Asymmetrical incest avoidance in the choice of social and genetic mates
Mating with close relatives generally results in reduced reproductive success (inbreeding depression) because it increases the risk that rare deleterious recessive alleles will be expressed in offspring. None the less, incest may occur when animals have incomplete knowledge about relatedness or when the costs of avoiding inbreeding are high. Over a 17-year period, Savannah sparrows, Passerculus sandwichensis, in an island population rarely paired incestuously (9 of 1110 pairs, 15 of 1609 nesting attempts). All but one case of close inbreeding (coefficient of inbreeding, F ≥ 0.25) involved 1-year-old males breeding for the first time, whereas more than half of the cases involved females that were 3 years old or older. Father–daughter matings were avoided completely. Incest avoidance was apparent in the choice of genetic as well as social mates. Paternity analysis using microsatellites revealed that birds nearly always refrained from choosing close relatives as genetic mates. These results support a model of asymmetrical incest avoidance, which predicts differences in the likelihood of incestuous matings as a function of sex, age and relationship, even when coefficients of inbreeding are identical. The model and results also emphasize the importance of distinguishing types of inbreeding and considering the social and historical context of animals' mating choices. The model may help to explain such patterns as female-biased natal dispersal in birds.