For the first time, humpback whales spotted having sex: Why homesexuality is ‘natural’ in the animal kingdom

Homosexuality is not uncommon in the natural kingdom. Its incidence has been reported in “over 1500 animal species” ranging from tiny insects and spiders to reptiles, birds, and mammals. It has been observed in both males and females, and in both captivity and the wild. (J M Gomez, A Gonzalez-Megias and M Verdu, “The evolution of same-sex sexual behaviour in mammals” in Nature, 2023).

A Darwinian paradox?

There is mounting evidence to suggest that biological and genetic factors are major drivers behind homosexual behaviour. While there is no one ‘gay gene’, and environmental factors too play a role, a 2019 study in Nature found that five genetic markers to be “significantly associated” with same-sex behaviour in humans. This, however, creates a paradox.

The very basis of Charles Darwin’s theory of evolution lies in the fact that the goal of all organisms, and thus of evolution itself, is reproduction i.e. the passing down of one’s genes to the next generation. Same-sex sexual behaviour does not serve this purpose. Instead, it actually diverts scarce resources and time into activity that actively precludes organisms from reproducing.

Thus, as per Darwin’s theory, the biological/genetic basis for homosexuality should then have been weeded out from the gene pool. But that is simply not the case. What then explains the prevalence of homosexuality?

The ‘indiscriminate mating’ hypothesis

There are many hypotheses which try to explain same-sex sexual behaviour in animals. One, which has gotten a lot of attention in recent times, is the “indiscriminate mating” hypothesis.

According to it, “the ancestral animal species mated indiscriminately with regard to sex … if only because it is unlikely that the other traits required to recognize a compatible mate — differences in size, shape, colour or odour, for example — evolved at exactly the same time as sexual behaviours”. (A Kamath, C E McDonough, Julia D Monk, Max R Lambert, and Erin Giglio, “An alternative hypothesis for the evolution of same-sex sexual behaviour in animals” in Nature, Ecology and Evolution, 2019).

The authors further point out that “mate recognition can require physiologically and cognitively costly adaptations, and being excessively discriminating in choosing mates can lead individuals to miss out on mating opportunities that lead to reproduction”.

Thus, according to this hypothesis, the present-day diversity in sexual behaviour in animals stems from an ancestral background of indiscriminate mating among individuals of all sexes.

Maintaining social bonds, avoiding conflict

Other hypotheses, however, attempt to explain homosexual behaviour in animals in terms of its adaptive functions.

Evolutionary biologists N W Bailey and M Zuk first hypothesised that “same-sex sexual behaviour contributes to establishing and maintaining positive social relationships”. (“Same-sex sexual behaviour and evolution” in Trends in Ecology and Evolution, 2009). Consequently, such behaviour is more common in social rather than non-social species.

Gomez, Gonzalez-Medias, and Verdu’s 2019 study found evidence for this hypothesis. They wrote that there is a “direct correlation” between the incidence of same-sex sexual relations and sociality, and that it “has been favoured evolutionarily as a way to establish, maintain and strengthen social relationships that may increase bonds and alliance between members of the same group.”

Bailey and Zuk also suggested that same-sex sexual behaviour contributed towards “diminishing intrasexual aggression and conflict”. Such interactions, they hypothesised, “may serve to communicate social status and establish and reinforce dominance hierarchies, thus preventing future conflicts, or may contribute to diverting aggressive behaviour toward courtship behaviour”. Consequently, same-sex sexual interactions should be more common in species with aggressive, often lethal, intrasexual relations.

This too was supported by the evidence gathered by Gomez, Gonzalez-Medias, and Verdu. They found that such behaviour “mitigates” adulticide (killing of other adults of the same species), but only among males. “This is so because adulticide in mammals seems to be the consequence of intrasexual conflicts only in males, whereas it seems to be displayed by females primarily to protect their progeny against infanticidal conspecifics,” they wrote.

Notably, they found that “rather than a maladaptive or aberrant behaviour, same-sex sexual behaviour in mammals is a convergent [evolving in multiple species, independently] adaptation” with specific survival benefits.