Homosexual and bisexual activity between animals has been well documented, with more than 1,500 species recorded in engaging in same-sex sexual behaviour.

But despite the large body of evidence, evolutionary biologists have struggled to explain what has become known as the “Darwin Paradox” – why are these behaviours so common when they result in no opportunity for species to reproduce.

And why, when animals have evolved over millennia, has same-sex sexual behaviour repeatedly evolved and persisted?

Researchers from the Yale School of Forestry & Environmental Studies suggest instead of examining the issue as a conundrum in need of a solution, the question ought to be reframed from “why do animals engage in same sex behaviour” to “why not?”

Writing in the journal Nature Ecology & Evolution, the authors suggest that these behaviours may actually have been part of the original, ancestral condition in animals and have persisted because they have few — if any — costs and perhaps some important benefits.

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“We argue that the frequently implicit assumption of [different-sex sexual behaviour] as ancestral has not been rigorously examined, and instead hypothesise an ancestral condition of indiscriminate sexual behaviours directed towards all sexes. By shifting the lens through which we study animal sexual behaviour, we can more fruitfully examine the evolutionary history of diverse sexual strategies.”

Lead author and F&ES doctoral candidate Julia Monk said: “We propose a shift in our thinking on the sexual behaviours of animals.

“We’re excited to see how relaxing traditional constraints on evolutionary theory of these behaviours will allow for a more complete understanding of the complexity of animal sexual behaviours.”

In the past, the researchers say research into species’ sexual behaviours has rested on two assumptions.

The first is that same-sex behaviour has high costs because individuals spend time and energy on activities offering no potential for reproductive success.

And the other assumption has been that same-sex behaviours emerged independently in different animal species and evolutionary lineages.

“If any trait other than homosexuality had been observed in such a diverse array of species it would be widely accepted as being part of our ancestral DNA rather than something that evolved later,” said Ms Monk.

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

They argue a combination of same-sex sexual behaviours (SSBs) and different-sex sexual behaviours (DSBs) is an original condition for all sexually producing animals — and that these tendencies likely evolved in the earliest forms of sexual behaviour.

The authors suggest not only that same-sex behaviours are often “not costly”, but can in fact be advantageous from a natural selection perspective because individuals are more likely to mate with more partners.

Many species aren’t inherently monogamous but instead try to mate with more than one individual. In many species it can be difficult for individuals to even discern between different sexes.

“So, if you’re too picky in targeting what you think is the opposite sex, you just mate with fewer individuals. On the other hand, if you’re less picky and engage in both SSB and DSB, you can mate with more individuals in general, including individuals of a different sex,” said co-author Max Lambert, a postdoctoral fellow at the University of California-Berkeley’s Departmental of Environmental Science, Policy, and Management.

He added: “So far, most biologists have considered SSB as extremely costly and, consequently, something that is aberrant.”