Same-sex behaviour is well-documented in more than 500 species of animals and birds. For as long as humans have lived, homosexuality has occurred in every culture. If the point of sex is procreation, why has evolution not weeded out non-productive sex?

The Laysan albatross is the epitome of monogamy, with the pair committed to each other for the long haul. But biologists discovered that about 60 per cent of the population in Oahu, Hawaii, is female, and during the breeding season, about 30 per cent of the pairs are female-female.

When there are too few males to go around, many females have no options to reproduce. A female albatross lays only one egg a year, and it takes two birds to incubate it successfully. Some females opt for an alternate strategy: They mate on the sly with males who already have partners, and incubate their eggs with their female partners.

These same-sex pairs perform typical courtship rituals: rubbing necks, kissing with their bills, and even mounting one another. In 2007, Brenda Zaun, a state biologist, reported that one same-sex relationship lasted 19 years on the neighbouring island of Kauai.

So is same-sex behaviour merely a response to a short supply of one gender?

Biologists speculate that female-female pairings in Laysan albatrosses continue even when the sex ratio evens out. The birds are the archetype of monogamy after all, even if the pairs are same-sex.

Male dolphins form brotherhoods, and goose each other to cement their relationships. The better they bond, the more effective their coalitions, and they enjoy better access to females.

Is it about improving one’s chances of mating with the opposite sex?

Much to the concern of sheep farmers, about eight per cent of all rams are homosexual, refusing to mate with ewes. These ram-preferring rams have a smaller hypothalamus, the part of the brain that controls reproductive functions, than heterosexual rams.

Although same-sex behaviour appears to be genetic in fruit flies, no gene for gayness has been found in mammals. Intriguingly, identical human twins share the same genetic material, but they don’t always share the same sexual orientation.

Some scientists say there is definitely a genetic bias as homosexuality runs in families. Gay men have more gay uncles than heterosexual men. However, this doesn’t hold true for lesbians.

An Italian study discovered the mothers, aunts, and grandmothers of gay men bore more children. A similar study of the Samoan families of fa’afafine, a tradition-sanctioned third gender of effeminate men, corroborated these findings. Whatever made the women good at producing babies appeared to modify the sexual orientation of some sons.

In December 2012, a team led by William Rice, an evolutionary biologist at University of California, Santa Barbara, theorised that epigenetic marks may hold the answer. Residing beside DNA, they direct how, when, and which genes switch on in response to the environment throughout one’s life. In the womb, epi-marks protect boys from underexposure and girls from overexposure to testosterone.

These switches are not typically hereditary, but sometimes, they get passed on. In such cases, girls inherit the marks from their fathers and become masculine, while boys receive from their mothers and become feminine.

Homosexuality may indirectly benefit other members of the family like increased fertility in women and appropriate hormonal controls in parents.

Other scientists wonder if homosexuality serves any purpose at all. Perhaps it rides piggyback on another beneficial adaptation that gets selected again and again, and homosexuality gets inadvertently chosen too.

There probably is no single explanation for a behaviour found in so many different creatures. Perhaps it is a result of gender shortage for some, male bonding, and genetic side-effect in others. Among dogs, not all same-sex mountings are sexual, but a show of dominance.

Most of our attempts to answer the paradox of homosexuality focus on the barrenness of same-sex. But many sexual acts are non-reproductive. Since sex also gives pleasure, to that end, homosexuality is no different from heterosexuality. Perhaps that’s all there is to it.