Same-sex sexual behavior in birds: expression is related to social mating system and state of development at hatching

We report the findings of a phylogenetic comparative analysisexamining patterns and frequency of occurrence of same-sex courtshipand mounting behavior in birds. Our analysis has shown associationsbetween same-sex sexual behavior and both mating system anddegree of precociousness at hatching. The patterns of expressionand frequency of occurrence of same-sex sexual behavior differedmarkedly for males and females. Patterns of same-sex sexualexpression reflected the competitive sexes that actively solicitsexual interactions in heterosexual encounters. Male–male(MM) sexual behavior occurred across all mating systems, butMM mounting was significantly more prevalent in those specieswith facultative polygamy. The frequency of MM sexual behaviorincreased with degree of polygamy. Female–female (FF)sexual behavior (especially courtship) occurred most frequentlyin socially monogamous species and rarely occurred in speciesthat display obligate polygamy (predominantly polygynous species).Both expression and frequency of FF sexual behavior was stronglyrelated to the precocial state of development at hatching. FFsexual behavior is more likely to occur in species in whichmonogamy occurs together with the production of precocial offspring;that is, in monogamous species that are exceptions to the morecommon altricial mode of development. We suggest that requirementof biparental care in monogamous species may influence the greaterexpression of FF sexual behavior and longer term associations.Both spatial and behavioral dispersion of females and engagementin uniparental care may be important in explaining the lowerincidence of FF sexual behavior in polygynous species. Socialcontexts where males congregate at communal leks or displayareas may influence the greater expression and frequency ofMM sexual behavior in polygynous species.