Charles Darwin argued that sexual preferences can shape the progress of evolution, creating displays, such as the peacock’s tail, that are inexplicable by natural selection alone.

It’s safe to say, however, that he did not anticipate the lesbian albatrosses of Hawaii. Nor bisexual bonobos. Let alone sadomasochistic bat bugs or the gay penguins of New York.

Homosexuality is so widespread among some animal species that it can reshape their social dynamics and even change their DNA, according to the first peer-reviewed survey of research on the subject.

From mammals to snails, and even nematode worms, homosexual behaviour is almost universal across the animal kingdom, and Californian scientists argue that it should be considered a selective force in its own right.

“The variety and ubiquity of same-sex sexual behaviour in animals is impressive — many thousands of instances of same-sex courtship, pair bonding and copulation have been observed in a wide range of species, including mammals, birds, reptiles, amphibians, insects, molluscs and nematodes,” write Nathan Bailey and Marlene Zuk of the University of California, Riverside.

Animals engage in same-sex activity for a variety of reasons, ranging from the need for an alternative child-rearing strategy to mistaken identity. “Male fruit flies may court other males because they are lacking a gene that enables them to discriminate between the sexes,” Dr Bailey said.

“But that is different from male bottlenose dolphins, who engage in same-sex interactions to facilitate group bonding, or female Laysan albatrosses that can remain pair-bonded for life and co-operatively rear young.”

Almost a third of chick-raising pairs of Laysan albatrosses were found to be all female in one Hawaiian colony. Although they did not raise as many chicks as male-female pairs, females that shared parenting did much better than solitary females.

If these birds seem like paragons of parental rectitude, in other animals, homosexual behaviour is more fleeting — but even more common. For bottlenose dolphins, about half of male sexual encounters are with other males, while one study of the bearded vulture showed that as many as a quarter of mountings were male-to-male.

Bonobos are known to have a penchant for same-sex genital rubbing and even oral sex, and male bat bugs pierce the bodies of other males with their penises and ejaculate into their blood, just as they do to females.

Dr Bailey argues that homosexual activity could act as a selective pressure. “Same-sex behaviour can have evolutionary consequences that are beginning to be considered. For example, male-male copulations in locusts can be costly for the mounted male, and this cost may increase selection pressure for males’ tendency to release a chemical which dissuades other males from mounting them,” he said.

The indiscriminate attempt of male common toads to mate with each other also seems to have affected the way their cries have evolved, he said.

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