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

Aldo Poiani*
School of Biological Sciences, Monash University, Australia

© 2008 Aldo Poiani

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: ( This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the School of Biological Sciences, Monash University, Victoria 3800, Australia; Tel: +61 3 99055665; Fax: +61 3 9905 5613; E-mail:


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 resolved. Here I introduce a novel model, the Synthetic Reproductive Skew Model of Homosexuality that aims at understanding 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 utilises 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 specific social circumstances and sociosexual interactions that, in birds, seem to be more affected by dominance conflicts than by affinitive relationships.

Keywords: Homosexuality, comparative analyses, birds, reproductive skew.