The basis of homosexuality remains complicated and controversial. Several researchers and writers who perceive homosexuality with positivity usually begin their discussions by implying that this sexual orientation is natural. Nonetheless, science has directed its attention toward other animals to explore how homosexuality occurs in the greater natural environment.
Bruce Bagemihl has been widely accredited for uncovering sexual diversity in nature. His 1999 review revealed that homosexual behaviour has been documented in about 500 species, ranging from primates to gut worms. He further noted that the animal kingdom demonstrates greater sexual diversity—including homosexual, bisexual, and non-reproductive sex. Since then, homosexual behaviours have been observed in about 1,500 species.
However, British-American neuroscientist Simon LeVay reminded that the existence of sexual behaviour in the animal kingdom remains rare considering that sexual behaviours leaning toward heterosexuality are significantly more frequent. Furthermore, Bagemihl also reminded that these observed homosexual behaviours in animals are often limited by human interpretations.
The use of homosexuality to describe the sexual behaviours of animals other than humans is also confusing to a certain extent. According to Volker Sommer and Paul L. Vasey, animals do not have sexual orientation and gender identities unlike humans. Their sexual behaviours are driven by sex alone. Thus, some researchers argue that homosexual behaviours observed in animals are not primarily motivated by sex but a socio-sexual purpose. In other words, some animals might be engaging in homosexual behaviours not because they are sexually pleasurable but because they have adaptive social functions.
In her piece published in the magazine Scientific American Mind, science writer Emily V. Discroll discussed several studies and literatures that investigated homosexuality in the animal kingdom. These literatures suggest that homosexual behaviours are natural part of the overall sexual behavioural repertoire of a particular animal species. Most of the time, animals that exhibited homosexual behaviours also exhibited heterosexual behaviours, thus making them bisexuals. Some believe that these observations are suggestive of the fact that bisexuality is a natural state among animal species, possibly including humans as well.
It is very important to take into account the aforementioned reminders. A study by Inon Scharf and Oliver Y. Martin involving 110 species of insects and arachnids revealed that apparent homosexual behaviours observed in active male species were a result of mere confusion. The passive or receiving males trigger this misidentification whenever they release sex pheromones or carry female pheromones that were attached to them during prior mating activities with female species. Scharf and Martin noted that insects and arachnids mate quickly and this result in greater predisposition toward misidentification. The cost to identify the gender of their mates or the cost of hesitations appears to be greater than the cost of making mistakes.
True homosexual behaviours observed in a significant number of species, especially in social animals, are still worth considering. Discroll has summed up several critical reasons why animal species apart from humans demonstrate varying degrees of homosexual behaviours.
First, some animal species might engage in homosexual acts to diffuse social tensions or maintain order in a group. Bonobos, for example, have exhibited high levels of promiscuousness. They are sexually active and about half of these activities involve same-sex partners. Some scientists believe that the frontally-placed clitoris of female bonobos have evolved to facilitate same-sex activities, particularly genital-genital rubbing. Male bonobos have been observed to mount, fondle, and perform oral sex on one another. Based on observations, these homosexual behaviours seem to ease social tension and promote social bonding. As mentioned, the more homosexuality, the more peaceful the species.
Some species of birds exhibit same-sex unions. Males tend to steal eggs from females and raise them with another males. Two male black swans, for example, are able to make better nests because they are bigger and stronger than male-female partnerships. Oystercatchers exhibit polygamous partnerships because of intense competition for male mates. Some oystercatcher nests have two females and one male. To promote bonding between the two females, they perform mounting. This polygamous trio produces more offspring because the nests are better tended and protected from predators.
Some studies suggest that homosexuality might be hardwired in several species. The study of wild fruit flies revealed that males carrying a particular mutation are prewired for both heterosexual and homosexual behaviours and either of these two is activated by external stimuli.
Homosexuality in some species appears to be far more common in captivity than in wild. Accordingly, the captivity effect influences the emergence of homosexual behaviours because of absence or lack of opposite-sex mates. This has been observed in penguins. Furthermore, the same behaviours might be a mechanism used to relieve stress as observed from koala bears. In domesticated cattle, homosexual behaviours in females signal sexual receptivity and in wild, they need not display this sexual readiness because they are free to roam and interact with males.
There is also a possibility that homosexual behaviours in human males are a result of being in similar environments that require easing of sexual tension, relief from stress, or promotion of social bonding. However, this should never be accounted as a sole cause of homosexuality in humans because sexual orientation and human sexuality are both complex. In addition, some studies suggest that humans have natural predisposition toward homosexuality because they too are also wired for bisexuality.
But exclusive homosexuality is somehow unique to human species to a certain extent, except from the fact that sheep have also demonstrated similar sexual behaviours. In his book, Aldo Poiani mentioned that the uniqueness of human homosexuality centres on having unique mental faculties that are absent from other species.
Nonetheless, homosexuality across several species, including humans and social animals both in wild and in captivity, might be a result of interplays between several factors and predispositions. These factors might include social benefits, environmental function and genetic mechanism, and biological including psychological and neurological mechanisms, among others.
Current research on homosexuality in the animal kingdom remains young and complicated. According to a book chapter by Paul L. Vasey, homophobia has played a role in dissuading researchers from studying this subject. During the 1990s, homophobia repressed the scholarly careers and research agenda of researchers. But apart from this, homosexual behaviours in the animal kingdom challenges prevailing understanding about animal behaviours. It is possible that the lack of research on this subject is due to the overall complications surrounding sexual behaviours.
Further details of the review of Bagemihl are in his 1999 book Biological exuberance: Animal homosexuality and natural diversity published by St. Martin Press. Discussions from LeVay appeared on his 1996 book Queer science: The use and abuse of research into homosexuality published by MIT Press. Further details of the discussion of Sommer and Vasey are in the introductory book chapter “Introduction: Topic, hypotheses, research trajectories” that appeared on the 2006 book Homosexual behaviour in animals: An evolutionary perspective published by Cambridge University Press.
More details of the study of Scharf and Martin are in the article “Same-sex sexual behaviour in insects and arachnids: Prevalence, causes, and consequences” published in 2013 in the journal Behavioural Ecology and Sociobiology.
Further details of the piece authored by Discroll are in the article “Bisexual species” that appeared on the magazine Scientific American Mind. Further details of the discussions and conclusions of Poiani are in his 2010 book Animal homosexuality: A biosocial perspective published by Cambridge University Press. Further details of the discussion of Vasey are in the book chapter “Where do we go from here, Research on the evolution of homosexual behaviour in animals” that appeared on the 2006 book Homosexual behaviour in animals: An evolutionary perspective edited by Sommer and Vasey and published by Cambridge University Press.