Same-Sex Mounting in Birds: Comparative Test of a Synthetic Reproductive Skew Model of Homosexuality

Same-sex mounting is an evolutionary paradox in that it is not directly conducive to fertilization in spite of its obvious sexual origin. Whether same-sex mounting is an adaptive behaviour that indirectly enhances reproductive success of self or close relatives through its mediation of dominance or cooperative interactions, or whether it is just a by-product of neuroendocrinological conditions manifested during breeding periods of the year is an issue that remains to be re- solved. Here I introduce a novel model, the Synthetic Reproductive Skew Model of Homosexuality that aims at understand- ing same-sex mounting as a result of the combined effect of a set of variables and processes that affect both sexual and sociosexual aspects of behaviour. I also provide a comparative test of the model, the test is circumscribed to birds and util- ises data from 72 avian taxa. Comparative analyses suggest that same-sex mounting in birds is an evolutionary result of inter-individual interactions associated with the dynamics of reproductive skew, direct effects of sexual readiness in spe- cific social circumstances and sociosexual interactions that, in birds, seem to be more affected by dominance conflicts than by affinitive relationships.