Visual signalling by asymmetry: a review of perceptual processes

Abstract

Individual levels of asymmetry in traits that display £uctuating asymmetry could be used as visual signals of phenotypic (and perhaps genotypic) quality, as asymmetry can often be negatively related to ¢tness parameters. There are some data to support this hypothesis but the experimental protocols employed have commonly resulted in asymmetries far larger than those observed in nature. To date, there has been little consideration of the ability of animals to accurately discriminate small asymmetries (of the magnitude observed in the wild) from perfect symmetry. This is key to assessing the plausibility of the asymmetry-signalling hypothesis. Here, I review the perceptual processes that may lead to the discrimination of asymmetry and discuss a number of ecologically relevant factors that may in£uence asymmetry signalling. These include: signal orientation, distance of trait elements from the axis of symmetry, trait complexity, trait contrast and colour, and the behaviour of both signaller and receiver. I also discuss the evolution of symmetry preferences and make suggestions as to where researchers should focus attention to examine the generality of asymmetry-signalling theory. In highly developmentally stable signalling systems the magnitude of asymmetry may be too small to be detected accurately and reliably, hence asymmetry signalling is unlikely to have evolved in these situations.

Symmetrical crypsis and asymmetrical signalling in the cuttlefishSepia officinalis

“…The deimatic display shown by S. officinalis to potential predators is strikingly symmetrical when expressed fully. Swaddle (1999) identifies several features of visual signals that amplify symmetry detection: the use of simple, conspicuous elements around the midline, a border around the edge, a vertical axis of bilateral symmetry, and orientation of that axis towards the receiver. The complete deimatic display comprises all of these features, suggesting that symmetry is important to the efficacy of this signal, and thus supporting the prediction that anti-predator displays will evolve towards a symmetrical design.…”

Behind the Looking-Glass: A Review on Human Symmetry Perception

“…These isometries consist of reflection symmetry (aka mirror symmetry or bilateral symmetry), rotational symmetry, translational symmetry (aka repetition), and of combinations of these three basic operations. Although the human visual system is sensitive to all of these symmetries, bilateral symmetry is probably the single most salient symmetry [24], the most investigated, and the most relevant to humans and other vertebrates [3]. The rest of this paper focuses on bilateral symmetry, for which the shorthand 'symmetry' will be adopted.…”

“…The literature contains a number of excellent overviews of human symmetry perception research [2][3][4][5][6]. Comprehensive reviews date back to the 1990s, however, and since then, advances have been made on all fronts of human symmetry research.…”

“…Consequently, whether or not sensitivity to symmetry evolved due to evolutionary pressure is still a matter of debate. For a more extensive treatment of symmetry in the animal world and the role of natural selection, refer to [3].…”

Asymmetry and Symmetry in the Beauty of Human Faces

“…Courtship displays by peacocks, birds of paradise, or butterflies are meaningless as far as attracting humans for reproduction, and the same applies to bowerbirds, or swallows, or rabbits, and so on. There is simplicity in ease of detection in the case of perceptual symmetry [45,46], without even invoking the issue of aesthetics in the brain of the animal observer. While the perceptual simplicity allows for detection of deviations arising from, say, bodily parasite invasion, diseases, toxins, unsuccessful fights, and can be made by simple brains, the sophisticated, highly evolved asymmetric human brain, in contrast, can handle -and does -elaborate communication signals, including multiple symbols, metaphors, and cultural considerations.…”