December 16, 2013
On the origins of sexuality

Homosexuality is a divisive, controversial topic. The controversy isn’t just confined to homosexuality of course, the same problems go for bisexuality, asexuality and transgender. Science and reason are prime vehicles for progress and it is through these I address controversial debates.


It is important to treat such issues with maturity

The most clear evidence suggesting a natural role for homosexuality is it’s existence across the animal kingdom. The earliest documented cases of animal homosexuality date back 2,300 years to the work of Aristotle. Up until now homosexual behaviours have been observed and documented in penguins, bats, seals, rats, walruses, sheep and chimpanzees, amongst other animals I can’t recall right now.

Research conducted in fruit flies by Grosjean et al in 2008 showed that specifically altering neuronal biochemistry (in particular, glutamate signalling) could render males ‘gender blind’. The resultant chemical change caused broadly bi-sexual behaviour to occur in male fruit flies. Of course, it is very important to note that expressing the same change in human brains would not have the exact same effects as happened in the fruit flies. Our brains contain greater complexity, it would seem. The principle the fruit fly study highlighted is the chemical nature of sexuality, based on interfering with basic neurochemicals and genes which are present in both flies and humans, and used for largely similar tasks.

Sheep are a prime animal model when studying sexuality. Rams have been characterised to naturally exhibit 4 sexualities; heterosexual, homosexual, asexual and bisexual. Males are generally studied as they are the active initiators of sexual interactions in sheep. Numerous studies have been conducted into this occurrence, suggesting that 8% of rams in a given population are ‘male oriented’, meaning they prefer to mount males instead of females.


Fus Roh Dah!

Understanding of how sexual behaviour is programmed into the mammalian brain has been unraveled through studies into rodents and rams. The earliest studies go as far back as the 1950’s where Phoenix et al demonstrated the presence of testosterone in guinea pigs could masculinise and defeminise the developing female brain. This study is not unique and more recent follow up studies in rats have since confirmed this. It is suggested there is a critical period in early development where hormonal feedback from the developing gonads ‘set’ the brain on its eventual course to sexual identity, which (from an evolutionary standpoint) should enable the organism to select a mate and continue the species. A study into female rats conducted 2011 (Henley et al, 2011, Hormones and Behaviour) showed that treatment of the developing rat brain with the oestrogen, oestradiol, influenced female rats to ignore sexually active males and spend time with other female rats, particularly those on oestrus.

The science underlying the hormonal effect on sexual preference explains that the developing testes release testosterone into circulation and back to the brain, resulting in the development of a male brain. The absence of testes, via the absence of the Y chromosome, would allow foetal development in the default female state. Altering this hormonal state could affect how the brain responds to hormonal cues. Elevating oestrogens in a male brain may influence some parts of it to feminise.

Studies in humans have shown differences in brain structure between hetero and homosexual males. Most notably in a region of the hypothalamus called the Suprachiasmatic Nucleus (SCN). This region is principally implicated in the body clock. This particular anatomical difference may simply be a consequence of another underlying process and not really anything to do with body clock function. A study by Swaab et al in 1995 showed an increased SCN volume in homosexual men (via post mortem examination of men who died from AIDS, it was concluded that AIDS did not affect SCN size). The study showed possible metabolic differences in the hypothalamus between homo and heterosexual men, though the study did not elaborate on why this occurred. Further studies by Swaab and colleagues induced this change in SCN size in rats, which subsequently grew up to exhibit bisexual behaviours. The results aren’t identical as the rats mated with females as well as males, but the important thing to note here is that male preference was generated in these rats (and there is the species difference to account for, too). The interesting aspect of this study was how they replicated the increased SCN size in the rats. This was achieved via interfering with developmental brain sex-hormone levels.


location of the SCN

In line with the aforementioned experiments on rats and rams, the prevailing hypothesis for formation of sexual preference in humans is that the sexual anatomy of the brain is likely influenced by the levels of sex hormones (testosterones and oestrogens) in the womb. We’re all mammals after all and share a common developmental biology. These hormone levels may fluctuate due to natural biological variation, the environment the mother is exposed to and a whole host of other factors. With homosexual males and females, the full development of a sexually different brain is likely NOT occurring. What appears to be happening is the masculinisation and feminisation of particular regions of the brain, so the person may retain other gender typical traits, but merely have a difference in preference. In humans, this is difficult to test fully at this stage.

 To throw another spanner in the works, the process is not completely straightforward as further sexual development occurs during puberty. It may be the case that the sexual development in the womb may predispose a certain sexual preference, but perhaps environmental and psychological input into the developing adolescent brain may also steer where the sexual preference ultimately rests. Finally, there is the inference that due to varying levels of sex hormones influencing brain development in the womb, sexuality may lie on a continuum. It may not just be strict categories of hetero – homo – bi etc. It may well be a gradient, or an assortment of sexual behaviours, dependent on developmental circumstance.

Atypical sexual preference may not simply be a persistent evolutionary accident (though it may have started out as one); such behaviours have been suggested to have useful social roles in social organisms.

 A lot more study needs to be done to provide more conclusive evidence in humans, but the results so far are promising. The wealth of findings from numerous laboratories from across the world are in conflict with old fashioned schools of thought that the atypical sexualities are unnatural, immoral choices. Perhaps it is time information on the science is disseminated more widely. I can see why such information isn’t widely distributed as it can be difficult to digest. Furthermore, the fabled ‘silver bullet’ hasn’t arrived for the science of sexuality, perhaps putting it low on the radar of news-science reporting. But science doesn’t always wait for a magic study which suddenly validates a field. It is often a long, arduous accumulation of evidence, peppered with criticism and reason, but ultimately relies on a consensus. Some science does get that single defining study, but at this point the science of sexuality is left wonting.


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