Homosexuality occurs in most animal species, from insects to mammals, and it is more common amongst males. But how could homosexual behaviour have evolved if same-sex copulations do not result in offspring and should therefore decrease fitness? Furthermore, copulation tends to be an energetically costly activity for both parties involved and even though it tends to be less so for males than for females, the fact remains that even male–male homosexuality appears to be a waste of energy and gametes. The truth though, is that exclusive homosexuality is rarely present in non-human animals, and most males that engage in homosexual encounters also mate with females. Could it be possible, then, that male homosexual behaviour leads to an increase in subsequent success with female mates? If this were the case, homosexual behaviour would have a direct positive effect on fitness.
When it comes to mate choices, females of the live-bearing fish Poecilia mexicana like to copy their female friends. That is, seeing a male consorting with someone else makes him a more desirable mate and they are more likely to mate with him later, probably because copulation in itself is an indication of male quality; but would the effect of this fishy voyeurism be different if the sexual participants were both males? A recent paper by David Bierbach and his colleagues from the University of Frankfurt in Germany provides insight into the secret sexual lives of bisexual male fish and the aphrodisiac effects of their sexual escapades on their heterosexual mates.
There are two types of P. mexicana males; drab, small males, and big, colourful ones. Not surprisingly, females generally prefer the larger, more handsome suitors. Before copulations, males of the species engage in a pre-mating behaviour called ‘nipping’, in which a male makes oral contact with either the female's genital opening or the male's copulatory organ depending on the sex of his chosen mate. To test the hypothesis that male–male sexual behaviour increases male attractiveness to females, Bierbach and his team presented females with a digital animation of either a drab male ‘nipping’ on another male or a drab male ‘nipping’ on a female. The team then determined whether the female bias for colourful males had changed based on the behavioural interactions they had observed.
The results of these experiments showed that females that had watched the drab males ‘nipping’ at either other males or females increased their preference towards these drab males. The aphrodisiac effect on the females was similar whether they had watched videos of homosexual or heterosexual interactions suggesting that, for males, sexual encounters, even if they are homosexual, will increase their fitness by increasing their chances of later finding a female mate.
In their study, Bierbach and his colleagues provide insight into the evolution of homosexual behaviour. So, what adaptive benefits could homosexual behaviour confer to individuals for it to be so ubiquitous across the animal kingdom? Well, this study doesn't talk about other animals but if you are a little ugly P. mexicana male and can't seem to find a female date, mating with another male might increase your chances of scoring a nice fertile female down the road.
