This month is LGBT History Month, an annual event which celebrates the lives of the LGBT community both past and present. The event helps draw attention to the ongoing work of organisations like Schools Out, which encourage teachers to give lessons on ‘significant’ gay people like Alan Turing, the forefather of modern computing whose work as a Cryptanalyst helped to defeat Nazi Germany. Alternatively, teachers might opt to take Science lessons based on research which shows that homosexual behaviour typically occurs in hundreds of species of animals. 
Any Science teachers who try to draw on Darwin’s work in this context will be rather disappointed with what they find. Despite dedicating his life to the observation of the natural world, no reports of homosexual behaviour appear ever to have been made by Darwin. This is surprising given that recent academic research shows overwhelmingly that “nearly all species” of animals exhibit some degree of homosexual behaviour both in the wild and in captivity.
Expert observations of Rhesus monkeys’ behaviour, for example – behaviour which Darwin studied very closely - has shown conclusively that both male and female members of the species demonstrate homosexual as well as heterosexual behaviour over the course of their lives. Thus, according to Professor Paul Vasey, same-sex Rhesus monkeys are typically “affectionate to each other, touching, holding and embracing” one another.
Darwin’s observations of Rhesus monkey behaviour were decidedly more hetero-normative, focussing exclusively on interactions between the sexes at the expense of interactions within single-sex pairings. As an animal of “the order to which man belongs“, Rhesus monkey courtship behaviour appeared to Darwin as identical to the human equivalent – a heterosexual, male-driven process in which “ornamental” females were selected according to their perceived physical attractiveness.
So why did Darwin and his contemporaries fail to observe the homosexual behaviour which naturally occurs in species like the Rhesus monkey? Part of the answer undoubtedly lies in Victorian Britain’s narrow definition of sexuality. While during the seventeenth and early eighteenth centuries sexual acts between people of the same sex were deemed “part and parcel of ‘normal’ sexual behaviour”, over the course of the late eighteenth and nineteenth centuries “natural” sexuality became more narrowly defined as productive, marital and thus heterosexual.
As a man who dedicated his life to analysing the selective forces which influenced the evolutionary process – from the weather, to geography, to physical strength and aesthetic adornments - one would expect Darwin to have taken a keen interest in the potentially critical evolutionary consequences of same-sex behaviour in animals and humans alike. In neither his public work nor private letters, however, did he describe sexuality as anything other than a heterosexual/reproductive phenomenon; “Sexuality”, he told Charles Lyell in 1861, should be defined as the uniting of “two elements” which “go to form the new being“.
In a world where homosexual behaviour was increasingly labelled “unnatural”, it might have been the case that Darwin simply did not see homosexual behaviour in the natural world – that he was somehow culturally blinded to its existence in nature. Alternatively, it might have been the case that as an aspiring respectable man of science whose work was considered by many to tread a fine line between “real” scientific endeavour and “indecent aestheticism”, Darwin lacked the freedom to observe, discuss and thus “naturalise” types of behaviour which had the potential to offend and alienate his middle-class audience.
While Darwin’s work does little to help teachers convey the complexity of human sexuality to their students, it nonetheless provides an extremely rich insight into the relationship between science and culture. Contrary to what we tend to be taught at school, it is impossible for even the most dedicated and “detached” of scientists to offer insights that are entirely objective and wholly a-political. In looking at Darwin’s work both as a piece of science and as a product of culture, students can learn valuable lessons about the subjectivity of what we “know” and about the ways and means by which heterosexuality has been constructed as the norm from which other sorts of sexuality diverge.
 For discussions of homosexual behaviour in the animal kingdom see Volker Sommer & Paul L. Vasey, Homosexual Behaviour in Animals, An Evolutionary Perspective, (Cambridge University Press, 2006), Bruce Bagemihl, Biological Exuberance: Animal Homosexuality and Natural Diversity, (London, 1999) and Frans De Waal & Frans Lanting, Bonobo: The Forgotten Ape, (California, 1998).
 There is a single reference to homosexual behaviour in animals in a rather distressing letter written to Darwin by Robert Swinhoe in 1865. Interestingly, in the letter Swinhoe interprets a violent dog fight as motivated by an innate homophobic instinct. There is no reply from Darwin that we know of, but it is significant that the issue of homosexuality does not feature in any of his discussions of dog behaviour in Variation, Descent or Expression.
 Sommer & Vasey, Homosexual Behaviour in Animals, (Cambridge University Press, 2006).
 See T. Hitchcock, ‘The Development of Sexuality’, English Sexualities 1700 – 1800, (London, 1997), p. 65. Generally speaking, while in the earlier period masculinity was measured by the quantity of sex a man had, by the early nineteenth century it was increasingly measured by its (heterosexual) quality.
 For a discussion of the cultural pressures under which Darwin laboured see G. Dawson, Darwin, Literature and Victorian Respectability,(Cambridge, 2007), introduction.