Visual signalling by asymmetry: a review of perceptual processes

Individual levels of asymmetry in traits that display fluctuating asymmetry could be used as visual signals of phenotypic land perhaps genotypic) qual ity, as asymmetry can often be negatively related to fitness parameters. Th ere are some data to support this hypothesis but the experimental protocols employed have commonly resulted in asymmetries far larger than those obser ved in nature. To date, there has been little consideration of the ability of animals to accurately discriminate small asymmetries (of the magnitude o bserved in the wild) from perfect symmetry. This is key to assessing the pl ausibility of the asymmetry-signalling hypothesis. Here, I review the perce ptual processes that may lead to the discrimination of asymmetry and discus s a number of ecologically relevant factors that may influence asymmetry si gnalling. These include: signal orientation, distance of trait elements fro m the axis of symmetry, trait complexity, trait contrast and colour, and th e behaviour of both signaller and receiver. I also discuss the evolution of symmetry preferences and make suggestions as to where researchers should f ocus attention to examine the generality of asymmetry-signalling theory. In highly developmentally stable signalling systems the magnitude of asymmetr y may be too small to be detected accurately and reliably, hence asymmetry signalling is unlikely to have evolved in these situations.