Why do some animals exhibit homosexual behaviour?

Once dismissed as a scientific anomaly, a growing body of research suggests that homosexual behaviour is surprisingly common throughout the natural world.

According to Petter Bøckman, Zoologist and Lecturer at the Natural History Museum in Oslo, ‘homosexual behaviour has been observed in over 1,500 species’ and the list includes ‘lions, wolves, deer, seagulls, elephants, monkeys, dolphins, a couple of frog species, some fish, a few insects and an octopus’.

For the majority of these species, homosexual activity is a rarity or case of mistaken identity, but for some, it’s a normal part of their lives.

On the surface, homosexuality in animals presents an evolutionary paradox. As it cannot result in offspring, it seems logical that the genes that cause it would die out through natural selection. However, there are several theories about why homosexual behaviour could actually benefit certain species, and none challenge evolution.  

Social bonding

Bonobos, our very sexually-active relatives Photo by Sergey Uryadnikov / Shutterstock
Bonobos, our very sexually-active relatives
Photo by Sergey Uryadnikov / Shutterstock

For animals that live in social groups, such as primates, pack carnivores and sea mammals, sex isn’t simply about reproduction; it’s about social harmony too. Non-conceptive sex, including both the heterosexual and homosexual kind, can be used to forge bonds between pairs or within a group, aid reconciliation or establish rank.

One particular species for whom sex performs an important social function is our close relative, the bonobo. Social order in female-dominated bonobo society is maintained by regular sexual contact between members of the same or opposite sex. Individuals caress or lick each other’s genitals, males engage in ‘penis fencing’ and females rub their genitals together in a variety of social situations, from greeting to easing tensions after a fight, and even when sharing food.

Paul L. Vasey, Professor of Psychology at the University of Lethbridge in Alberta, Canada, believes that, far from contradicting Darwin, bonobos’ sex lives may give them an evolutionary advantage.

He says: ‘Homosexual behaviour in animals has been categorised as an evolutionary puzzle, but if it helps animals to function better as a social group, then it also helps them to survive and ultimately reproduce.’

Sexual reward

Two macaques motivated by sexual reward Photo by pzAxe / Shutterstock
Two macaques very much being motivated by sexual reward
Photo by pzAxe / Shutterstock

Sex can be a risky business for many animals, carrying the threat of infection, confrontation and attack, and taking valuable time away from other tasks like finding food. It is not surprising then, says Vasey, that ‘animals are highly motivated by immediate sexual reward’. The motivation, he believes, may be so strong that it overflows into sexual activity with members of the same sex.

This certainly seems to be true for female macaques, who have been found by Vasey’s research to regularly mount each other, rub their genitals together and even defend each other from rivals. The females also mount males to encourage mating, and once they have learnt that this behaviour is pleasurable, it is natural for them to apply it to each other.

A strong sex drive may also explain why the males of some species engage in homosexual behaviour when the females are unobtainable. Typical examples, says Bøckman, include ‘outside of the mating season’ or ‘if the alpha male has an absolute hegemony on sex, leaving the lower ranking males no option but to enjoy each other’.

‘Animals don’t worry about sexuality and morality,’ he adds, ‘if they want to have sex they go for it’.

Rearing young

A nesting pair of Magellan penguins on an island in Chile Photo by Yevgenia Gorbulsky
A nesting pair of Magellan penguins on an island in Chile
Photo by Yevgenia Gorbulsky

For some species, it makes sense to pair up with a member of the same sex to rear young if a partner of the opposite sex isn’t available. The best known example of this occurs on the Hawaiian island of Oahu, where roughly a third of Laysan albatross nesting pairs are female-female.

Research by Professor Marlene Zuk from the University of Minnesota suggests that, in the absence of enough males, a female albatross may sneakily mate with a male from a male-female pair before bonding with another female to raise the resulting chick. Besides overcoming the male shortage, such behaviour allows these birds to get their eggs fertilised by the fittest males in the colony.

It’s not just females that form same sex partnerships to rear young. Male-male penguin pairs living in zoos have been known to successfully incubate adopted eggs. The most famous case is of male chinstrap penguin couple, Roy and Silo, who hatched and raised a female chick called Tango at New York City Central Park Zoo.

‘Under certain circumstances two males or two females are more successful at rearing chicks than their two gender counterparts, making homosexuality an important alternative reproductive strategy,’ says Bøckman.

Genetic basis

A Dorset Ram Photo by Molly Marshall
One in ten domestic rams demonstrate a lifelong preference for other males
Photo by Molly Marshall

Whether it helps animals live together, satisfy urges or raise young, homosexual behaviour usually seems to be a learned response, and the individuals that practise it could be more accurately described as bisexual, as they also seek out members of the opposite sex. However, there are animals that exhibit strict homosexuality, and in these cases, there may be a genetic explanation.

For example, around one in ten domestic rams demonstrate a lifelong preference for other males. Research has shown that the part of the brain that controls sex hormones—the hypothalamus—is half the size in gay rams than in their heterosexual counterparts, suggesting that their behaviour is biologically determined.

According to Vasey, we need to use the ‘sexually antagonistic gene hypothesis’ to understand what’s going on. The thinking is, that the same genes that lead to same-sex attraction in males, result in a useful evolutionary trait in females, such as increased fertility or sexual desire – for the opposite sex that is.

‘If research suggests that female relatives of homosexual males produce more offspring, this would solve the puzzle of how the gene for homosexuality could be selected for over time and not die out,’ he says.  

Whichever theory applies, it is evident that homosexual behaviour plays an important role in the lives of several species. And as technology allows scientists to observe wild animals ever closer, it is likely that more and more cases of homosexuality in the animal kingdom will come to light.