Most species of animal display homosexual behaviour to some extent. It’s been reported in everything from barn owls to bison.
But theory tells us that traits that don’t help an individual pass on its genes to the next generation, such as homosexuality, should soon disappear from a population. So how has it been favoured by evolution? Herein has always lain something of a mystery.
A new study may provide some clues. It shows that males inclined to display overt homosexual behaviour actually increase their chances of getting a girl who might otherwise be uninterested.
The study concentrates on Atlantic mollies, small fish related to guppies. Female mollies prefer to mate with big, brightly coloured males, and ignore small, relatively drab males.
But as David Bierbach of the University of Frankfurt and colleagues show in new research published in the journal Biology Letters, female mollies will go for drabber males that have previously been involved in same-sex interactions, favouring them over drab males that have had no prior sexual interactions at all.
In the world of Atlantic mollies, it seems the girls prefer males with a history of successful sexual conquest. (Read 1,500 animal species practice homosexuality)
So it’s easy to see how homosexuality can persist among fishes and other animals that swing both ways. Things get a bit more tricky when we try to explain how exclusive homosexuality is maintained.
Humans, sheep and some birds display exclusively gay behaviour, in which members of one sex consistently pair only with individuals of their own sex. But again, a new study may explain this.
Andrea Camperio Ciani and Elena Pellizzari of the University of Padova in Italy, have spotted an interesting trend. They have found that the maternal grandmothers and aunts of homosexual men produce more offspring, overall, than the maternal grandmothers and aunts of heterosexual men. (read Homosexual Animals Out of the Closet)
In other words this research, published in the online journal PLOS ONE, appears to show that homosexuality is the result of having female relatives who are unusually fecund. This supports the idea of a model called sexual conflict.
Males and females largely share the same genes, but genes that have one effect in one sex may have another effect in the other: genes that promote female fecundity might, for instance, result in homosexuality in male relatives. Provided that the overall number of offspring is maintained, the tendency for some males in the family to be gay will therefore remain.
It goes to show that our genes work in complex and subtle ways.