The famous primatologist Frans de Waal once tried to explain to me the finer points of bonobo etiquette. “You know how we shake hands? They shake genitals.”

The majority of sexual activity among bonobos, most of which are bisexual, is done purely for pleasure and social bonding rather than procreation. They really do make love, not war. But homosexual matings lead to a reproductive dead-end, so make little sense from an evolutionary point of view.

In most animals, the sexual urge and its payoff, orgasm, is a strong driver of behaviour — so that a species reproduces and ultimately survives. This urge may compel animals to seek satisfaction even when there is no reproductive benefit, such as when an animal is in an all-male group. It also might promote bonding in such a troop, according to Professor Robin Dunbar, an evolutionary expert who has pointed out that, in Ancient Greece, Spartans encouraged homosexuality among its elite. The emotional ties would strengthen commitment to each other, raising their chances of survival in battle.

Lesbianism in macaques, while not directly resulting in offspring, might still ultimately be of reproductive benefit. Sexual activity might, primatologists have suggested, be a way for a junior female to form a strategic alliance or bond with a senior female. This might raise her status within the group, so that she attracts a high-status male to mate with.

The kind of same-sex parenting seen in Laysan albatrosses makes sense when we remember that chicks reared by two parents are more likely to survive than the chicks of lone parents, since they have twice the resources and twice the protection. The ultimate goal is the survival of the colony, because it is only under the group’s protective roof that an individual can propagate its own genetic line.

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