Fitness and asymmetry under different environmental conditions in the barn swallow

Individual differences in developmental stability are often estimated using individual asymmetry for bilaterally symmetric traits. These are expected to reflect intrinsic abilities to cope with adverse environmental conditions, an important aspect of genotypic quality which should be associated with fitness, but this supposition has never been tested. Two experiments which manipulated environmental conditions were used to investigate whether individual asymmetry under good environmental conditions adequately predicts a genotype's capacity to buffer development across environments, and further to test whether individuals with superior buffering capacity against environmental stress, as reflected in less increase in asymmetry between poor and good environments, had higher fitness than those with poor buffering capacity, after controlling for the influence of asymmetry per se on fitness. Individual asymmetry of the outer tail feathers of male barn swallows, Hirundo rustica, was compared between two different kinds of good and poor environments. Although some individuals changed ranks for asymmetry values between environmental qualities, individual asymmetry under good conditions correlated with this trait in poor environmental conditions. Furthermore, asymmetry under good environmental conditions was positively correlated to change in asymmetry in one of the two experiments, indicating that individual asymmetry is a good predictor of developmental buffering ability. Individuals that increased in asymmetry less under poor environmental conditions had higher reproductive success, even after controlling for asymmetry phenotype, indicating that developmental stability reflects individual quality, as expressed in high relative fitness. These experiments demonstrate that intrinsic differences in developmental stability reliably predict an important fitness component.