Same-sex pairings have previously been observed in more than 1,000 species including dolphins and penguins, but now scientists say they also happen among worms, frogs, birds and many, many others.
The findings fly in the face of conventional views of how animal species thrive and are bound to ruffle the feathers of those who believe homosexuality is unnatural.
Evolutionary biologist Dr Nathan Bailey said: ‘It’s clear same-sex sexual behaviour extends far beyond the well-known examples that dominate both the scientific and popular literature – for example, bonobos, dolphins, penguins and fruit flies.’
Almost a third of Laysan albatrosses on the Hawaiian island of Oahu have been raised by two females.
The ‘lesbian’ pairs have got together to raise broods because there has been a shortage of male albatrosses.
They raise fewer chicks than heterosexual pairs, but their efforts are helping to restore the dwindling population of the birds on the island, the study says.
Other animal species tend to show homosexual pairings in lower proportions of about 10 per cent, the same fraction that has long been controversially claimed for humans.
A pair of male penguins recently hatched an egg at Bremerhaven Zoo in northern Germany after its biological parents rejected it.
Half the time male bottlenose dolphins have sex they are having it with other males, while for male bearded vultures same-sex encounters make up a quarter of all couplings.
The existence of homosexuality among animals inspired a museum exhibition three years ago in Oslo, Norway, which displayed photographs of male-on-male action among whales and giraffes.
But the scientists behind the new study argue that the reasons for same-sex coupling can vary from species to species, with some being effectively blind to gender while others practice ‘swinging’ as a social glue.
Dr Bailey, of California University in Riverside, Southern California, said: ‘For example, male fruit flies may court other males because they are lacking a gene that enables them to discriminate between the sexes.
‘But that is very different from male bottlenose dolphins, who engage in same-sex interactions to facilitate group bonding, or female Laysan albatross that can remain pair-bonded for life and cooperatively rear young.’
Writing in the scientific journal Trends in Ecology & Evolution, Dr Bailey and colleague Professor Marlene Zuk say homosexuality can also affect survival prospects for the worse ‘by removing some individuals from the pool of animals available for mating’.
Dr Bailey said: ‘Same-sex behaviour can have evolutionary consequences that are just now beginning to be considered. For example male-male copulations in locusts can be costly for the mounted male, and this cost may in turn increase selection pressure for males’ tendency to release a chemical called panacetylnitrile, which dissuades other males from mounting them.’
He said more work had to be done to understand how much homosexuality in animals is an inherited factor, and to determine any genetic reasons for it.