

Building on older investigations of female macaque monkeys in Japan, researchers studied 236 adult males within a colony of 1,700 rhesus macaques living freely on the tropical island of Cayo Santiago, Puerto Rico. They found that a whopping 72% of the males engaged in same-sex behavior and that all of them, except for one individual, were observed mounting females as well. Even more eye-popping than the fact that almost three-fourths of the male monkeys are bisexual is the fact that the assumed evolutionary paradox did not prove to be true at all. Instead, same-sex behavior was a predictor of more offspring, not fewer.

The team approached their data and assessed the strength of several popular theories about the evolutionary purpose of same-sex behavior: establishing dominance in groups, shortages of different-sex partners, a response to unusual environmental conditions, and reducing tension following aggression. They found that, for this colony of macaques at least, none of those explanations held up. Instead, same-sex behavior in males was strongly correlated with “coalitionary bonds.” That is, the more often two males bonded sexually, the more likely they were to support each other during conflicts within the group, providing them both with an advantage.

The rhesus monkeys on Cayo Santiago are one of the most studied populations of animals in the world, with data going back to 1938 and the pedigree of every individual recorded back to 1956. The team used that pedigree data and found that bisexuality in male macaques was 6.4% heritable — marking the first genetic link in primate same-sex behavior discovered outside of humans. As a side note, a preference for mounting or being mounted (what humans would call topping or bottoming) was also found to be inherited, paralleling previous findings in humans.

"Unfortunately there is still a belief amongst some people that same-sex behavior is 'unnatural', and some countries sadly still enforce the death penalty for homosexuality,” said lead researcher Vincent Savolainen in the study’s press release. “Our research shows that same-sex behavior is in fact widespread amongst non-human animals.”

Upon reviewing the study, psychologist Lisa Diamond, who specializes in the study of sexual orientation, remarked, “Research results like these highlight how common beliefs about what’s normal in nature, including human nature, may not be scientifically accurate or complete.”

“This would imply that what’s ‘unnatural’ may really be our societal norms and expectations, not bisexuality itself,” added John Sylla, President of the American Institute of Bisexuality, which co-funded the Imperial College study.

Indeed, the general public hears about same-sex behavior in nature and still imagines “gay” animals. But bisexuality in nature is vastly more common than homosexuality. The only animal species in which a significant percentage of the population is known to engage in exclusive same-sex behavior is the domesticated sheep. Approximately 8% of rams show a clear homosexual orientation, alone among the 1,500 species found to exhibit same-sex behavior. The balance of scientific research on non-human animals strongly suggests that bisexuality is, by definition, quite natural.

All of this begs the question: What can we infer about humans on the basis of animal data? Researchers urge caution about drawing direct comparisons, but the discovery of both a genetic link and reproductive advantage of bisexuality in primates has clear implications for our species. At the very least, it provides a plausible pathway to explaining the possible evolutionary roots of same-sex behavior, which has long baffled scientists as a seemingly maladaptive trait, one they assumed should get weeded out by natural selection.

As it happens, we don’t need to rely on animal data to begin to understand human bisexuality. We have sex research going back nearly a century that attests to bisexuality being not only prevalent but possibly even the norm. From Li Shui Tong’s research in the 1920s and 30s to the Kinsey Reports of the 1940s and 50s, to the work of Fritz Klein, J. Michael Bailey, the aforementioned Lisa Diamond, and an extensive and growing body of polling and surveys, a legitimate case can be made that a majority of humans could actually be bisexual. About 58%, in fact, by some rough calculations. Public attitudes, however, continue to lag far behind the science, and bi erasure remains rampant.

As the data piles higher and higher, when will we stop reflexively discussing sexuality in terms of “straight and gay” with no mention of bi? When will people stop assuming that everyone in same-sex (or for that matter, opposite-sex) relationships is either gay or straight? When will stereotypes about bisexuality as unnatural, “a phase”, or a stepping-stone to being gay finally crumble once and for all under the colossal weight of evidence to the contrary? To be sure, attitudes on this front have been gradually improving over time, but they still have a long way to go. In light of everything, our collective mind-block on all things bi seems like something out of the Twilight Zone.

In my own experience as a “professional bisexual,” I cannot begin to tell you how many people that appear straight or gay to all the world ask for confidentiality, and then get a load off their chest by coming out to me as bi: A once-powerful C-suite woman in New York, now a grand- and great-grandmother, revealing her intimate relationships with other women when she was younger; the “queeny” gay man who professed his love of performing cunnilingus; a prominent gay activist who boasted he hooks up once or twice a year with women when he’s on the road; the conservative man who waxed nostalgically for his college roommate-slash-lover; one half of a “lesbian power couple” who confessed that she could just as easily have settled down with a man. In general, when I mention to people how much data there is to suggest a high prevalence of bisexuality, they are rarely shocked. The most common reaction I hear is, “Yeah, that makes sense.” It’s as though we’ve known all along on some subconscious level just how pervasive bisexuality is, but for some reason we need it pointed out to us over and over.

In the end, while building ever greater mountains of high-quality data can benefit us in ways both big and small, research on the behavior of rhesus monkeys isn’t going to be what finally tips the scales of how we talk about sexuality. We need to examine the ways we interpret the world without baking in false assumptions that lead us off course from the get-go. The way people have struggled to find an answer to the supposed “Darwinian paradox” of same-sex behavior reveals a great deal about the side effects of bi erasure. The reality is simple: Whether human or animal, most individuals who engage in same-sex behavior are not “homosexual”; they are bi. In our animal relatives, bisexuality has now been shown to be a heritable, evolved trait that increases reproductive success. From a reproductive and evolutionary standpoint, bisexuality may be beneficial for humans as well, in the absence of social stigmas. When we forget about bisexuality, there are fundamental truths about life that we cannot understand. Why did it take us over 100 years of sex research to figure that out?

Published Aug 2, 2023