El homosexual behavior in animals It has been documented for decades, and yet it continues to generate heated debates both in the scientific community and among the general public. For a long time, it was considered a rarity, a kind of curious anecdote that researchers overlooked, sometimes due to pure personal bias, other times for fear of being questioned by their colleagues. Today, with thousands of observations accumulated across dozens of species, the picture is quite different.
Far from being a marginal phenomenon, same-sex sexual behavior appears in birds, mammals, reptiles, insects, and even aquatic invertebratesAnd in many cases, it serves very clear functions: reducing tension, cementing alliances, practicing for future matings, or simply obtaining pleasure. Evolutionary biology, ethology, and neuroscience have had to work hard to explain how all this fits into the theory of natural selection, and the truth is that the answers that are accumulating are becoming increasingly solid.
What do scientists understand by “homosexuality” in animals
The word “Homosexual” was born in 19th-century Prussia to describe attraction and sexual behavior between people of the same sex. Applying it as is to other species is problematic for two reasons: firstly, because in animals we cannot ask about desires, identity, or orientation; secondly, because the term carries an enormous cultural weight in humans that simply does not exist in the rest of the animal kingdom.
That's why, in biology, we often talk about “sexual conduct between individuals of the same sex” or SSB (same-sex sexual behavior). This label is operational: it is used when copulation, mounting, genital stimulation, courtship games, displays, or even ejaculation are observed between two animals of the same sex, repeatedly and in a context where heterosexual interactions also exist.
Sexual motivations in animals can only be inferred Based on what is observed—there are no interviews or self-reports—scientists are cautious when speaking of “homosexual animals” in the human sense. What they do is describe whether an individual shows a stable preference for same-sex partners, whether the behavior is sporadic or frequent, and what social or reproductive consequences it has.
For decades, many naturalists ignored these behaviors or reinterpreted them as “play,” “domination,” or “confusion,” especially when they clashed with their own prejudices about sexuality. Now it is recognized that The same actions can have several functions at the same timeSexual contact can be pleasurable, affirm a bond, and at the same time reinforce hierarchies within the group.
In modern literature it is considered correct to say that an animal “exhibits homosexual behavior” when, in an observable way, it engages in sexual activities with individuals of the same sex, without that implying attributing to it a fixed orientation as we do in humans.
Why same-sex sex is an evolutionary challenge
From the perspective of natural selection, anything that doesn't contribute to producing offspring seems, at first glance, like a dead end. The logic is simple: for a trait to be maintained Generation after generation, behavior must favor, directly or indirectly, the transmission of genes. If a behavior increases the probability that an individual will mate with a fertile female and produce viable offspring, that behavior will tend to persist.
To explain this apparent contradiction, biologists have proposed several hypotheses. One of the most influential is the one that sees these behaviors as a adaptive social toolSex between individuals of the same sex would help reduce aggression, forge alliances, reconcile after a conflict, or consolidate bonds that, in the long run, increase the survival of the group and the indirect reproduction of genes.
Another proposal suggests that certain genes or hormonal environments that favor homosexuality in some individuals may trigger fertility or sexual desire in othersIn other words, the same genetic basis that causes a minority of males to have a preference for other males could make many females more receptive or fertile, more than compensating for the reproductive "loss" of those males.
In any case, the major current comparative analyses agree on one key point: Homosexual behavior is not randomly distributedIt is more common in lineages with complex societies, high levels of interaction, and often greater internal conflict, which fits well with the idea that it would act as a social glue.
What major studies in mammals and primates say
One of the most striking recent advances comes from a team of CSIC and several Spanish universities, which analyzed the evolution of homosexual behavior in mammals. They examined records in hundreds of species and concluded that same-sex sexual behavior occurs more frequently in species with intense social life within stable groups.
According to that study, this type of behavior has already been observed in about 5% of all mammal species and in approximately half of the families. Furthermore, males and females participate with similar prevalence. In approximately 40% of the species where it appears, it is not a simple isolated incident: it occurs moderately or even frequently during the breeding season.
The study detects a clear association between sociability and sex among individuals of the same sexHighly social species are much more likely to exhibit these types of interactions, in both males and females. This supports the idea that sex, beyond fertilization, is a tool for building and maintaining relationships within the group.
A striking relationship was also found between this behavior and the Intrasexual violence in malesIn species where males are particularly aggressive toward one another, sexual contact between males is more likely to be observed at some point in their lives. Homosexual behavior, in this context, could serve as a release valve to mitigate tensions and reduce the risk of serious injuries that would impair the reproductive capacity of all.
In non-human primates, another large meta-analysis published in high-impact journals compiled data from 491 species and documented same-sex sexual behavior in 59 of them. Acts such as mounting, genital stimulation, and even ejaculation were taken into account, and it was assessed whether the behavior was repeated in a population or anecdotal.
SSB in primates: sex as social glue
By cross-referencing this data with a battery of ecological and social traits, the researchers found that SSB is more common in primates with highly complex societies, marked hierarchies and shifting alliances. It is also associated with species that have strong sexual dimorphism (large size differences between males and females) and long lifespans, where maintaining stable relationships for years is key to survival.
The environment is also not irrelevant. Sexual behavior between individuals of the same sex is more likely to be observed in habitats dry, with scarce resources or high predator pressureIn such scenarios, maintaining the group's internal cohesion and minimizing conflicts can mean the difference between life and death, and sex becomes another tool in that game.
Bonobos are a textbook example: they use sex to channel virtually any social tension, including frequent contact between females and between malesBut they are not the only ones. Macaques, chimpanzees, vervet monkeys, and several lemurs show similar patterns, suggesting that this solution has appeared time and again in different branches of primates.
The authors of these large studies insist that they are not claiming that every specific homosexual encounter has a direct benefit. Rather, the idea is that in complex social contexts The sexual repertoire expands and is reused for various functions: reconciliation, alliance, practice, negotiation…
They also emphasize that the conclusions are based on observable behaviors in non-human primates. Human sexuality, shaped by culture, moral norms, and identities, is a different world. Directly comparing what a bonobo does with what a gay person feels is an oversimplification, although it does help to dismantle the myth that homosexuality is "against nature."
Bonobos, macaques and other primates: sex for more than just reproduction
In the bonobosAmong chimpanzees, close relatives of the pygmy marmoset, sex is practically everywhere. Females engage in genital contact with each other very frequently, and males rub their genitals together (the famous "penis fight") after fights or tense situations. Approximately 60% of the species' sexual activity involves two or more females, and much of it has no direct reproductive function.
In their matriarchal society, these contacts serve to reduce aggression, consolidate coalitions and climb the social ladder without resorting to potentially lethal fights. It's a clear example of how homosexual sex can stabilize social life and, in the long run, benefit the entire group.
The Japanese macaques They are another much-studied case. In some populations, homosexual behavior among females is not only common, but has become the norm during certain periods. They form same-sex "consort pairs" characterized by affectionate contact, repeated mounting, and play, and in some groups up to a quarter of the females maintain these types of bonds for days or weeks.
Paul Vasey's research showed that females explore more postures and movements with other females than with males, which suggests a conscious pursuit of pleasure. But that doesn't imply that they are "lesbians" in the human sense of the term: they also copulate with males frequently and, in fact, may use those encounters to encourage them to mate more.
In many primates, same-sex sexual contact arises precisely from behaviors that have previously developed in heterosexual contextsOnce the repertoire has been learned, it is reused with other members of the group to strengthen relationships, calm aggression, or explore hierarchies.
Sheep, rams and the role of the brain
One of the few cases in which a stable preference for same-sex partners has been described is that of the domesticated sheepIn some flocks, approximately 8% of rams prefer to mount other males even when fertile females are available. This indicates a true long-term homosexual preference.
Neuroscientist Charles E. Roselli and his team studied the brains of these animals and discovered a region, the sexually dimorphic ovine nucleus (oSDN)This was approximately twice as high in rams that preferred females as in those that preferred males or in ewes. Furthermore, the activity of the aromatase enzyme, which converts testosterone to estradiol, was also higher in heterosexual rams.
These results suggest that the differences in brain anatomy and biochemistry These traits are linked to mate preferences in these animals. The oSDN of homosexual rams showed intermediate levels of aromatase expression, suggesting that the ability to synthesize estrogens in this region could modulate typical male sexual behavior.
From an evolutionary perspective, it remains a mystery why this pattern persists. Simon LeVay has proposed that the same genetic or hormonal factors that cause a minority of rams to be homosexual could to increase fertility or receptivity to mating in femalesIf that positive effect on sheep is strong enough, it would compensate for the direct loss of offspring from homosexual rams.
Interestingly, this stable preference has been documented primarily in domestic sheep selected for producing many breeding females. It is not so clear that the same occurs in wild populations, which reinforces the idea that domestication and human selection They could have altered the frequency of this trait.
Lions, dolphins and other large mammals: more than a “game”
The lions They also exhibit homosexual behaviors, both males and females. Males have been observed pairing for days, caressing, mounting, and performing pelvic thrusts, although without visible penetration or ejaculation. Some popular articles have attempted to downplay these behaviors, claiming they are "not sexual" but simply rituals of social bonding or dominance.
However, from a rigorous ethological perspective, the fact that contact reinforces bonds or hierarchies does not make it any less sexual. The fact that a male does not ejaculate does not imply that the behavior lacks an erotic component. Animal sexuality is not limited to reproduction nor to end in ejaculation, just as in humans there are multiple sexual practices without a reproductive purpose.
In the bottlenose dolphinsMales form “alliances” of two or more individuals that cooperate to court or control the movement of females for days or weeks. Within these alliances, numerous sexual games between males have been observed: mounting, genital rubbing, and prolonged contact. Studies led by Janet Mann have shown that strong personal relationships between juveniles, often cemented through sex, later translate into highly effective adult alliances to protect themselves and to access females.
On Amazon River Dolphin (Boto), groups of 3 to 5 individuals are described, usually young males and some females, that engage in group sex in multiple combinations. Stimulation is carried out with the snout, fins, and genital rubbing. They even practice homosexual and heterosexual penetrations of the blowhole, the respiratory opening equivalent to the nostrils, resulting in the only documented case of "nasal sex" in the animal kingdom.
Both in the African elephant as in the Asian Affectionate bonds and homosexual copulation are known. Encounters between males are often accompanied by clearly affectionate gestures: intertwining trunks, kissing, placing a trunk in the other's mouth. During the interaction, one of them rests his trunk on the other's back and pushes forward, an unmistakable sign of mounting intent. In captive Asian elephants, it has been estimated that up to 45% of sexual contacts They are between individuals of the same sex.
From hyenas to ferrets: matriarchies, hormones and domination
La spotted hyena It is a paradigmatic case of how biology can break our usual categories. The females possess a unique, elongated urogenital system that closely resembles a penis, leading early naturalists to believe they were hermaphrodites. The social structure is matriarchal, and dominance relationships include a marked sexual component, with same-sex mounting in both males and females.
Recent research has shown that females are more masculine in appearance, larger in size, and considerably more aggressive than males. They have been observed to be “masculinized” by high levels of testosterone during gestation, without losing female reproductive traits. This extra aggressiveness and bulk seems to give them an advantage in the competition for resources and mates.
In these societies, individuals with higher prenatal testosterone levels tend to mount other members of the group, including those of the same sex, while those exposed to fewer androgens adopt more submissive behaviors. Thus, homosexual behavior intersects with hierarchies, hormones, and resource access strategiesand cannot be reduced to a single explanation.
El European polecat (Mustela putorius) It also exhibits homosexual relationships between unrelated males, including mounting and anal penetration. Given that it is a solitary species without a complex social structure, and that these behaviors do not appear to provide clear adaptive benefits, it has been suggested that they could be a byproduct of other traits, such as hormonal sensitivity or the way in which the sexual response is activated.
In the case of domestic dogsHomosexuality is widely documented. Mounting between males, often with obvious erections, is relatively easy to observe. In many cases, it is interpreted as playful or dominance behavior, but that does not eliminate the sexual component: it is, once again, a multifunctional behavior.
Bison, giraffes, and other herbivores
On American bison Courtship, mounting, and complete anal penetration between males have been documented. Some Indigenous cultures of North America, such as the Lakota, had specific terms to refer to these animals with "two-spirit" behaviors, such as pte winktewhich reflects that they had been observing these behaviors for centuries.
Among the giraffesIt has been described that up to nine out of ten observed pairings occur between males. For a long time, this fact was overlooked or reported very discreetly in studies, probably due to the taboo associated with the topic. Ethologist Petter Bockman has pointed to this case as a classic example of how observer bias can literally erase part of the natural behavior of animals.
In both cases, homosexual behavior appears to be related to mating behavior testing, tension reduction and strengthening of ties between competitors who, under other circumstances, could end up dangerously clashing.
Birds: swans, ducks, penguins, vultures and pigeons
Birds offer some of the best-known and most publicized examples of same-sex pairing. In the black swansIt is estimated that around a quarter of the pairs consist of two males. These pairs may steal eggs from other nests or form temporary trios with a female to lay eggs, after which they expel her and remain in charge of incubation and raising the chicks.
Paradoxically, chicks raised by two males have greater chances of surviving Even in adulthood, male-female pairs tend to outlast males, probably because two males collaborate better in defending territory and accessing resources. In terms of fitness, these same-sex pairs can fare quite well.
In many ducksSexual activity rates among males are unusually high, reaching up to 19% of all pairings in a population in some cases. Since males typically abandon the female once she has laid eggs, much of their sexual energy is channeled into non-reproductive interactions, including mounting between males.
The penguins They have repeatedly made headlines. At the Central Park Zoo in New York, a pair of male chinstrap penguins formed a stable bond, built a nest, and successfully raised a female chick from an egg provided by staff. Zoos in Japan, Germany, and other countries have documented similar situations: male-male pairs that They build nests, incubating stones as if they were eggs and they appear indifferent, or even unreceptive, when females are introduced to them.
At the Bremerhaven Zoo in Germany, an attempt was made to separate several gay chinstrap penguin pairs by importing females from Sweden to encourage breeding. The experiment failed: the male pairs maintained their bond, and the zoo director admitted that the relationships between these males were too strong to be broken so easily.
The griffon vultures They have also provided striking examples. At the Biblical Zoo in Jerusalem, two males named Dashik and Yehuda built a nest as part of a breeding program. Keepers provided them with an artificial egg to incubate and later replaced it with a real chick, which the two males raised together. At another German zoo (Allwetter, Münster), two male vultures repeatedly built their nest, which other vultures stole, until they were separated in an attempt to pair one of them with a female—a decision that drew criticism from LGBT groups.
Parthenogenetic reptiles: sex without males, but with mounting
Among reptiles, some lizards of the Teiidae family They reproduce by parthenogenesis: populations consist almost entirely of females, which can produce offspring without fertilization by a male. In theory, no sexual behavior is necessary for reproduction to occur.
However, it has been observed that these females engage in sexual behaviors among themselves, with mounting and courtship rituals very similar to those observed in other species between males and females. Females experiencing a cycle with lower estrogen levels adopt “Masculine” roles in interactionwhile those at a high estrogen peak play the “feminine” role.
The most interesting thing is that the lizards that perform these rituals have higher ovulation and fertility rates than those kept in isolation. Sexual stimulation between females triggers certain hormones that favor the production of viable eggs, so, evolutionarily, the behavior makes sense even without males involved.
A similar pattern has been observed in some species of parthenogenetic geckos, reinforcing the idea that sex, even in populations without males, can still fulfill key physiological functions.
Insects and other invertebrates: useful mistakes, genes and “traumatic sex”
In insects and arachnids, homosexual behavior has been the subject of intense debate. One of the best-known hypotheses maintains that many of these interactions are due to identification errorsMales will mate with anything that comes their way because the cost of making a mistake (mounting another male) is less than the cost of missing an opportunity with a receptive female.
In some species of beetles (Tribolium o DarknessIt has been observed that males mount other males and deposit their sperm. If the "recipient" male later copulates with a female, he can transfer some of that other male's sperm to her. In this way, a male can... reproduce indirectlywithout investing time or energy in courting the female.
In the fruit fly, Drosophila melanogasterThe genetic basis of homosexual behavior has been thoroughly studied. Mutations have been identified that cause some males to exhibit courtship behavior almost exclusively toward other males. These genes act by modulating pheromones, neural circuits, and brain structure, and experiments have also explored how the environment (temperature, population density, previous experiences) influences the emergence of these behaviors.
Among the dragonflies and damselfliesResearchers have inferred the presence of homosexual copulation from the marks left by the males' copulatory claws on the females' heads. In a study of 11 species, between 20 and 80% of the males showed these characteristic marks, indicating a surprisingly high proportion of male-male mounting, likely resulting from misrecognition or the intensity of sexual competition.
bed bugs (Cimex lectularius) They take things to another level with their famous "traumatic insemination." Males pierce the recipient's body with a saber-like penis and inject sperm directly into the body cavity. Approximately half of these copulations occur between males, 30% with females, and 20% with individuals of other closely related species.
When a male is penetrated by another, the sperm survives inside him, migrates through the circulatory system to the vas deferens, and mixes with his own sperm. In this way, when that male then penetrates a femaleIt transmits both its own genetic material and that of the person with whom it had homosexual relations. It is another twisted form of indirect reproduction that demonstrates the extent to which evolution can be creative.
Pollutants and changes in sexual behavior
Not all homosexual behavior is necessarily the result of spontaneous adaptive processes. In the American white ibisExposure to methylmercury, an environmental contaminant, has been shown to increase the prevalence of male homosexual behavior. In controlled experiments, chickens exposed to increasing doses of this compound showed, as adults, higher levels of sexual interactions between males.
Methylmercury acts as endocrine disruptorinterfering with the normal function of the hormonal system. It has been suggested that similar blockages or alterations could affect sexual differentiation and behavior in other birds and vertebrates, opening the door to investigating how human pollution is modulating animal sexuality in nature.
The weight of prejudice and the limits of human labels
Throughout history, many researchers have treated animal homosexuality as a taboo subject, whether because lack of knowledge, discomfort, or fear of ridiculeThis has led to cases like that of giraffes, in which most of the observed mountings were between males, being barely mentioned in classical literature.
The recent debate about “gay lions” is a good example of the tensions that still exist. Some experts have claimed that these behaviors are not sexual in nature, but merely a form of dominance or social bonding; others have even gone so far as to suggest that perhaps the animals were not actually two males. These explanations, when used to deny the sexuality of the behavior itself, stem from project human categories of gender and orientation about animals that simply don't fit into our mental categories.
Two male lions mounting each other doesn't make them "less male," just as a female mounting another female doesn't cease to be female. The key is accepting that Sex in animals serves very diverse functions. and that it's okay to acknowledge that these acts are simultaneously sexual, social, and sometimes reproductive.
In humans, talking about “homosexual,” “bisexual,” or “heterosexual” involves a whole social construct linked to identity, desire, community, and rights. In other animals, the labels have to be more descriptive and less ideological: we simply describe what they do, with whom they do it, and in what contexts, and from there we try to understand why evolution has allowed those behaviors to persist.
Ultimately, what all this data shows is that Sexual diversity is a normal part of animal behaviorFar from being a rarity or a deviation, same-sex sex appears time and again as a social tool, an outlet, and, in some cases, a means of indirect reproduction. Acknowledging this complexity helps us not only to better understand other species but also to question many of the myths we still hold about our own sexuality.