On avian asymmetry: evidence of natural selection for symmetrical tails and wings in birds
Abstract
Fluctuating asymmetry (fa), or minor deviations from perfect bilateral symmetry in otherwise symmetrical structures, may reflect exposure to environmental or genetic stresses during trait development. Consequently, several authors have recently suggested that FA in elaborate secondary sexual characters such as the long tails of some birds could play an important role in sexual selection, perhaps by being used as a reliable signal of phenotypic quality during mate choice. Here we examine an alternative possibility: that the extent of fa in long tails and wings of birds is constrained primarily by natural selection for aerodynamic efficiency. Predictions developed from aerodynamic theory were tested by using comparative data from 63 long-tailed species. Data on fa in wing lengths confirmed that aerodynamic requirements may underlie patterns of fa in lifting surfaces: species that spend much of their time flying and those that migrate had more symmetrical wings than other birds. There was also substantial variation in the extent of tail fa. As predicted, long-tailed species with aerodynamically functional forked tails had more symmetrical outer tail feathers than those with other long tail types in which feather symmetry has less impact on flight performance. However, in common with several other recent studies, our data failed to support the predictions of the sexual selection hypothesis. In sexually dimorphic species, there was no evidence that tail asymmetry was greater in the longer-tailed sex, nor was tail fa negatively correlated with tail length. Our results therefore suggest that patterns of fa in long tails and wings may often be better understood in a context of natural rather than sexual selection.
