Social mating systems and extrapair fertilizations in passerine birds

Two alternative hypotheses have been proposed to explain how social and gen etic mating systems are interrelated in birds. According to the first (male trade-off) hypothesis, social polygyny should increase extrapair fertiliza tions because when males concentrate on attracting additional social mates, they cannot effectively protect females with whom the); have already paire d from being sexually assaulted. According to the second (female choice) hy pothesis, social polygyny should decrease extrapair fertilizations because a substantial proportion of females can pair with the male of their choice, and males can effectively) guard each mate during her fertile period. To d iscriminate these alternatives, we comprehensively reviewed information on social mating systems and extrapair fertilizations in temperate zone passer ine birds. We found significant inverse relationships between proportions o f socially polygynous males and frequencies of extrapair young, whether eac h species was considered as an independent data point (using parametric sta tistics) or phylogenetically related species were treated as nonindependent (using contrasts analyses). When social mating systems were dichotomized, extrapair chicks were twice as frequent in monogamous as in polygynous spec ies (0.23 vs. 0.11). We hypothesize that in socially polygynous species, (1 ) there is less incentive for females and males to pursue extrapair matings and (2) females incur higher costs for sexual infidelity (e.g., due to phy sical retaliation or reduction of paternal efforts) than in socially monoga mous species.