The Evolution Of Homosexuality

June is LGBTQ+ month.

Thousands of species have been shown to exhibit some form of homosexual behaviour. It’s no rare phenomenon and should not be a taboo. In fact, zoologist Petter Bøckman says that “No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all, such as sea urchins and aphids. Moreover, a part of the animal kingdom is hermaphroditic, truly bisexual. For them, homosexuality is not an issue“.

This month, I will be giving examples of sexual fluidity in animals as well as examples of ancient human homosexuality. However this week, I wanted to start by asking the question ‘WHY has homosexuality evolved in the animal kingdom?’

 

 

Quick Definitions

Before I get to the sciency bit, I thought I would just explain some of the more complicated terms that I will be using. For those of you who may not be familiar with them or may just need to refresh your memory!

What is fitness? – the word fitness doesn’t only relate to how athletically fit you are, or how good-looking you are. When zoologists use the word fitness, they’re usually using it in the context of evolution. In this context, fitness is related to the passing on of one’s genes to the next generation. It can be directly – through the number of offspring. Or it can indirectly – through the genes that one shares with other related individuals. For a trait to evolve over time, it must have a high fitness level. For example, brightly coloured birds may secure more matings, and so pass on more genes and have higher fitness, leading to the evolution of birds with brighter colourations.

What is evolution? – the process by which an organism’s genetic and/or physical characteristics develop and change over time.

What is selection? – when an individual’s traits (such as height or aggressiveness) evolve over other traits because of their higher fitness benefits (how they affect survival and reproductive success). There are several types of selection that can act upon an individual’s traits.

Defining homosexual behaviour – In this context, homosexual behaviour may or may not involve a sexual act. It can include many interactions such as courtship, pair-bonding, co-parenting, play and dominance interactions, as well as sexual acts. The term homosexual behaviour is used in these studies to describe same-sex interactions that an animal exhibits throughout any point in its lifetime, even if the animal goes on to reproduce heterosexually. Therefore in this sense, bisexuality or sexual fluidity may be a more fitting term, as human society generally defines homosexuality as exclusively same-sex interactions.

Phew, now that we’re all caught up, let’s delve into the fascinating world of evolution and animal behaviour!

Does homosexual behaviour go against evolution?

Homosexual behaviour has often been considered a “Darwinian paradox.” This is because scientists have argued that sexual behaviour which does not result in offspring, would have a lower fitness level. So in theory, individuals that show homosexual behaviour would not pass on as many genes, and their genetic information would not be able to persist in the population and would eventually go extinct. It basically contradicts Darwin’s theory of evolution.

However, recent evidence suggests that homosexuality does, in fact, have evolutionary benefits.

In an article by Savolainen and Hodgson (2016), the authors counter the idea that homosexuality has negative fitness consequences.

Firstly, they explain that in social species, nonreproductive sexual behaviour is common even in heterosexual pairings. This is true in humans too, as females remain sexually receptive throughout nonfertile phases of their menstruation cycle, and in pregnancy too (Gangestad and Thornhill 2008). Savolainen and Hodgson (2016) go on to say that in humans and other primates, nonreproductive sex (including homosexual sex) is thought to have important social implications. This means that it may be disadvantageous in terms of fitness for one to display extreme heterosexuality due to reduced sociality. It would be equally disadvantageous to show extreme homosexuality too, due to a reduction in mating opportunities. If that is the case, sexual fluidity, or being somewhere in the middle of the sexuality spectrum may provide the optimum fitness.

Secondly, studies have suggested that homosexuality can be linked partly to genetics (more about that below). Savolainen and Hodgson (2016) explain the reasons why there might be a fitness advantage in individuals who carry a gene that increases the chance of homosexuality. They also say that homosexuality could be due to environmental variation, which may be unavoidable anyway.

Savolainen and Hodgson (2016) argue that even if homosexual behaviour has real negative costs to an individual’s fitness, this doesn’t make the behaviour unique or unusual. This is because there are many variable traits in humans that have fitness differences depending on their expression (such as physical attractiveness or tall stature – which are both linked to increased reproductive success). Yet the general public doesn’t ask why those short or unattractive traits exist. Sexuality isn’t binary, it is a trait that comes in a spectrum of forms, like attractiveness or height, and yet it has many negative associations. This negative focus on homosexuality might be largely driven by cultural taboos against the behaviour. (Savolainen and Hodgson, 2016).

Why has homosexuality evolved in the animal kingdom?

The paper by Savolainen and Hodgson (2016) then pulls together 5 different possible explanations for the evolution of homosexual behaviour in nature. These include:

• Kin altruism selection.

First of all, just so we’re clear, let me explain that ‘kin’ means individuals that are genetically related to one another (like your mum or brother). And ‘altruism’ is the behaviour of an animal that benefits another, at its own expense.

The kin altruism selection theory suggests that homosexual individuals forego their own reproduction in order to altruistically aid in the rearing or reproductive efforts of relatives (Kirkpatrick 2000). However, foregoing reproduction doesn’t explain why individuals would engage in homosexual behaviour. Furthermore, kin selection only works when the benefits to kin outweigh the costs to the individual. Yet the fitness cost of not reproducing is quite high, and so may not support this hypothesis.

• Overdominance selection.

The overdominance selection theory proposes that individuals who carry the ‘hypothetical’ gene allele for homosexuality as well as the gene for non-homosexuality (heterozygotes), will have increased fitness over individuals who carry two of the same allele (homozygotes). Although, there is currently little evidence to support this hypothesis. (Savolainen and Hodgson, 2016).

• Sexually antagonistic selection.

In sexually antagonistic selection, the hypothetical allele for homosexuality would result in an evolutionary cost when expressed in one sex, but would be counteracted by an advantage when expressed in the other sex. Therefore the allele would persist within the genetic population. In this scenario, it is hypothesized that the homosexual allele would be either ‘masculinizing’ or ‘feminizing’ (for lack of a better word) when expressed in either sex. So if it was ‘masculinizing’, the allele would increase the fitness of males, but decrease the fitness of females, or vice versa for ‘feminising’. Again, limited evidence is available that supports this explanation. (Savolainen and Hodgson, 2016).

Whilst I understand the concept, I’m not sure about the use of the words ‘masculine’ and ‘feminine’ in this explanation, as I don’t believe such things can be measured or quantified. I think feminity and masculinity are very subjective concepts.

• Paternal and maternal effects.

Epigenetic effects may also play a role in sexuality. Meaning, a mother can pass to her son ‘epigenetic marks’ that cause the development of a female’s sexual preference and results in a male homosexual. The same logic could be applied to the father of a homosexual daughter. (Savolainen and Hodgson, 2016).

Without getting too technical, how this works is that there are parts on the DNA called ‘epigenetic marks’ that can influence the sensitivity to androgen hormones of developing genitals and brains. So when the contradicting epigenetic marks are passed down through generations, a child could develop genitals that physically match the appropriate sex chromosome, but their brain development for sexual preference will be directed towards the opposite gender. (Savolainen and Hodgson, 2016).

• The Bisexual advantage.

Savolainen and Hodgson’s (2016) final suggestion as to why homosexuality evolved is the bisexual advantage. This is the idea that sexuality is a continuously variable and heritable trait that ranges from exclusively homosexual attraction to exclusively heterosexual attraction, and all areas in between. The bisexual advantage hypothesizes that the optimum trait value with respect to reproductive fitness is actually intermediate, in the bisexual range.

This is because they suggest that individuals with exclusive homosexual attraction would fail to reproduce and be selected against (no fitness), while individuals with exclusive heterosexual attraction would also have fewer offspring (lower fitness) because they are poor at forming social alliances (as we already know that nonreproductive sex has many social implications in the animal kingdom). Or because of another yet unknown reason. (Savolainen and Hodgson’s, 2016).

There are studies with behavioural as well as genetic support for the bisexual advantage hypothesis. You can be read about it in more detail in the original article here.

Final thoughts from me

I hope that wasn’t too confusing, personally I have found it fascinating to learn about!

I think it’s important to remember that not one single explanation can fit all. The reason why one species exhibits homosexual acts may not be the same reason behind another species. Still, it’s an incredibly interesting topic which has unfortunately been overlooked in the past due to the fear of publishing controversial information, or simply due to societies cultural ignorance and widespread taboos. Society has unsurprisingly been hesitant to admit that animals could be gay. Thankfully, lgbtq+ is becoming less of a taboo among the scientific community these days.

It’s no doubt that all traits, including behavioural ones, have evolved for a reason. Everything has a purpose or is the result of having a past purpose. A purpose that enhances one’s fitness. Therefore, I can without a doubt say that whatever the evolutionary reason is behind an animals homosexual behaviour (including humans), one thing that’s for damn sure is homosexuality is not, and never has been, a crime against nature.

Thanks for reading! Be sure to follow the blog in order not to miss another post!

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References:

Bawagan, J. (2019). Scientists explore the evolution of animal homosexuality. [online] Phys.org. Available at: https://phys.org/news/2019-05-scientists-explore-evolution-animal-homosexuality.html [Accessed 7 Jun. 2019].

News.bbc.co.uk. (2006). BBC NEWS | World | Europe | Oslo gay animal show draws crowds. [online] Available at: http://news.bbc.co.uk/1/hi/world/europe/6066606.stm [Accessed 7 Jun. 2019].

Savolainen, V. and Hodgson, J.A., 2016. Evolution of Homosexuality. Encyclopedia of Evolutionary Psychological Science, pp.1-8.

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Featured image: ‘ rainbow flag on white background : harvey milk plaza, san francisco (2012) ‘ by torbakhopper