Genetic similarity, extrapair paternity, and offspring quality in Savannah sparrows (Passerculus sandwichensis)

The occurrence of extrapair paternity (EPP) in birds is often attributed to the action of good-genes sexual selection whereby females "trade up" on male genetic quality by allocating fertilizations to males with better genes than those possessed by their social mate. To date, most studies of EPP in birds focus on absolute measures of male quality as a criterion for female choice, although multiple mating by females in other taxa is more commonly ascribed to benefits associated with the individual optimization of offspring genotypes. Here, we examine whether the genetic similarity of social mates predicts female mating patterns in a population of Savannah sparrows (Passerculus sandwichensis) where as many as 70% of adults produce extrapair young (EPY). We consider the influence of genetic similarity across all stages of a female's decision-making process, from pair formation through the decision to produce EPY, to the allocation of fertilizations to specific extrapair sires. In a 4-year study of 175 males, 206 females, and 506 offspring, females were more likely to produce EPY when paired to genetically similar males, but they did not appear to be influenced by the size, age, mass, individual heterozygosity, and genetic diversity of their social mates. In paired comparisons, females were almost twice as likely to decrease their genetic similarity to males when producing EPY as they were to increase it. Nonetheless, females did not select especially dissimilar males when mating outside the pair-bond nor did they pair disassortatively with respect to genetic similarity. Relative measures of male quality may influence mating patterns in birds, but only at some points in a female's decision-making process.