Far from being "unnatural," homosexuality is well-documented across the animal kingdom.
In the middle of the Pacific Ocean, two Laysan albatrosses meet on the Hawaiian island of Oahu. They perform their courtship dance, bobbing their heads, puffing up their chests and extending their wings and necks in a way that only the really attractive birds can. Like all the rest, this pair are embarking on a lifetime partnership. Together, they will rear a new chick almost every year, and may be lucky enough to live into their sixties. They are also both female.
This might seem a little unusual, but it's not worth a glance on Oahu. Here, 31% of the Laysan albatross pairs are female-female.
Homosexuality is not something we generally consider as part of the natural world. After all, we measure the biological ‘fitness’ of an individual animal by its reproductive success. How could homosexual pairings or behaviours possibly result in high reproductive success?
Yet homosexual behaviours occur across the animal kingdom. Bonobos engage in various homosexual acts, including kissing, fellatio and genital rubbing; 8-10% of domestic rams show a preference for male partners; and female Japanese macaques mount each other regularly.
So why are these behaviours relatively unexplored and unexplained? Homosexuality was observed in animals as early as 1911, and was recognised as a “basic mammalian capacity” by 1951, yet work in the area seems to be limited, even now.
It's easy to attribute this to social and cultural aversions; homosexuality is sometimes termed an ‘unnatural’ behaviour by political and religious groups, so it's likely that these views previously blinded people to the behaviour occurring in animal systems.
Indeed, even today, starting purely scientific conversations about homosexual behaviours in non-human animals can be incredibly difficult. Research on an animal may be extrapolated by the media and lobby groups and inaccurately applied to human sexuality.
This works both ways. The liberal media may anthropomorphise a male-male penguin couple with a pro-gay marriage ethos in mind. On the other hand, conservatives may use research showing that social experience or ‘gene misexpression’ can trigger homosexuality to delegitimise human homosexuality. Both parties can use objective research to justify emotional or political tirades that often only act to fuel stereotypes.
This complex and emotive societal issue may have discouraged researchers from exploring homosexual behaviours in the past – or, at the very least, discouraged them from publicising it. Even today, the controversy, combined with the power of social media, causes issues for researchers; but it can also provide a chance to educate the public.
“[The controversy] draws in a larger audience which is great, but it also has the potential to be sensationalised," said Lindsay Young, Vice President and Executive Director of Pacific Rim Conservation. "Despite this, it presents an exceptionally compelling opportunity to convey the message that same-sex behaviour can be viewed scientifically, without a social or political agenda, and to be clear about the limitations of extrapolating from animal to human behaviour.”
There are also more practical reasons behind the lack of research into animal homosexuality. For example, in animals where males and females are not easily distinguished, sexual behaviours between two individuals may automatically cause the observer to assume the two individuals are of opposite sex.
While this assumption may be due to expected societal ‘norms’, it is not biologically unreasonable. Reproduction is the predominant function of sexual behaviour, so it's logical to assume that two animals engaging in these behaviours are of the opposite sex. Additionally, while homosexual behaviours have been observed in many thousands of species, they do not appear to be frequent behaviours.
“In most species, homosexual behaviour rarely occurs,” said Paul Vasey, a professor of psychology at the University of Lethbridge. “This makes it difficult to study. Long periods of observation are necessary just to get a few observations, which might amount to a single publication.”
Since zoos provide an easier way to observe animals over a long period of time, many documented homosexual behaviours are seen in captive animals. However, this produces a confounding factor whereby we don’t know if the behaviour is truly one that would occur in a natural population.
“There are so few examples of [homosexual behaviours] occurring in the wild that not many inferences could be made that excluded human influences," said Dr Young.
In species where homosexual behaviour is relatively common, as in Japanese macaques, researchers face the additional challenge of accurately interpreting these behaviours.
“[The] theoretical frameworks for interpreting such behaviours are nowhere near as developed as they are for reproductive sex,” said Prof Vasey. “This leaves researchers… scratching their heads about what the behaviours mean.”
There are two broad strategies for determining why an animal behaves as it does. The ‘proximate’ cause is the immediate, instant cause of the behaviour. This generally incorporates the animal's environment and its motivation for acting, either through hunger, fear, sexual arousal or any other feeling or emotion relevant to the particular situation.
The ‘ultimate’ cause of behaviour is the advantage that the behaviour provides to the species as a whole. It views the behaviour as an adaptation to the species’ long-term environment and life history.
For Japanese macaques, homosexual behaviour appears to provide no evolutionary benefits, no ‘ultimate’ cause. While homosexual interactions do have an impact on alliances within macaque groups, their principal function is for pleasure. Therefore, the motivation or ‘proximate’ cause of the individual is seen as the triggering factor: in this case, a desire for sexual satisfaction or enjoyment. “The animals engage in the behaviour simply because it is pleasurable,” said Prof Vasey.
Of course, a behaviour can be motivated by pleasure while still serving an evolutionary function. Some social species, such as bonobos and bottlenose dolphins, engage in homosexual genital rubbing and mounting, which helps foster a strong group bond. “For [these] species… homosexual behaviour serves various social functions such as alliance formation and reconciliation,” said Prof Vasey.
In these species, a strong social structure is a key feature of their life history. It provides safety in numbers, increases foraging ability, and provides an environment where young can learn over time from the experience of their elders. Without this social stability, the survival and successful reproduction of the individuals would be compromised.
Perhaps counter-intuitively, homosexual behaviours can also assist directly in improving an individual's reproductive fitness. This brings us back to our Laysan albatross pair.
In cooperatively breeding species, individuals help to raise offspring that are not their own. The reproductive benefit from this is usually due to kin selection ‒ animals that help raise their siblings or cousins are passing on their genes in an indirect way. Yet our same-sex albatross pair are unrelated. How could this relationship benefit their reproductive output?
On the island of Oahu, the unusual number of female-female pairs appears to result from a skewed sex ratio. In a study published in 2008, Dr Young and her colleagues found that the island population was 59% female due to female-biased immigration. This situation left many females without males to pair with. These females could still mate with a male, but would not have the support of a lifelong partnership.
By pairing together, two females could still provide the care necessary to raise a chick, and by alternating whose egg was incubated each year both individuals would benefit. Female-female pairs did raise fewer chicks than male-female pairs, but they were better off than not breeding at all.
Successful mothers in female-female pairs had a 4% chance of transitioning to a male-female partnership in the following year, while no unsuccessful female-female mothers did even if males were available. Dr Young and her co-author Eric VanderWerf suggest this is due to the skewed sex ratio allowing males to select only the best quality females, leaving lower quality females to partner with each other or not at all.
It is difficult for us to distinguish through pure observation whether a sexual relationship between two animals is for pleasure, reproduction or due to an attraction to a specific sex. We may know that two female albatrosses pair for life, but we cannot know if they do so out of mutual attraction or out of necessity.
Female macaques that mount each other still engage in sexual behaviours with males, causing some to describe this as bisexual behaviour rather than homosexual. Yet the apparent motivation of the macaques appears to be sexual pleasure rather than attraction to either or both sexes.
“Genital stimulation has been documented during some same-sex interactions,” said Prof Vasey, “and this strongly suggests that immediate sexual reward is a proximate mechanism motivating the behaviour.”
Very few species have been found in which individuals preferentially mate with the same sex. 8-10% of domestic rams display a preference for a male partner, and refuse to mate with a female. This makes them one of the few known species with exclusively homosexual individuals. However, this still doesn’t explicitly prove that truly homosexual animals exist in the wild.
Homosexual behaviours are particularly interesting to study in animals, since they seem paradoxical with our idea of evolution and the necessity of increasing reproductive success. "Studying such behaviours provides a more complete picture of how behavioural evolution works,” said Prof Vasey.
This knowledge can be pivotal when it comes to wildlife management and conservation work. Dr Young said, “same-sex [Laysan albatross] pairs have much lower reproductive success, which we had incorrectly attributed to predators. For conservation, knowing the cause of a poor reproductive outcome is critical to determining how to manage it.”
Without recognising and studying homosexual behaviours, we run the risk of missing interesting and sometimes vital relationships and mechanisms within our natural systems. By continuing to observe and carefully interpret these behaviours, we can achieve a greater understanding of the life history of differing species and further explore the complex process of evolution.
Edited by Andrew Katsis and Ellie Michaelides, and sponsored by Mike McRae