Social mating systems and extrapair fertilizations in passerine birds

Two alternative hypotheses have been proposed to explain howsocial and genetic mating systems are interrelated in birds.According to the first (male trade-off) hypothesis, socialpolygyny should increase extrapair fertilizations because whenmales concentrate on attracting additional social mates, theycannot effectively protect females with whom they have already paired from being sexually assaulted. According to the second(female choice) hypothesis, social polygyny should decreaseextrapair fertilizations because a substantial proportion offemales can pair with the male of their choice, and males caneffectively guard each mate during her fertile period. To discriminatethese alternatives, we comprehensively reviewed informationon social mating systems and extrapair fertilizations in temperatezone passerine birds. We found significant inverse relationshipsbetween proportions of socially polygynous males and frequenciesof extrapair young, whether each species was considered asan independent data point (using parametric statistics) orphylogenetically related species were treated as nonindependent (using contrasts analyses). When social mating systems weredichotomized, extrapair chicks were twice as frequent in monogamousas in polygynous species (0.23 vs. 0.11). We hypothesize thatin socially polygynous species, (1) there is less incentivefor females and males to pursue extrapair matings and (2) femalesincur higher costs for sexual infidelity (e.g., due to physical retaliation or reduction of paternal efforts) than in sociallymonogamous species.