Monkeys do it. Birds do it. Even Giraffes do it! Animal homosexual behaviour is a lot more prevalent than you might think.
Birds, reptiles, mammals and even snails engage in same-sex sexual activities. Homosexuality among animals is just one of a number of sexual behaviours. It's widespread, and in some species, frequent.
In the animal kingdom, male-female monogamy is in the minority. Animals just don't have Victorian morals. They engage in polygyny (one male and several females), polyandry (one female and several males), promiscuity, where both sexes have multiple partners, threesomes, 'moresomes', and cheating spouses.
"There is a whole variety of arrangements and different ways of having sex in the animal kingdom," said Dr Geoff MacFarlane, biologist from the University of Newcastle.
Homosexual behaviour has been scientifically observed in almost every kind of animal. Worldwide 450 species have been described as engaging in sex with same-sex partners.
MacFarlane searched through the scientific literature for reports of animal homosexuality. He stuck to a conservative list of same-sex behaviours, notably genital rubbing and mounting. Although these two alone sound more like the activities of a one night stand than a steady relationship, it also ensures sex means sex, and other behaviours such as grooming aren't included.
McFarlane found that same-sex behaviour is more common in primates and other mammals, but is also found in birds, reptiles, amphibians, fish, insects and other invertebrates. And it isn't restricted to wildlife, it's also common in domestic animals.
In some domestic animals, especially rams, it can cause problems. Farmers pay to have their ewes serviced and a randy ram more interested in its fellow mates is not helping the economy.
Most same-sex behaviour described in the literature occurs in mammals. Giraffes live in all male groups and only associate with females to reproduce. Around 80% of the interactions between giraffe males are sexual, usually mounting behaviour. Male lions also mount in about 8% of interactions.
Primates especially engage in complex homosexual relations. Over thirty species of primate have been documented engaging in same-sex coupling, some forming long-term bonds.
Bonobos use sex to dissipate aggression and maintain social cohesion. Not only are more than 50% of their sexual interactions same-sex, between both females and males, but same-sex coupling also tends to last longer.
Over 130 bird species, including that terribly pink creature - the flamingo - spend part of their lives in steady same-sex relationships.
"Flamingos are a good example" says MacFarlane "Six percent of them actually form male-male pairs that engage in all the same things a male-female couple will engage in, including courtship and parenting. They may actually take over a nest and raise the young."
Suprisingly, male waterbirds from same-sex couples are often more successful than their heterosexual counterparts, as they are more aggressive and can occupy more territory, he said.
Amongst Roseate tern couples, one third are both females. They pair together and raise young, sitting on double the amount of eggs. Both birds lay eggs, and often at least some of these are fertilised, indicating that the females are happy to sit on both sides of the fence.
In Australia, over forty species of birds have been recorded in the scientific literature as engaging in homosexual behaviour. One lyrebird study found between eighty to ninety percent of lyrebird mating displays were directed towards immature males.
The tendancy towards same-sex partners can be ongoing too. A behavioural study of a galah population by Professor Lesley Rogers from the University of New England found that amongst some galahs, the preference for same-sex relationships remained after their mate died, even when the size of the flock and the ratio of males to females changed.
Why? Evolutionarily speaking...
Darwinian evolution says that animals evolve because particular traits develop that give the animal a reproductive advantage. These traits are passed on to the next generation, which again develops traits that if reproductively successful are passed on to their offspring.
Historically, biologists have had a variety of explanations for homosexual behaviour in animals. These have included a case of mistaken identity; 'playful' activity; or practise for the real thing.
Other suggestions were that homosexuality arose because a lack of females or males meant there weren't enough heterosexual couples. It was also thought possible that homosexual behaviour was zoological behaviour - the result of putting animals in captivity.
"When you are confronted with animals engaging in same sex activity it falls outside the tenet of what natural selection is all about," said MacFarlane. Homosexual activities do not lead to reproduction, so it's difficult to fit in with that cornerstone of biology, Darwinian evolution.
One of the latest theories on the block is the 'pleasure principle' puts forward by Paul Vasey from the University of Lethbridge in Canada.
Vasey tested a number of the pervading ideas about the evolutionary significance of homosexual behaviour in a study on Japanese macaques. He found that up to 33% of sexual activity between Japanese macaques is female-female. To explain this, he formulated an idea that no one had suggested before: the monkeys engaged in homosexual sex for pleasure.
People are now becoming more interested in these hypotheses and are more open in looking at the adaptive significance of homosexual behaviour, MacFarlane said.
Learned or innate?
Like many other forms of animal behaviour, there is a complex interplay between the roles of nature and nurture. And it's not likely that any single theory holds for all species, MacFarlane said.
One of the explanations offered by the 'nurture' side is that homosexual behaviour is about dominance and hierarchy. For example, a lot of both play and homosexual behaviour goes on between male and younger male dolphins in Shark Bay in Western Australia, and in captivity.
According to Luciana Möller from Macquarie University, this is "probably based on dominance and hierarchy".
On the other hand, 'nature' or biological theories look to elevated hormone levels occurring either before or after birth.
Cornell University researcher Elizabeth Atkins-Regan exposed pregnant female finches to an oestrogen-synthesis inhibitor. The females of the chicks that hatched from these eggs preferred other females. But the same species, when deprived of fathers, produced male chicks that grew up to prefer males.
Also on the nature side are controversial genetic theories behind homosexuality in humans and homosexual behaviour in animals, including attempts to finding a gene for homosexual behaviour.
Many scientists have debated the genetic contribution to sexual orientation, including U.S. scientist Dr Dean Hamer, who discovered a greater incidence of the gene Xq28 in homosexual men.
Hamer and his colleagues identifed a gene potentially linked to same-sex courtship in male fruit-flies. When inserted in male fruit-flies from the genus Drosphila, this gene caused the flies to increase same-sex behaviour from 8% to 90%.
But rather than putting in a 'homosexual gene', MacFarlane suggests this process may have just removed the fly's ability to discriminate between males and females.
Few scientists are yet to commit to studies on homosexual behaviour, and most observations come about in the course of other research. Until these studies are done, whether the scientific jury remains in or out on the question of animal homosexual behaviour, there's no doubt that the animals themselves are content to remain 'out'.
Published 16 September 2004