The discovery of ‘lesbian’ seagulls in California in the 1970s shook outdated beliefs that homosexuality was unnatural. Since then, scientists have documented cases of homosexuality in hundreds more species (Image Credit: JanBirdie, Pixabay licence)

Darwin’s work on evolution, natural selection and “survival of the fittest” is probably the most well-known scientific hypothesis out there.

Survival of the fittest means that the “fittest” have the highest reproductive success – whether that is achieved by roaring the loudest, building the most beautiful nest, camouflaging the best, or performing the most impressive mating dance. Passing on their genes to the next generation is what makes an individual successful in this context.

However, Darwin himself already realized that homosexuality could not be explained with this hypothesis – if mating success is the determining factor, how come homosexuality hasn’t gone extinct? 

Disclaimer: While we can observe the behavior of an animal, for instance same-sex sexual behavior, it is certainly problematic to anthropomorphize and label them “homosexual.” It is important to note that we cannot assume an animal’s sexual orientation from their behavior – we can only observe the behavior itself and search for possible explanations: improving social bonds, simply for pleasure, or to more effectively raise babies. Furthermore, my intention in writing this article is to simply counter the outdated and misguided belief that homosexuality is ‘unnatural’. If you feel I’ve mischaracterised your gender or sexuality, please let me know.

First and foremost, no sexual orientation is unnatural. There is such an amazing diversity of animal behavior and modes of reproduction, humans and their sexuality could come off as quite boring in comparison.

For starters, there are purely female species that reproduce asexually, for instance a species of lizard (Leiolepis ngovantrii) found in the US and Mexico that simply clone themselves. Komodo dragons are also capable of this, with the two lizard species making up what is thought to be only about 70 or so other vertebrates that can reproduce asexually.

Not all species are capable of such a feat, but many have found ways to minimise male involvement. There are ‘lesbian’ seagulls (famously the western seagulls studied off the coast of California) that have a one-night-stand with a male and raise the chick together. Of course, there are also intersex animals which possess both male and female reproductive organs. Especially in remote places like the deep sea, where you may not meet another member of your species for months, it’s very useful to have both male and female reproductive organs in case you bump into a potential partner. 

Lesbian Seagulls: The Discovery that Shook Up Conservatives’ Core Beliefs in the 70s

Speaking of the deep sea, often among fish we find an analogue of transexuality – sequential hermaphroditism – whereby a male or female will transition to one sex if there is a scarcity of that sex. A female sheepshead wrasse will transition to male if she feels there is space for one, and clownfishes can transform from male to female within weeks if the female of a school disappears (really changes the plot of Finding Nemo).

Other animals are not assigned a sex through their genes and develop it throughout their lifetime, determined by environmental factors. One such case is the European eel, which Sigmund Freud – yes, that one – spent many months dissecting until he finally found one specimen that had testes. Until that point, the theory on how eels reproduce was devised by Aristotle: through “spontaneous generation,” as nobody had seen a male eel and their reproduction was quite the mystery.

But back to that initial question: if success for a species means successfully passing on traits to your offspring, why does homosexual behaviour crop up so often? There are thousands of observed instances of homosexual behavior between animals, especially in other mammals and primates. There are many reports, but to list a few, it occurs in birds, such as swans and penguins, as well as mammals, such as bats, elephants and sheep, but also in reptiles and insects (we’ve linked the Wikipedia article below which contains an extensive list). Scholars have thought of many hypotheses as to how homosexuality has persisted both in animals and humans – and as it often is the case with science, there likely isn’t one definite answer, but rather a mix of many factors.

Homosexual Behaviour in Animals

While twin studies have tried to find genetic links to homosexuality in humans, no definite genome regions could be identified so far – but it is likely that there are a number of genes responsible, similar to eye color or height. This in turn means that an individual can inherit only some “homosexual” genes, but not express the homosexual phenotype, i.e. behave mostly heterosexual. Those individuals may express traits that increase their attractiveness towards the opposite gender, and their genes may be passed on more successfully, therefore keeping the “homosexual” genes in the mix.

Bi- and homosexual behavior is hypothesized to aid the wellbeing of the whole group, for instance by decreasing aggression. And while homosexual behavior may reduce the number of offspring an individual has, that does not mean their genes do not get passed on to the next generation. Helping out their siblings with child-rearing and foraging for others in the group increases the fitness of the whole group. Also, if male siblings compete for access to a female, homosexuality likely decreases conflict within the family and increases reproductive success for heterosexual brothers. 

Studying sexuality in our closest living relatives – primates – gives us insight into our own past. Homosexual behavior in bonobos seems to have a de-escalating function in social situations, especially during food sharing. It may aid bonding between individuals in a group and mediates after outbreaks of aggression. Other primates, for example Japanese macaques, seem to engage in same-sex sexual behaviors simply as the result of mutual sexual attraction. 

So, next time you overhear someone trying to argue that it’s “just not natural” to be anything but heterosexual, you can smile, safe in the knowledge that humans are, if anything, unnaturally boring in comparison to the full spectrum of sexuality out there in the rest of the animal kingdom. And if you’re feeling brave, point them to this article (and maybe also tell them that others’ sexuality is REALLY none of their business).

Eva Paulus is a marine biologist and very thankful for all the amazing LBGTQ+ people in her life, and for the support and proofreading of this particular article to make sure it’s as inclusive and supportive as possible. Follow her on Twitter @Deep_Sea_Dirndl.