Homosexual behavior in animals has been puzzling zoologists since Darwin. If sex is simply for reproduction, the reasoning goes, animals that engage in it with those of the same sex are wasting energy and reducing their chances of passing on their genes. Vast efforts have been made to explain why same-sex sexual behavior (SSB) has been recorded in 1,500 species of animals and rising, but a new paper proposes everyone has been looking at the question the wrong way around.

As it has become less socially acceptable to use religious texts to justify condemning others’ personal lives, those seeking an excuse to condemn have taken to calling same-sex attraction “unnatural”. The enormous evidence for widespread SSB in animals shows this is untrue. Nevertheless biologists, whatever their stance on human sexual diversity, have struggled to explain its presence in animals, with theories like the "gay uncles" hypothesis and talk of mistaken identity or the “prison effect”.

Yale PhD student Julia Monk has provided another explanation. She questions whether heterosexuality really should be seen as the default, with everything else a deviation, as zoologists and non-scientists alike have assumed.

“We argue that the frequently implicit assumption of different-sex sexual behavior (DSB) as ancestral has not been rigorously examined, and instead hypothesize an ancestral condition of indiscriminate sexual behaviors directed towards all sexes,” Monk and co-authors write in Nature Ecology & Evolution.

Monk challenges the implicit assumption: “The costs of SSB are high… benefits must be even greater to account for its persistence.” Likewise, the paper points out that common “hypotheses seem to assume that SSB has independent origins in many animal lineages.” Ancestral animals are envisaged as exclusively engaged in DSB.

Instead, the authors propose: “The ancestral condition for sexual behavior in animals included both DSB and SSB, and that various evolutionary processes, adaptive or otherwise, have shaped the persistence and expression of SSB in different lineages, but need not explain its origins.” This, they note, is the way biologists explain other traits that are common across a variety of species with a common ancestor, yet it has not previously been applied to SSB.

“Put simply,” they write, “we are proposing a shift from asking ‘Why engage in SSB?’ to ‘Why not?’”

The paper notes sexual activity with no chance of leading to offspring is not restricted to SSB. Animals have been recorded having sex with “different species, dead bodies, inanimate objects [as well as] self-stimulatory behaviors,” the authors write. Often the animals performing the most SSB are also particularly into DSB – they're just horny, and not always that picky. Just because a dog is humping your leg doesn’t mean it loves you.

The paper includes a sidebar where the authors stress they have kept “a distinction between human attributes of gender and sexuality… and the scientific terminology of sex and sexual behaviors, which can be used to describe specific characteristics and traits observed in non-human animals.” Nevertheless, the tendency to view animal sex through the lens of human hangups about our own sexuality probably accounts for why this idea was not considered before.

“The notion that SSB has arisen convergently in so many different lineages only makes intuitive sense from a heteronormative world view in which ‘heterosexual’ behaviour is framed as the ‘natural order’ for sexually reproducing species, and ‘homosexuality’ is viewed as a recent aberration whose existence must be explained and justified,” the authors conclude. It's a view that seems natural to straight scientists coming from homophobic societies, but science should be about reconsidering such blinkers.