A new theory argues same-sex sexual behaviour is an evolutionary norm

WHEN IT COMES to sexual behaviour, the animal kingdom is a broad church. Its members indulge in a wide variety of activities, including with creatures of the same sex. Flying foxes gather in all-male clusters to lick each other’s erect penises. Male Humboldt squid have been found with sperm-containing sacs implanted in and around their sexual organs in similar quantities to female squid. Female snow macaques often pair off to form temporary sexual relationships that includes mounting and pelvic thrusting. Same-sex sexual behaviour has been recorded in some 1,500 animal species.The mainstream explanations in evolutionary biology for these behaviours are many and varied. Yet they all contain a common assumption: that sexual behaviours involving members of the same sex are a paradox that does indeed need explaining. Reproduction requires mating with a creature of the opposite sex, so why does same-sex mating happen at all?