Sexual Conflict and Genital Evolution in Birds

In collaboration with postdoc Dr. Patricia Brennan, we have been investigating the evolution of genitalia in basal lineages of living birds. In 2007, we described anatgonistic sexual coevolution in genitalia of waterfowl (Brennan et al. 2007)– the first example of this phenomenon in vertebrates. Waterfowl have complex breeding systems that include elaborate male courtship display, intersexual female preferences for social mates to form pair bonds, forced extra-pair copulations (FEPCs), and female resistance behaviors and anatomies. Genital coevolution in waterfowl is dynamic, including multiple phylogenetically independent instances of genital elaboration and simplification (Brennan et al. 2007). A phylogenetically controlled comparative analysis of 16 species showed that the degree of vaginal elaboration is positively correlated with phallus length (Brennan et al. 2007). The vaginal complexity in waterfowl appears to function in making phallus penetration more difficult during forced copulations. Waterfowl genitalia are also highly seasonal– regrowing and regressing every year. Waterfowl are behaviorally complex organisms that provide a new opportunity to investigate the role of sexual conflict between female mating preferences and coercive male-male competition in the evolution of genital morphology.

Additional comparative anatomical and phylogenetic analyses have documented two novel, phylogenetically independent reductions in phallus complexity within the tinamous and the cracids (Brennan et al. 2008).

Current research is focusing on the functional morphology of the avian phallus, the role of social environment in the development of male genitalia, and development of sexual conflict theory to genital evolution.


Brennan, P. L. R., Prum, R. O., McCracken, K. G., Sorenson, M. D., Wilson, R. E., and Birkhead, T. R. 2007. Coevolution of male and female genital morphology in waterfowl. PLoS One 2(5): e418 (6 pp).

Brennan P.L.R., Birkhead T., Zyskowski K., Van deer Waag J. & Prum R.O. 2008. Independent evolutionary reductions of the phallus in basal birds. Journal of Avian Biology 39:487-492.

Brennan, P. L. R., and Prum, R. O. In Review. Sensory bias meets girls gone wild: Limits of sexual conflict in the narrow sense. Behavioral Ecology.