Elizabeth claimed oestrogen used by transgender people is making fish change gender. (Sherry Zhao2012/Flickr)

German scientists say they have found a tropical male fish that becomes more attractive to females when it displays same-sex behaviour.

Their research has shown the female Atlantic molly prefer mating partners whom they saw having sex with other males.

The University of Frankfurt study concludes that male fish can therefore increase their attractiveness towards females by displaying same-sex behaviour and thereby improve their chances of future heterosexual mating.

David Bierbach, of the University of Frankfurt, and colleagues said this new mechanism might explain the occurrence and persistence of homosexuality also in other animal species.

The researchers said: “Male homosexual behaviour – although found across the animal kingdom – remains a conundrum, as same-sex mating should decrease male reproductive fitness.

“In most species, however, males that engage in same-sex sexual behaviour also mate with females, and in theory, same-sex mating could even increase male reproductive fitness if males improve their chances of future heterosexual mating.